DOS 4.00 Technical Reference and Application Programming

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DOS 4.00: TECHNICAL REFERENCE AND APPLICATION PROGRAMMING                                                         
                                                                                
August 6th, 1988                                                                
                                                                                
FIRST EDITION (JULY 1988)                                                       
                                                                                
(c) Copyright International Business Machines Corporation 1981, 1988            
All rights reserved.                                                            
                                                                                
PREFACE                                                                         
                                                                                
AUDIENCE                                                                        
                                                                                
This book is written for programmers who develop applications for IBM           
Personal Computers and Personal System/2(R) computers.(1)                       
                                                                                
The program developer should be competent on the IBM Personal Computer and/or   
the Personal System/2 and should be familiar with DOS and at least one          
personal computer programming language.                                         
                                                                                
CONTENT                                                                         
                                                                                
Some parts of the book have been rewritten and reorganized to make reference    
and system architecture information more concise and to present programming     
information in a task-oriented manner.  Information about new and enhanced      
functions has been added, providing some guide material.                        
                                                                                
RELATED PUBLICATIONS                                                            
                                                                                
Other manuals that provide detailed information about DOS 4.00 are:             
                                                                                
    Getting Started with DOS 4.00                                               
    Using DOS 4.00                                                              
    DOS 4.00 Command Reference                                                  
    IBM Keyboard Layouts for Your PC and PS/2 systems.                          
                                                                                
                                                                                
(1) Personal System/2 is a registered trademark of the International Business   
Machines Corporation.                                                           
                                                                                
CONTENTS                                                                        
                                                                                
CHAPTER 1.  INTRODUCTION  TRD41  7
Organization of this Book  TRD41  16
New DOS 4.00 Services  TRD41  43
The Utilities Diskette  TRD41  57
Minimum Hardware Configuration  TRD41  79
CHAPTER 2.  ACCESSING DISKS  TRD42  11
The Disk Format  TRD42  20
The Boot Record  TRD42  41
The File Allocation Table (FAT)  TRD42  49
The Disk Directory  TRD42  114
The Data Area  TRD42  143
Accessing the Disk  TRD42  150
Reading and Writing Data Directly to the Disk  TRD42  168
Requesting Drive and Disk Information  TRD42  180
CHAPTER 3.  ACCESSING FILES WITH FILE HANDLES  TRD43  13
Filenames  TRD43  48
File Handles  TRD43  62
Special File Handles  TRD43  88
Reading and Writing Data to a File  TRD43  119
Requesting and Specifying File Attributes  TRD43  140
Accessing Subdirectories  TRD43  152
Accessing Directories  TRD43  266
Finding Files in Directories  TRD43  282
Requesting and Specifying National Language Support (NLS)  TRD43  294
Controlling Network Operations  TRD43  307
CHAPTER 4.  ACCESSING FILES USING FILE CONTROL BLOCKS  TRD44  10
The File Control Block (FCB)  TRD44  21
The Extended FCB  TRD44  128
The Disk Transfer Area (DTA)  TRD44  149
Accessing Files  TRD44  172
Accessing Sequential Records  TRD44  218
Accessing Random Records  TRD44  231
Finding Files in Directories  TRD44  250
CHAPTER 5.  MANAGING DEVICE I/O  TRD45  9
Managing Display I/O  TRD45  20
Managing Keyboard I/O  TRD45  43
Managing Miscellaneous I/O  TRD45  71
Managing File System Activities  TRD45  84
Accessing the System Device Drivers' Control Channel  TRD45  105
Reading and Writing Data in Binary and ASCII Modes  TRD45  155
CHAPTER 6.  CONTROLLING PROCESSES  TRD46  17
Allocating Memory  TRD46  32
DOS 4.00 Memory Management  TRD46  47
The DOS 4.00 Memory Map  TRD46  71
Identifying a Program at Load Time  TRD46  134
The Program Segment  TRD46  147
Loading and Executing Overlays  TRD46  198
The Parameter Block  TRD46  216
Terminating a Program/Subprogram  TRD46  260
Loading an Overlay without Executing It  TRD46  307
Calling a Command Processor  TRD46  328
Responding to Errors  TRD46  352
Responding to a Control-Break Action  TRD46  379
Requesting and Specifying the System Date and Time  TRD46  411
Requesting and Specifying the Interrupt Vectors  TRD46  426
CHAPTER 7.  CREATING OBJECT CODE LIBRARIES  TRD47  15
The IBM Library Manager/2  TRD47  20
Starting the LIB.EXE Utility  TRD47  52
Entering Input at the Command Line  TRD47  216
Using a Response File  TRD47  308
Creating and Maintaining Libraries  TRD47  359
Creating a Library File  TRD47  364
Modifying a Library File  TRD47  395
Combining Libraries  TRD47  457
Creating a Cross-Reference Listing  TRD47  475
Performing Consistency Checks  TRD47  500
Setting the Library Page Size  TRD47  513
Library Manager Error Messages  TRD47  540
CHAPTER 8.  CREATING AN EXECUTABLE FILE  TRD48  35
The IBM Linker/2  TRD48  40
Starting the LINK.EXE Program  TRD48  56
Entering LINK Input at the Command Line  TRD48  246
Using a Response File to Supply LINK Input  TRD48  340
Using Linker Options  TRD48  436
Preparing Files for CodeView /CODEVIEW  TRD48  512
Reserving Paragraph Space /CPARMAXALLOC  TRD48  530
Ordering Segments /DOSSEG  TRD48  560
Controlling Data Loading /DSALLOCATE  TRD48  583
Packing Executable Files /EXEPACK  TRD48  607
Viewing the Options List /HELP  TRD48  640
Controlling Run File Loading /HIGH  TRD48  661
Displaying LINK-Time Information /INFORMATION  TRD48  679
Copying Line Numbers to the Map File /LINENUMBERS  TRD48  692
Producing a Public Symbol Map /MAP  TRD48  731
Ignoring Default Libraries /NODEFAULTLIBRARYSEARCH  TRD48  770
Preserving Compatibility /NOGROUPASSOCIATION  TRD48  793
Preserving Lowercase /NOIGNORECASE  TRD48  807
Setting the Overlay Interrupt /OVERLAYINTERRUPT  TRD48  836
Pausing to Change Disks /PAUSE  TRD48  856
Setting the Maximum Number of Segments /SEGMENTS  TRD48  900
Setting the Stack Size /STACK  TRD48  939
Reading the Map File  TRD48  976
Creating an Overlaid Version of Your Program  TRD48  1032
Specifying an Overlay Structure to LINK  TRD48  1050
How LINK Formats the .EXE File  TRD48  1075
Ordering Segments  TRD48  1083
Segment Combine-Types  TRD48  1106
Groups  TRD48  1143
Instruction and Data Reference Errors  TRD48  1159
Linker Error Messages  TRD48  1215
Linker Limits  TRD48  1681
CHAPTER 9.  CONVERTING FILE FORMATS  TRD49  8
The EXE2BIN.EXE Utility  TRD49  13
Entering Input to EXE2BIN  TRD49  34
Two Types of Conversion  TRD49  61
Device Drivers  TRD49  66
Standard .COM File  TRD49  100
CHAPTER 10.  DEBUGGING A PROGRAM  TRD410  32
The DEBUG Utility  TRD410  37
Starting the DEBUG.COM Program  TRD410  46
Entering Commands at the DEBUG Prompt  TRD410  67
DEBUG Command Summary  TRD410  103
The DEBUG Work Space  TRD410  181
A (Assemble) Command  TRD410  225
C (Compare) Command  TRD410  348
D (Dump) Command  TRD410  400
E (Enter) Command  TRD410  504
F (Fill) Command  TRD410  620
G (Go) Command  TRD410  669
H (Hexarithmetic) Command  TRD410  771
I (Input) Command  TRD410  789
L (Load) Command  TRD410  811
M (Move) Command  TRD410  919
N (Name) Command  TRD410  973
O (Output) Command  TRD410  1019
P (Proceed) Command  TRD410  1040
Q (Quit) Command  TRD410  1090
R (Register) Command  TRD410  1113
S (Search) Command  TRD410  1263
T (Trace) Command  TRD410  1321
U (Unassemble) Command  TRD410  1390
W (Write) Command  TRD410  1513
XA (EMS Allocate) Command  TRD410  1645
XD (EMS Deallocate) Command  TRD410  1672
XM (EMS Map) Command  TRD410  1699
XS (EMS Status) Command  TRD410  1727
DEBUG Error Messages  TRD410  1759
CHAPTER 11.  WRITING AN INSTALLABLE DEVICE DRIVER  TRD411  26
Types of Device Drivers  TRD411  39
Character Device Drivers  TRD411  48
Block Device Drivers  TRD411  55
How DOS 4.00 Installs Device Drivers  TRD411  84
The Basic Parts of a Device Driver  TRD411  134
The Device Driver Header  TRD411  148
The Strategy Routine  TRD411  274
The Interrupt Routines  TRD411  283
How DOS 4.00 Passes a Request  TRD411  300
Responding to Requests  TRD411  392
Initialization Request  TRD411  456
Media Check Request  TRD411  511
Build BPB Request  TRD411  630
Input and Output Requests  TRD411  815
Nondestructive Input No Wait Request  TRD411  879
Character Input and Output Status Requests  TRD411  901
Character Input and Output Flush Requests  TRD411  935
Open and Close Requests  TRD411  951
Removable Media Request  TRD411  983
Generic IOCTL Request  TRD411  1008
Get Logical Device Request  TRD411  1037
Set Logical Device Request  TRD411  1055
CLOCK$ Device Driver Example  TRD411  1073
APPENDIX A.  DOS 4.00 INTERRUPTS  TRD412  13
20H Program Terminate  TRD412  21
21H Function Request  TRD412  41
22H Terminate Address  TRD412  46
23H Ctrl-Break Exit Address  TRD412  53
24H Critical Error Handler Vector  TRD412  83
25H/26H Absolute Disk Read/Write  TRD412  287
27H Terminate but Stay Resident  TRD412  353
28H-2EH Reserved for DOS 4.00  TRD412  392
2FH Multiplex Interrupt  TRD412  396
30H-3FH Reserved for DOS 4.00  TRD412  695
APPENDIX B.  DOS 4.00 FUNCTION CALLS  TRD413  100
Using DOS 4.00 Function Calls  TRD413  219
Program Code Fragments  TRD413  230
.COM Programs  TRD413  236
DOS 4.00 Registers  TRD413  256
Responding to Errors  TRD413  340
Extended Error Codes  TRD413  355
00H --  Program Terminate  TRD413  518
01H --  Console Input with Echo  TRD413  546
02H --  Display Output  TRD413  581
03H --  Auxiliary Input  TRD413  603
04H --  Auxiliary Output  TRD413  631
05H --  Printer Output  TRD413  655
06H --  Direct Console I/O  TRD413  672
07H --  Direct Console Input Without Echo  TRD413  720
08H --  Console Input Without Echo  TRD413  753
09H --  Display String  TRD413  786
0AH --  Buffered Keyboard Input  TRD413  815
0BH --  Check Standard Input Status  TRD413  848
0CH --  Clear Keyboard Buffer and Invoke a Keyboard Function  TRD413  868
0DH --  Disk Reset  TRD413  885
0EH --  Select Disk  TRD413  904
0FH --  Open File  TRD413  937
10H --  Close File  TRD413  985
11H --  Search for First Entry  TRD413  1022
12H --  Search for Next Entry  TRD413  1099
13H --  Delete File  TRD413  1149
14H --  Sequential Read  TRD413  1193
15H --  Sequential Write  TRD413  1240
16H --  Create File  TRD413  1286
17H --  Rename File  TRD413  1330
19H --  Current Disk  TRD413  1380
1AH --  Set Disk Transfer Address  TRD413  1400
1BH --  Allocation Table Information  TRD413  1425
1CH --  Allocation Table Information for Specific Device  TRD413  1453
21H --  Random Read  TRD413  1489
22H --  Random Write  TRD413  1541
23H --  File Size  TRD413  1599
24H --  Set Relative Record Field  TRD413  1642
25H --  Set Interrupt Vector  TRD413  1668
26H --  Create New Program Segment  TRD413  1695
27H --  Random Block Read  TRD413  1725
28H --  Random Block Write  TRD413  1787
29H --  Parse Filename  TRD413  1847
2AH --  Get Date  TRD413  1918
2BH --  Set Date  TRD413  1946
2CH --  Get Time  TRD413  1979
2DH --  Set Time  TRD413  2015
2EH --  Set/Reset Verify Switch  TRD413  2051
2FH --  Get Disk Transfer Address (DTA)  TRD413  2085
30H --  Get DOS Version Number  TRD413  2108
31H --  Terminate Process and Remain Resident  TRD413  2138
33H --  Get/Set System Value  TRD413  2175
35H --  Get Interrupt Vector  TRD413  2235
36H --  Get Disk Free Space  TRD413  2262
38H --  Get or Set Country Dependent Information  TRD413  2318
39H --  Create Subdirectory (MKDIR)  TRD413  2431
3AH --  Remove Subdirectory (RMDIR)  TRD413  2466
3BH --  Change the Current Directory (CHDIR)  TRD413  2498
3CH --  Create a File (CREAT)  TRD413  2530
3DH --  Open a File  TRD413  2577
3EH --  Close a File Handle  TRD413  2812
3FH --  Read from a File or Device  TRD413  2840
40H --  Write to a File or Device  TRD413  2886
41H --  Delete a File from a Specified Directory (UNLINK)  TRD413  2938
42H --  Move File Read Write Pointer (LSEEK)  TRD413  2971
43H --  Change File Mode (CHMOD)  TRD413  3035
44H --  I/O Control for Devices  TRD413  3092
45H --  Duplicate a File Handle (DUP)  TRD413  3103
46H --  Force a Duplicate of a Handle (FORCDUP)  TRD413  3137
47H --  Get Current Directory  TRD413  3170
48H --  Allocate Memory  TRD413  3202
49H --  Free Allocated Memory  TRD413  3237
4AH --  Modify Allocated Memory Blocks (SETBLOCK)  TRD413  3263
4BH --  Load or Execute a Program (EXEC)  TRD413  3303
4CH --  Terminate a Process (EXIT)  TRD413  3442
4DH --  Get Return Code of a Subprocess (WAIT)  TRD413  3466
4EH --  Find First Matching File (FIND FIRST)  TRD413  3494
4FH --  Find Next Matching File (FIND NEXT)  TRD413  3567
54H --  Get Verify Setting  TRD413  3606
56H --  Rename a File  TRD413  3628
57H --  Get/Set File's Date and Time  TRD413  3669
59H --  Get Extended Error  TRD413  3714
5AH --  Create Unique File  TRD413  3785
5BH --  Create New File  TRD413  3839
5CH --  Lock/Unlock File Access  TRD413  3878
5E00H --  Get Machine Name  TRD413  3972
5E02H --  Set Printer Setup  TRD413  4006
5E03H --  Get Printer Setup  TRD413  4051
5F02H --  Get Redirection List Entry  TRD413  4096
5F03H --  Redirect Device  TRD413  4169
5F04H --  Cancel Redirection  TRD413  4259
62H --  Get Program Segment Prefix Address  TRD413  4305
65H --  Get Extended Country Information  TRD413  4328
66H --  Get/Set Global Code Page  TRD413  4451
67H --  Set Handle Count  TRD413  4494
68H --  Commit File  TRD413  4539
6CH --  Extended Open/Create  TRD413  4562
APPENDIX C.  I/O CONTROL FOR DEVICES (IOCTL)  TRD414  4
44H --  I/O Control for Devices (IOCtl)  TRD414  50
APPENDIX D.  EXPANDED MEMORY SUPPORT  TRD415  3

Chapter 1

CHAPTER 1.  INTRODUCTION 7
Organization of this Book 16
New DOS 4.00 Services 43
The Utilities Diskette 57
Minimum Hardware Configuration 79
  
CHAPTER 1.  INTRODUCTION                                                        
                                                                                
This chapter provides information about this book, including the following:     
                                                                                
    Organization of the book for quick information retrieval                    
    New and enhanced DOS 4.00 services                                          
    Contents of the utilities diskette                                          
    Minimum hardware configuration.                                             
                                                                                
ORGANIZATION OF THIS BOOK                                                       
                                                                                
This book is organized by logical application program development stages        
necessary to develop an application program on DOS 4.00.  You must:             
                                                                                
    Write the source code for the program                                       
    Translate the source code to executable code                                
    Load and execute the program                                                
    Debug the program                                                           
    Create object code libraries (optional)                                     
    Convert your file formats (optional).                                       
                                                                                
In addition, the book tells how to make best use of the operating system by     
writing your own device driver or by using the system extensions.               
                                                                                
Each chapter describes a particular subject.  You do not need to read the       
entire book to create programs or solve problems.  Key topics also can be       
found by referring to the index and the table of contents.                      
                                                                                
The appendixes contain reference information for quick retrieval.  They         
contain the entire numerical list of DOS 4.00 services, including interrupts,   
function calls, and device driver services.                                     
                                                                                
The Technical Quick Reference contains abbreviated information about            
interrupts.  Function calls are listed, along with input/output information     
and error codes.                                                                
                                                                                
NEW DOS 4.00 SERVICES                                                           
                                                                                
DOS 4.00 replaces IBM DOS Version 3.30 and incorporates all services            
previously provided by IBM DOS Version 3.30.  Major new features include:       
                                                                                
    Enhanced country support                                                    
    Double-byte character support                                               
    A device driver                                                             
    File system support of large disk media                                     
    Extended Memory Support (EMS)                                               
    Performance improvements to the file system.                                
                                                                                
Several function requests within interrupt 21H have been enhanced.              
                                                                                
THE UTILITIES DISKETTE                                                          
                                                                                
A utilities diskette is included with this book.  It contains a listing of      
the following utilities, and the description of each utility, to help           
programmers develop an application program:                                     
                                                                                
DEBUG.COM           A utility to isolate and determine errors in executable     
programs.                                                                       
                                                                                
EXE2BIN.EXE         A utility to convert executable file formats (.EXE) to      
.COM formats to make them more compact and, therefore,                          
load more quickly.                                                              
                                                                                
LIB.EXE             A utility that allows the programmer to build and edit      
object libraries.                                                               
                                                                                
LINK.EXE            A utility to translate object code to executable code.      
                                                                                
VDISK.ASM           A fully documented programming example of a device          
driver.  However, this example does not reflect the                             
current level of VDISK.SYS.                                                     
                                                                                
MINIMUM HARDWARE CONFIGURATION                                                  
                                                                                
DOS 4.00 supports the following family of IBM Personal Computers and Personal   
System/2 computers:                                                             
                                                                                
The IBM Personal Computer                                                       
The IBM Personal Computer XT(TM)                                                
The IBM Personal Computer XT(TM) Model 286(1)                                   
The IBM Personal Computer AT(R) (all versions)(2)                               
The IBM PC Convertible                                                          
The IBM Personal System/2(R) Model 25 (all versions)                            
The IBM Personal System/2(R) Model 30 (all versions)                            
The IBM Personal System/2(R) Model 50 (all versions)                            
The IBM Personal System/2(R) Model 60 (all versions)                            
The IBM Personal System/2(R) Model 80 (all versions).                           
                                                                                
NOTE:  DOS 4.00 does not support the IBM PCjr.                                  
                                                                                
The minimum memory requirement is 256KB.                                        
                                                                                
                                                                                
(1) Personal Computer XT is a trademark of the International Business           
Machines Corporation.                                                           
                                                                                
(2) Personal Computer AT and Personal System/2 are registered trademarks of     
the International Business Machines Corporation.                                
                                                                                
PART 1.  WRITING PROGRAMS                                                       
                                                                                

Chapter 9

CHAPTER 9.  CONVERTING FILE FORMATS 8
The EXE2BIN.EXE Utility 13
Entering Input to EXE2BIN 34
Two Types of Conversion 61
Device Drivers 66
Standard .COM File 100
  
CHAPTER 9.  CONVERTING FILE FORMATS                                             
                                                                                
This chapter describes how to use EXE2BIN.EXE on your utilities diskette to     
convert an .EXE file to a .COM file format.                                     
                                                                                
THE EXE2BIN.EXE UTILITY                                                         
                                                                                
EXE2BIN is useful if you have a simple .EXE file that doesn't need the DOS      
preparation that an .EXE file usually requires, such as telling the program     
its location in memory and setting up a stack for it to use.  Converting an     
.EXE file to a .COM format makes the file more compact and enables it to load   
faster.                                                                         
                                                                                
To qualify for EXE2BIN conversion, your .EXE file has to meet these criteria:   
                                                                                
    The file format must be a valid .EXE one, as produced by the IBM Linker.    
    Actual code and data in the file (the resident portion of the program)      
must be less than 64KB.                                                         
    Only one segment can be declared; therefore, the file cannot define a       
STACK segment.                                                                  
    If the file is to be loaded by the DOS 4.00 command processor, the IBM      
Macro Assembler ORG statement must set the location pointer of the file         
at 100H.                                                                        
    The IBM Macro Assembler END statement must specify the first location as    
the starting address.                                                           
                                                                                
ENTERING INPUT TO EXE2BIN                                                       
                                                                                
Input to EXE2BIN must be entered at the command line and has the following      
format:                                                                         
                                                                                
Note that the only parameter required by EXE2BIN is the filename of the input   
file.  If you specify only a filename, EXE2BIN looks for a file with an .EXE    
extension in the current directory of the default drive.  The output file       
created by a successful conversion has the same filename with a .BIN            
extension.                                                                      
                                                                                
For example, suppose you have a file called MYFILE.EXE in your current          
working directory that meets EXE2BIN's conversion criteria. To have EXE2BIN     
convert the file, you can enter:                                                
                                                                                
EXE2BIN MYFILE                                                                  
                                                                                
EXE2BIN creates a memory image of MYFILE.EXE and copies it to the output        
file.  The new file named MYFILE.BIN is placed in your current working          
directory.                                                                      
                                                                                
Of course, you can specify any of these optional parameters.  If you specify    
a drive and path for your input file and do not specify an output               
specification, the output file is placed on the current drive and directory.    
                                                                                
NOTE:  This is a change from previous versions of DOS.                          
                                                                                
TWO TYPES OF CONVERSION                                                         
                                                                                
Two types of .EXE file conversion are possible, depending on how the entry      
point of your program is defined.                                               
                                                                                
DEVICE DRIVERS                                                                  
                                                                                
The first type of conversion is for a program that will be loaded at the        
absolute memory address specified by a user application or by DOS 4.00 at       
system initialization.  An example of this kind of program is a device          
driver.                                                                         
                                                                                
CS:IP is not specified in the program (the .EXE file header contains 0:0 as     
the program's entry point).  Segment fixups are allowed in this type of         
conversion.  If segment fixups are necessary, (that is, your program contains   
instructions requiring segment relocation), you are prompted for the fixup      
value.  Here are some examples of code that will cause prompting for fixup      
values:                                                                         
                                                                                
    You have used the SEG operator:                                             
                                                                                
symbol1  db  "e:\filename",0                                                    
symbol2  dw  SEG symbol1  ;explicit use of SEG                                  
symbol3  dw  symbol1      ;implicit use of OFFSET                               
                                                                                
    You have used a segment name as an immediate field of instruction:          
                                                                                
myseg    SEGMENT    PUBLIC                                                      
                                                                                
.                                                                               
.                                                                               
.                                                                               
MOV      AX,MYSEG          ; fixup                                              
                                                                                
The fixup value is the absolute segment at which the program is to be loaded.   
                                                                                
The DOS 4.00 command processor is not capable of properly loading this type     
of program.                                                                     
                                                                                
STANDARD .COM FILE                                                              
                                                                                
To produce standard .EXE files that are suitable for EXE2BIN conversion, you    
must use the Macro Assembler ORG statement to set the location pointer of the   
file at 100H and specify the first location as the start address in the END     
statement.                                                                      
                                                                                
ORG   100H                                                                      
START                                                                           
.                                                                               
.                                                                               
.                                                                               
END START                                                                       
                                                                                
When EXE2BIN sees that CS:IP is specified as 0000:100H, it assumes that the     
file is to be run as a standard .COM file, with the location pointer set at     
100H by the ORG statement.  No segment fixups are possible because standard     
.COM files must be segment relocatable.  Once the conversion is complete, you   
may rename the resultant file to a .COM extension.  Then the command            
processor is capable of loading and executing the program in the same manner    
as the .COM programs supplied on your DOS 4.00 diskette.                        
                                                                                
If your file does not fit one of these two descriptions, or if it is similar    
to the standard .COM file description but has segment fixups, the following     
message is displayed:                                                           
                                                                                
File cannot be converted                                                        
                                                                                
This message is also displayed if the file is not a valid .EXE file.            
                                                                                
See the IBM Macro Assembler manual for a comparison of .EXE and .COM file       
structures.  Skeleton structures for .EXE files and .COM files are provided     
on the MASM diskette.

Chapter 10

CHAPTER 10.  DEBUGGING A PROGRAM 32
The DEBUG Utility 37
Starting the DEBUG.COM Program 46
Entering Commands at the DEBUG Prompt 67
DEBUG Command Summary 103
The DEBUG Work Space 181
A (Assemble) Command 225
C (Compare) Command 348
D (Dump) Command 400
E (Enter) Command 504
F (Fill) Command 620
G (Go) Command 669
H (Hexarithmetic) Command 771
I (Input) Command 789
L (Load) Command 811
M (Move) Command 919
N (Name) Command 973
O (Output) Command 1019
P (Proceed) Command 1040
Q (Quit) Command 1090
R (Register) Command 1113
S (Search) Command 1263
T (Trace) Command 1321
U (Unassemble) Command 1390
W (Write) Command 1513
XA (EMS Allocate) Command 1645
XD (EMS Deallocate) Command 1672
XM (EMS Map) Command 1699
XS (EMS Status) Command 1727
DEBUG Error Messages 1759
  
CHAPTER 10.  DEBUGGING A PROGRAM                                                
                                                                                
This chapter describes how to use the DEBUG.COM program on the utilities        
diskette to identify and fix problems in your programs.                         
                                                                                
THE DEBUG UTILITY                                                               
                                                                                
DEBUG provides a controlled testing environment that enables you to monitor     
the execution of a program.  You can make changes directly to a .COM or an      
.EXE file and execute the file immediately to determine whether your changes    
fixed a problem.  You do not need to reassemble source code files first.        
DEBUG allows you to load, alter, or display any file and to execute object      
files as well.                                                                  
                                                                                
STARTING THE DEBUG.COM PROGRAM                                                  
                                                                                
To start DEBUG, enter information in the following format:                      
                                                                                
You can enter just the DEBUG command, or you can include a file                 
specification.  The parameters parm1 and parm2 represent input and output       
specifications of the program you are debugging.  For example, suppose you      
wanted to monitor the execution of the DOS 4.00 DISKCOMP utility.  You enter:   
                                                                                
DEBUG DISKCOMP.COM A: B:                                                        
                                                                                
The DEBUG program loads DISKCOMP into memory and displays the DEBUG prompt:     
                                                                                
                                                                                
The hyphen (-) tells you DEBUG is ready to accept commands to alter, display,   
or execute the contents of the program in memory.                               
                                                                                
If you enter just DEBUG without a file specification, you can either work       
with the present memory contents or you can load a required file into memory    
using the DEBUG NAME and LOAD commands.                                         
                                                                                
ENTERING COMMANDS AT THE DEBUG PROMPT                                           
                                                                                
A DEBUG command consists of a single letter, usually followed by one or more    
parameters.  For example, the NAME command is entered at the DEBUG prompt as    
a single letter followed by a file specification:                               
                                                                                
-N MYPROG                                                                       
                                                                                
A command and its parameters can be entered in uppercase, lowercase, or a       
combination of both.  The command and its parameters can be separated by        
delimiters; however, delimiters are only required between two consecutive       
hexadecimal values.  Thus, the following DUMP commands are equivalent:          
                                                                                
dcs:100 110                                                                     
d cs:100 110                                                                    
d,cs:100,110                                                                    
                                                                                
A command is activated only after you press the Enter key.  If you want to      
terminate a command and return to the DEBUG prompt, simultaneously press the    
Ctrl and Break keys.                                                            
                                                                                
For commands producing a large amount of output, you can simultaneously press   
the Ctrl and Num Lock keys (or Pause key if available) to suspend the display   
and then press any key to restart the display, or you can redirect the          
command's output to a file.                                                     
                                                                                
When DEBUG encounters a syntax error in a line, it displays the line with the   
error identified as follows:                                                    
                                                                                
d cs:100 CS:100                                                                 
&hat.error 110                                                                  
                                                                                
In this example, the DUMP command expects the second address to contain only    
a hexadecimal offset value.  It finds the S, which is not a hexadecimal         
character.                                                                      
                                                                                
DEBUG COMMAND SUMMARY                                                           
                                                                                
The table below lists the DEBUG commands and describes the debugging            
operations you can perform with them.  Complete format descriptions and         
examples for each command can be found starting on page 10-4.                   
                                                                                
+---------------------------------------------------------------------------+   
| COMMAND     TASK DESCRIPTION                                              |   
+---------------------------------------------------------------------------+   
| A           Assemble IBM Macro Assembler statements directly into memory. |   
| (Assemble)                                                                |   
+---------------------------------------------------------------------------+   
| C           Compare the contents of two blocks of memory.                 |   
| (Compare)                                                                 |   
+---------------------------------------------------------------------------+   
| D (Dump)    Dump the contents of a portion of memory to the display or    |   
|             redirect it to a file.                                        |   
+---------------------------------------------------------------------------+   
| E (Enter)   Make changes to bytes in memory.                              |   
+---------------------------------------------------------------------------+   
| F (Fill)    Fill a range of memory with byte values.                      |   
+---------------------------------------------------------------------------+   
| G (Go)      Execute the program in memory from one address to the         |   
|             breakpoint address and then display the next instruction.     |   
+---------------------------------------------------------------------------+   
| H (Hex)     Add and subtract two hexadecimal values and display the       |   
|             results.                                                      |   
+---------------------------------------------------------------------------+   
| I (Input)   Display the input in the first byte next to the port.         |   
+---------------------------------------------------------------------------+   
                                                                                
+---------------------------------------------------------------------------+   
| COMMAND     TASK DESCRIPTION                                              |   
+---------------------------------------------------------------------------+   
| L (Load)    Load the contents of absolute disk sectors or a file          |   
|             specified by the NAME command into memory.                    |   
+---------------------------------------------------------------------------+   
| M (Move)    Copy the contents of a block of memory to another location.   |   
+---------------------------------------------------------------------------+   
| N (Name)    Set up file control blocks and file specification information |   
|             for LOAD and WRITE commands.                                  |   
+---------------------------------------------------------------------------+   
| O (Output)  Send a byte to an output port.                                |   
+---------------------------------------------------------------------------+   
| P           Execute a subroutine call, loop instruction, interrupt, or    |   
| (Proceed)   repeat string instruction and return control to DEBUG at the  |   
|             next instruction.                                             |   
+---------------------------------------------------------------------------+   
| Q (Quit)    End the DEBUG session without saving the debugged program.    |   
+---------------------------------------------------------------------------+   
| R           Display the contents of registers and the settings of flags.  |   
| (Register)                                                                |   
+---------------------------------------------------------------------------+   
| S (Search)  Search a range of memory for characters.                      |   
+---------------------------------------------------------------------------+   
| T (Trace)   Execute one or more instructions in your program and display  |   
|             the contents of registers and flags after each instruction.   |   
+---------------------------------------------------------------------------+   
| U           Translate the contents of memory into Assembler-like          |   
| (Unassemble)statements, displaying their addresses and hexadecimal        |   
|             values.                                                       |   
+---------------------------------------------------------------------------+   
| W (Write)   Write the debugged program to absolute disk sectors or to the |   
|             original file loaded with DEBUG.                              |   
+---------------------------------------------------------------------------+   
| XA          Allocate a specified number of expanded memory pages to an    |   
| (Allocate)  EMS handle.                                                   |   
+---------------------------------------------------------------------------+   
| XD          Deallocate an EMS handle.                                     |   
| (Deallocate)                                                              |   
+---------------------------------------------------------------------------+   
| XM (Map)    Map an EMS logical page to an EMS physical page from an EMS   |   
|             handle.                                                       |   
+---------------------------------------------------------------------------+   
| XS          Display the status of expanded memory.                        |   
| (Status)                                                                  |   
+---------------------------------------------------------------------------+   
                                                                                
THE DEBUG WORK SPACE                                                            
                                                                                
When the DEBUG program starts, the registers and flags are set to the           
following values for the program being debugged:                                
                                                                                
    The segment registers (CS, DS, ES, and SS) are set to the bottom of free    
memory; that is, the first segment after the end of the DEBUG program.          
    The Instruction Pointer (IP) is set to hex 0100.                            
    The Stack Pointer (SP) is set to the end of the segment, or the bottom of   
the transient portion of the program loader, whichever is lower.  The           
                                                                                
segment size at offset 6 is reduced by hex 100 to allow for a stack that        
size.                                                                           
    The remaining registers (AX, BX, CX, DX, BP, SI, and DI) are set to 0.      
However, if you start the DEBUG program with a file specification, the CX       
register contains the length of the file in bytes.  If the file is              
greater than 64KB, the length is contained in registers BX and CX (the          
high portion in BX).                                                            
    The initial state of the flags is:                                          
                                                                                
NV UP EI PL NZ NA PO NC                                                         
                                                                                
    The default disk transfer address is set to hex 80 in the code segment.     
                                                                                
All of available memory is allocated.  At this point, the loaded program is     
unable to allocate memory.                                                      
                                                                                
.EXE Files                                                                      
                                                                                
If a file loaded by DEBUG has an extension of .EXE, DEBUG does the necessary    
relocation and sets the segment registers, stack pointer, and instruction       
pointer to the values defined in the file.  The DS and ES registers, however,   
point to the program segment prefix at the lowest available segment.  The BX    
and CX registers contain the size of the program that is smaller than the       
file size.                                                                      
                                                                                
The program is loaded at the high end of memory if the appropriate parameter    
was specified when the linker created the file.                                 
                                                                                
.HEX Files                                                                      
                                                                                
If a file loaded by DEBUG has an extension of .HEX, the file is assumed to be   
in INTEL hex format, and is converted to executable form while being loaded.    
                                                                                
A (ASSEMBLE) COMMAND                                                            
                                                                                
PURPOSE                                                                         
                                                                                
Assembles IBM Macro Assembler language statements directly into memory.         
                                                                                
FORMAT                                                                          
                                                                                
A[address]                                                                      
                                                                                
PARAMETERS                                                                      
                                                                                
address     Use any of the following formats:                                   
                                                                                
    A segment register plus an offset, such as CS:0100                          
    A segment address plus an offset, such as 4BA:0100                          
    An offset only, such as 100.  In this case, the default                     
segment is used.                                                                
                                                                                
COMMENTS                                                                        
                                                                                
All numeric input to the Assemble command is in hexadecimal. The assembly       
statements you enter are assembled into memory at successive locations,         
starting with the address specified in address. If no address is specified,     
the statements are assembled into the area at CS:0100, if no previous           
Assemble command was used, or into the location following the last              
instruction assembled by a previous Assemble command.  After all desired        
statements have been entered, press the Enter key when you are prompted for     
the next statement to return to the DEBUG prompt.                               
                                                                                
DEBUG responds to invalid statements by displaying:                             
                                                                                
&hat.error                                                                      
                                                                                
and re-displaying the current assemble address.                                 
                                                                                
DEBUG supports standard 8086/8088 assembly language syntax (and the 8087        
instruction set), with the following rules:                                     
                                                                                
    All numeric values entered are hexadecimal and can be entered as 1          
through 4 characters.                                                           
                                                                                
    Prefix mnemonics are entered in front of the opcode to which they refer.    
They can also be entered on a separate line.                                    
                                                                                
    The segment override mnemonics are CS:, DS:, ES:, and SS:.                  
                                                                                
    String manipulation mnemonics must specify the string size. For example,    
MOVSW must be used to move word strings and MOVSB must be used to move          
byte strings.                                                                   
                                                                                
    The mnemonic for the far return is RETF.                                    
                                                                                
     The assembler will automatically assemble short, near, or far jumps and    
calls depending on byte displacement to the destination address. These          
can be overridden with the NEAR or FAR prefix. For example:                     
                                                                                
0100:0500 JMP 502   ;a 2 byte short jump                                        
0100:0502 JMP NEAR 505 ;a 3 byte near jump                                      
0100:0505 JMP FAR 50A  ;a 5 byte far jump                                       
                                                                                
The NEAR prefix can be abbreviated to NE, but the FAR prefix cannot be          
abbreviated.                                                                    
                                                                                
    DEBUG cannot tell whether some operands refer to a word memory location     
or a byte memory location.  In this case, the data type must be                 
explicitly stated with the prefix WORD PTR or BYTE PTR. DEBUG will also         
accept the abbreviations WO and BY.  For example:                               
                                                                                
NEG   BYTE PTR [128]                                                            
DEC   WO [SI]                                                                   
                                                                                
    DEBUG also cannot tell whether an operand refers to a memory location or    
to an immediate operand. DEBUG uses the common convention that operands         
enclosed in square brackets refer to memory. For example:                       
                                                                                
MOV   AX,21   ;Load AX with 21H                                                 
MOV   AX,[21]  ;Load AX with the                                                
contents of memory                                                              
location 21H                                                                    
                                                                                
    Two popular pseudo-instructions have also been included. The DB opcode      
assembles byte values directly into memory. The DW opcode assembles word        
values directly into memory. For example:                                       
                                                                                
DB   1,2,3,4,'THIS IS AN EXAMPLE'                                               
DB   'THIS IS A QUOTE: ''                                                       
DB   'THIS IS AN APOSTROPHE:''                                                  
                                                                                
DW   1000,2000,3000,'BACH:'                                                     
                                                                                
    All forms of the register indirect commands are supported. For example:     
                                                                                
ADD   BX,34[BP+2][SI-1]                                                         
POP   [BP+DI]                                                                   
PUSH  [SI]                                                                      
                                                                                
    All opcode synonyms are supported. For example:                             
                                                                                
LOOPZ   100                                                                     
LOOPE   100                                                                     
                                                                                
JA      200                                                                     
JNBE    200                                                                     
                                                                                
    For numeric co-processor opcodes the WAIT or FWAIT prefix must be           
explicitly specified. For example:                                              
                                                                                
FWAIT FADD ST,ST(3)    ;This line will                                          
;assemble a                                                                     
;FWAIT prefix                                                                   
FLD TBYTE PTR [BX]      ;This line will                                         
;not                                                                            
                                                                                
EXAMPLE                                                                         
                                                                                
C>debug                                                                         
-a200                                                                           
08B4:0200 xor ax,ax                                                             
08B4:0202 mov [bx],ax                                                           
08B4:0204 ret                                                                   
08B4:0205                                                                       
                                                                                
C (COMPARE) COMMAND                                                             
                                                                                
PURPOSE                                                                         
                                                                                
Compares the contents of two blocks of memory.                                  
                                                                                
FORMAT                                                                          
                                                                                
C range address                                                                 
                                                                                
PARAMETERS                                                                      
                                                                                
range       Either of these two formats:                                        
                                                                                
    An address followed by an offset, such as CS:100 110.                       
    An address followed by L value, where value is the number of                
hexadecimal bytes to be processed.  For example, CS:100 L 10.                   
                                                                                
The limit for range is hexadecimal 10000 or decimal 64KB.  To                   
specify a range of 64KB within 4 hexadecimal characters, enter                  
0000 or 0 for value.                                                            
                                                                                
address     Any of these three formats:                                         
                                                                                
    A segment register plus an offset, such as CS:0100.                         
    A segment address plus an offset, such as 4BA:0100.                         
    An offset only, such as 100.  In this case, the default                     
segment is used.                                                                
                                                                                
COMMENTS                                                                        
                                                                                
The contents of the two blocks of memory are compared;  the length of the       
comparison is determined from the range.  If unequal bytes are found, their     
addresses and contents are displayed, in the form:                              
                                                                                
addr1  byte1  byte2  addr2                                                      
                                                                                
where, the first half (addr1  byte1) refers to the location and contents of     
the mismatching locations in range, and the second half (byte2  addr2) refers   
to the byte found in address.                                                   
                                                                                
If you enter only an offset for the beginning address of range, the C command   
assumes the segment contained in the DS register.  To specify an ending         
address for range, enter it with only an offset value.                          
                                                                                
EXAMPLE                                                                         
                                                                                
C  100 L20 200                                                                  
                                                                                
The 32 bytes (hex 20) of memory beginning at DS:100 are compared with the 32    
bytes beginning at DS:200.  L20 is the range.                                   
                                                                                
D (DUMP) COMMAND                                                                
                                                                                
PURPOSE                                                                         
                                                                                
Displays the contents of a portion of memory.                                   
                                                                                
FORMAT                                                                          
                                                                                
D [address]                                                                     
                                                                                
or                                                                              
                                                                                
D [range]                                                                       
                                                                                
PARAMETERS                                                                      
                                                                                
address     Any of the following formats:                                       
                                                                                
    A segment register plus an offset, such as CS:0100.                         
    A segment address plus an offset, such as 4BA:0100.                         
    An offset only, such as 100.  In this case, the default                     
segment is used.                                                                
                                                                                
range       Either of these two formats:                                        
                                                                                
    An address followed by an offset, such as CS:100 110.                       
    An address followed by L value, where value is the number of                
hexadecimal bytes to be processed.  For example, CS:100 L 10.                   
                                                                                
The limit for range is hexadecimal 10000 or decimal 64K bytes.                  
To specify a range of 64K bytes within 4 hexadecimal characters,                
enter 0000 or 0 for value.                                                      
                                                                                
COMMENTS                                                                        
                                                                                
The dump is displayed in two parts:                                             
                                                                                
1.  A hexadecimal portion.  Each byte is displayed in hexadecimal.              
                                                                                
2.  An ASCII portion.  The bytes are displayed as ASCII characters.             
Unprintable characters (ASCII 0 to 31 and 127 to 255) are indicated by a        
period.                                                                         
                                                                                
With a 40-column system display format, each line begins on an 8-byte           
boundary and shows 8 bytes.                                                     
                                                                                
With an 80-column system display format, each line begins on a 16-byte          
boundary and shows 16 bytes.  There is a hyphen between the 8th and 9th         
bytes.                                                                          
                                                                                
NOTE:  The first line may have fewer than 8 or 16 bytes if the starting         
address of the dump is not on a boundary.  In this case, the second             
line of the dump begins on a boundary.                                          
                                                                                
The Dump command has two format options.                                        
                                                                                
OPTION 1                                                                        
                                                                                
Use this option to display the contents of hex 40 bytes (40-column mode) or     
hex 80 bytes (80-column mode).  For example:                                    
                                                                                
D address                                                                       
                                                                                
or                                                                              
                                                                                
D                                                                               
                                                                                
The contents are dumped starting with the specified address.                    
                                                                                
If you do not specify an address, the D command assumes the starting address    
is the location following the last location displayed by a previous D           
                                                                                
command.  Thus, it is possible to dump consecutive 40-byte or 80-byte areas     
by entering consecutive D commands without parameters.                          
                                                                                
If no previous D command was entered, the location is offset hex 100 into the   
segment originally initialized in the segment registers by DEBUG.               
                                                                                
NOTE:  If you enter only an offset for the starting address, the D command      
assumes the segment contained in the DS register.                               
                                                                                
OPTION 2                                                                        
                                                                                
Use this option to display the contents of the specified address range.  For    
example:                                                                        
                                                                                
D range                                                                         
                                                                                
NOTE:  If you enter only an offset for the starting address, the D command      
assumes the segment contained in the DS register.  If you specify an            
ending address, enter it with only an offset value.                             
                                                                                
For example:                                                                    
                                                                                
D cs:100 10C                                                                    
                                                                                
A 40-column display format might look like this:                                
                                                                                
04BA:0100  42 45 52 54 41 20 54 00                                              
BERTA T.                                                                        
                                                                                
04BA:0108  20 42 4F 52 47                                                       
BORG                                                                            
                                                                                
E (ENTER) COMMAND                                                               
                                                                                
PURPOSE                                                                         
                                                                                
    Replaces the contents of one or more bytes, starting at the specified       
address, with the values contained in the list (see Option 1).                  
                                                                                
    Displays and allows modification of bytes in a sequential manner (see       
Option 2).                                                                      
                                                                                
FORMAT                                                                          
                                                                                
E address [list]                                                                
                                                                                
PARAMETERS                                                                      
                                                                                
address     Any of the following formats:                                       
                                                                                
    A segment register plus an offset, such as CS:0100.                         
    A segment address plus an offset, such as 4BA:0100.                         
    An offset only, such as 100.  In this case, the default                     
segment is used.                                                                
                                                                                
list        A string of byte values.  If you include a character string,        
enclose the characters in single or double quotation marks.  To                 
specify a quotation mark as a character within the string when it               
is also used to delimit the string, type it twice.                              
                                                                                
'These ''quotes'' are correct.'                                                 
'This one''s okay, too.'                                                        
                                                                                
COMMENTS                                                                        
                                                                                
If you enter only an offset for the address, the E command assumes the          
segment contained in the DS register.                                           
                                                                                
The Enter command has two format options.                                       
                                                                                
OPTION 1                                                                        
                                                                                
Use this option to place the list in memory beginning at the specified          
address.                                                                        
                                                                                
E  address list                                                                 
                                                                                
For example:                                                                    
                                                                                
E ds:100 F3 'xyz' 8D                                                            
                                                                                
Memory locations ds:100 through ds:104 are filled with the 5 bytes specified    
in the list.                                                                    
                                                                                
OPTION 2                                                                        
                                                                                
Use this option to display the address and the byte of a location, then the     
system waits for your input.                                                    
                                                                                
For example:                                                                    
                                                                                
E  address                                                                      
                                                                                
Enter a 1- or 2-character hexadecimal value to replace the contents of the      
byte; then take any one of the following actions:                               
                                                                                
1.  Press the space bar to advance to the next address.  Its contents are       
displayed.  If you want to change the contents take option 1, above.            
                                                                                
To advance to the next byte without changing the current byte, press the        
space bar again.                                                                
                                                                                
2.  Enter a hyphen to back up to the preceding address.  A new line is          
displayed with the preceding address and its contents.  If you want to          
change the contents, take option 1, above.                                      
                                                                                
To back up one more byte without changing the current byte, enter another       
hyphen.                                                                         
                                                                                
3.  Press the Enter key to end the Enter command.                               
                                                                                
NOTE:  Display lines can have 4 or 8 bytes of data, depending on whether the    
system display format is 40- or 80-column.  Spacing beyond an 8-byte            
boundary causes a new display line, with the beginning address, to be           
started.                                                                        
                                                                                
For example:                                                                    
                                                                                
E cs:100                                                                        
                                                                                
might cause this display:                                                       
                                                                                
04BA:0100 EB.                                                                   
                                                                                
To change the contents of 04BA:0100 from hex EB to hex 41, enter 41.            
                                                                                
04BA:0100 EB.41                                                                 
                                                                                
To see the contents of the next three locations, press the space bar three      
times.  The screen might look like this:                                        
                                                                                
04BA:0100  EB.41  10. 00.  BC.                                                  
                                                                                
To change the contents of the current location (04BA:0103) from hex BC to hex   
42, enter 42.                                                                   
                                                                                
04BA:0100  EB.41  10. 00.  BC.42                                                
                                                                                
Now, suppose you want to back up and change the hex 10 to hex 6F.  This is      
what the screen looks like after entering two hyphens and the replacement       
byte:                                                                           
                                                                                
04BA:0100 EB.41 10.00.  BC.42-                                                  
04BA:0102 00.-                                                                  
04BA:0101 10.6F                                                                 
                                                                                
Press the Enter key to end the Enter command.  The hyphen prompt will appear.   
                                                                                
F (FILL) COMMAND                                                                
                                                                                
PURPOSE                                                                         
                                                                                
Fills the memory locations in the range with the values in the list.            
                                                                                
FORMAT                                                                          
                                                                                
F range list                                                                    
                                                                                
PARAMETERS                                                                      
                                                                                
range       Either of these two formats:                                        
                                                                                
    An address followed by an offset, such as CS:100 110                        
    An address followed by L value, where value is the number of                
hexadecimal bytes to be processed.  For example, CS:100 L 10.                   
                                                                                
The limit for range is hexadecimal 10000 or decimal 64K bytes.                  
To specify a range of 64K bytes within 4 hexadecimal characters,                
enter 0000 or 0 for value.                                                      
                                                                                
list        A string of byte values.  If you include a character string,        
enclose the characters in single or double quotation marks.  To                 
specify a quotation mark as a character within the string when it               
is also used to delimit the string, type it twice.                              
                                                                                
'These ''quotes'' are correct.'                                                 
'This one''s okay, too.'                                                        
                                                                                
COMMENTS                                                                        
                                                                                
If the list contains fewer bytes than the address range, the list is used       
repeatedly until all the designated memory locations are filled.                
                                                                                
If the list contains more bytes than the address range, the extra list items    
are ignored.                                                                    
                                                                                
NOTE:  If you enter only an offset for the starting address of the range, the   
Fill command assumes the segment contained in the DS register.                  
                                                                                
EXAMPLE                                                                         
                                                                                
F 4BA:100 L 5 F3 'XYZ' 8D                                                       
                                                                                
Memory locations 04BA:100 through 04BA:104 are filled with the 5 bytes          
specified.  Remember that the ASCII values of the list characters are stored.   
Thus, locations 100-104 will contain F3 58 59 5A 8D.                            
                                                                                
G (GO) COMMAND                                                                  
                                                                                
PURPOSE                                                                         
                                                                                
Executes the program you are debugging.                                         
                                                                                
Stops the execution when the instruction at a specified address is reached      
(breakpoint), and displays the registers, flags, and the next instruction to    
be executed.                                                                    
                                                                                
FORMAT                                                                          
                                                                                
G [=address] [address [address...]]                                             
                                                                                
PARAMETERS                                                                      
                                                                                
address     Any of the following formats:                                       
                                                                                
    A segment register plus an offset, such as CS:0100.                         
    A segment address plus an offset, such as 4BA:0100.                         
    An offset only, such as 100.  In this case, the default                     
segment is used.                                                                
                                                                                
COMMENTS                                                                        
                                                                                
Program execution begins with the current instruction, whose address is         
determined by the contents of the CS and IP registers, unless overridden by     
the =address parameter (the = must be entered).  If =address is specified,      
program execution begins with CS:=address.                                      
                                                                                
The Go command has two format options.                                          
                                                                                
OPTION 1                                                                        
                                                                                
Use this option to execute the program you are debugging without breakpoints.   
For example:                                                                    
                                                                                
G [=address]                                                                    
                                                                                
This option is useful when testing program execution with different             
parameters each time. (Refer to the Name command.)  Be certain the CS:IP        
values are set properly before issuing the G command, if not using =address.    
                                                                                
OPTION 2                                                                        
                                                                                
This option performs the same function as Option 1 but, in addition, allows     
breakpoints to be set at the specified addresses.  For example:                 
                                                                                
G [=address] address                                                            
[address...]                                                                    
                                                                                
This method causes execution to stop at a specified location so the system or   
program environment can be examined.                                            
                                                                                
You can specify up to ten breakpoints in any order.  You may wish to take       
advantage of this if your program has many paths, and you want to stop the      
execution no matter which path the program takes.                               
                                                                                
The DEBUG program replaces the instruction codes at the breakpoint addresses    
with an interrupt code (hex CC).  If any one breakpoint is reached during       
execution, the execution is stopped, the registers and flags are displayed,     
and all the breakpoint addresses are restored to their original instruction     
codes.  If no breakpoint is reached, the instructions are not restored.         
                                                                                
NOTES:                                                                          
                                                                                
1.  Once a program has reached completion (DEBUG has displayed the "Program     
terminated normally" message), you must reload the program before it can        
be executed again.                                                              
                                                                                
2.  Make sure that the address parameters refer to locations that contain       
valid 8086/8088 instruction codes.  If you specify an address that does         
not contain valid instruction in the first byte, unpredictable results          
occur.                                                                          
                                                                                
3.  The stack pointer must be valid and have 6 bytes available for the Go       
command, otherwise, unpredictable results occur.                                
                                                                                
4.  If only an offset is entered for a breakpoint, the G command assumes the    
segment contained in the CS register.                                           
                                                                                
5.  Do not set breakpoints at instructions in read-only memory (ROM BIOS or     
ROM BASIC).                                                                     
                                                                                
For example:                                                                    
                                                                                
G 102 1EF 208                                                                   
                                                                                
Be careful not to set a breakpoint between a segment override indication        
(such as ES; alone on a line), and the instruction that the override            
qualifies.                                                                      
                                                                                
Execution begins with the current instruction, whose address is the current     
values of CS:IP.  The =address parameter was not used.                          
                                                                                
Three breakpoints are specified; assume that the second is reached.             
Execution stops before the instruction at location CS:1EF is executed, the      
original instruction codes are restored, all three breakpoints are removed,     
the display occurs, and the Go command ends.                                    
                                                                                
Refer to the Register command for a description of the display.                 
                                                                                
H (HEXARITHMETIC) COMMAND                                                       
                                                                                
PURPOSE                                                                         
                                                                                
Adds the two hexadecimal values, then subtracts the second from the first.      
Displays the sum and difference on one line.                                    
                                                                                
FORMAT                                                                          
                                                                                
H value value                                                                   
                                                                                
EXAMPLE                                                                         
                                                                                
H 0F 8                                                                          
0017 0007                                                                       
                                                                                
The hexadecimal sum of 000F and 0008 is 0017, and their difference is 0007.     
                                                                                
I (INPUT) COMMAND                                                               
                                                                                
PURPOSE                                                                         
                                                                                
Inputs and displays (in hexadecimal) 1 byte from the specified port.            
                                                                                
FORMAT                                                                          
                                                                                
I portaddress                                                                   
                                                                                
PARAMETERS                                                                      
                                                                                
portaddress A 1-4 character hexadecimal value specifying an 8- or 16-bit port   
address.                                                                        
                                                                                
EXAMPLE                                                                         
                                                                                
I 2F8                                                                           
6B                                                                              
                                                                                
The single hexadecimal byte read from port 02F8 is displayed (6B).              
                                                                                
L (LOAD) COMMAND                                                                
                                                                                
PURPOSE                                                                         
                                                                                
Loads a file or absolute disk sectors into memory.                              
                                                                                
FORMAT                                                                          
                                                                                
L [address[drive sector sector]]                                                
                                                                                
PARAMETERS                                                                      
                                                                                
address     Any of the following formats:                                       
                                                                                
    A segment register plus an offset, such as CS:0100.                         
    A segment address plus an offset, such as 4BA:0100.                         
    An offset only, such as 100.  In this case, the default                     
segment is used.                                                                
                                                                                
drive       A decimal number that indicates a particular drive.  For example,   
drive A is 0, drive B is 1, and so on.                                          
                                                                                
sector      1-3 character hexadecimal values that specify the starting          
relative sector number and the number of sectors to be loaded or                
written.                                                                        
                                                                                
NOTE:  Relative sector numbers are obtained by counting the                     
sectors on the disk surface.  The first sector on the disk                      
is at track 0, sector 1, head 0, and is relative sector 0.                      
The numbering continues for each sector on that track and                       
head, then continues with the first sector on the next                          
head of the same track.  When all sectors on all heads of                       
the track have been counted, numbering continues with the                       
first sector on head 0 of the next track.                                       
                                                                                
COMMENTS                                                                        
                                                                                
The maximum number of sectors that can be loaded with a single Load command     
is hex 80.  A sector contains 512 bytes.                                        
                                                                                
NOTE:  DEBUG displays a message if a disk read error occurs.  You can retry     
the read operation by pressing the F3 key to re-display the Load                
command.  Then press the Enter key.                                             
                                                                                
The Load command has two format options.                                        
                                                                                
OPTION 1                                                                        
                                                                                
Use this option to load data from the disk specified by drive and place the     
data in memory beginning at the specified address.  For example:                
                                                                                
L  address drive sector sector                                                  
                                                                                
The data is read from the specified starting relative sector (first sector)     
and continues until the requested number of sectors is read (second sector).    
                                                                                
NOTE:  If you only enter an offset for the beginning address, the L command     
assumes the segment contained in the CS register.                               
                                                                                
For example, to load data, you might enter:                                     
                                                                                
L DS:100 1 0F 6D                                                                
                                                                                
The data is loaded from the diskette in drive B and placed in memory            
beginning at DS:100.  Consecutive sectors of data are transferred, 6DH (109),   
starting with relative sector hex 0F (15) (the 16th sector on the diskette).    
                                                                                
NOTE:  Option 1 cannot be used if the drive specified is a network drive.       
                                                                                
OPTION 2                                                                        
                                                                                
When issued without parameters, or with only the address parameter, use this    
option to load the file whose file specification is at CS:80. For example:      
                                                                                
L                                                                               
                                                                                
or                                                                              
                                                                                
L address                                                                       
                                                                                
This condition is met by specifying the file name when starting the DEBUG       
program, or by using the Name command.                                          
                                                                                
NOTE:  If DEBUG was started with a file specification and subsequent Name       
commands were used, you may need to enter a new Name command for the            
proper file specification before issuing the Load command.                      
                                                                                
The file is loaded into memory beginning at CS:100 (or the location specified   
by address), and is read from the drive specified in the file specification,    
or from the default drive, if none was specified.  Note that files with         
extensions of .COM or .EXE are always loaded at CS:100.  If you specified an    
address, it is ignored.                                                         
                                                                                
The BX and CX registers are set to the number of bytes read; however, if the    
file being loaded has an extension of .EXE, the BX and CX registers are set     
to the actual program size.  The file may be loaded at the high end of          
memory.  Refer to "The DEBUG Work Space" on page 10-3 for the conditions that   
are in effect when .EXE or .HEX files are loaded.                               
                                                                                
For example:                                                                    
                                                                                
DEBUG                                                                           
-N myprog                                                                       
-L                                                                              
                                                                                
The file named MYPROG is loaded from the default directory and placed in        
memory beginning at location CS:0100.                                           
                                                                                
M (MOVE) COMMAND                                                                
                                                                                
PURPOSE                                                                         
                                                                                
Moves the contents of the memory locations specified by range to the            
locations beginning at the address specified.                                   
                                                                                
FORMAT                                                                          
                                                                                
M range address                                                                 
                                                                                
PARAMETERS                                                                      
                                                                                
range       Either of these two formats:                                        
                                                                                
    An address followed by an offset, such as CS:100 110.                       
    An address followed by L value, where value is the number of                
hexadecimal bytes to be processed.  For example, CS:100 L 10.                   
                                                                                
The limit for range is hexadecimal 10000 or decimal 64KB.  To                   
specify a range of 64KB within 4 hexadecimal characters, enter                  
0000 or 0 for value.                                                            
                                                                                
address     Any of the following formats:                                       
                                                                                
    A segment register plus an offset, such as CS:0100.                         
    A segment address plus an offset, such as 4BA:0100.                         
    An offset only, such as 100.  In this case, the default                     
segment is used.                                                                
                                                                                
COMMENTS                                                                        
                                                                                
Overlapping moves are always performed without loss of data during the          
transfer.  (The source and destination areas share some of the same memory      
locations.)                                                                     
                                                                                
The data in the source area remains unchanged unless overwritten by the move.   
                                                                                
NOTES:                                                                          
                                                                                
1.  If you enter only an offset for the beginning address of the range, the M   
command assumes the segment contained in the DS register.  If you specify       
an ending address for the range, enter it with only an offset value.            
                                                                                
2.  If you enter only an offset for the address of the destination area, the    
M command assumes the segment contained in the DS register.                     
                                                                                
EXAMPLE                                                                         
                                                                                
M CS:100 L10 500                                                                
                                                                                
The 17 bytes of data from CS:100 through CS:110 are moved to the area of        
memory beginning at DS:500.                                                     
                                                                                
N (NAME) COMMAND                                                                
                                                                                
PURPOSE                                                                         
                                                                                
    Formats file control blocks for the first two file specifications, at       
CS:5C and CS:6C.  (Starting DEBUG with a file specification also formats        
a file control block at CS:5C.)                                                 
                                                                                
The file control blocks are set up for the Load and Write commands and to       
supply required file names for the program being debugged.                      
                                                                                
    All file specifications and other parameters, including delimiters, are     
placed exactly as entered in a parameter save area at CS:81, with CS:80         
containing the number of characters entered.  Register AX is set to             
indicate the validity of the drive specifiers entered with the first two        
file specifications.                                                            
                                                                                
FORMAT                                                                          
                                                                                
N [d:][path]filename[.ext]                                                      
                                                                                
COMMENTS                                                                        
                                                                                
If you start the DEBUG program without a file specification, you must use the   
Name command before a file can be loaded with the L command.                    
                                                                                
EXAMPLE                                                                         
                                                                                
DEBUG                                                                           
-N myprog                                                                       
-L                                                                              
                                                                                
To define file specifications or other parameters required by the program       
being debugged, enter:                                                          
                                                                                
DEBUG myprog                                                                    
-N file1 file2                                                                  
                                                                                
In this example, DEBUG loads the file MYPROG at CS:100, and leaves the file     
control block at CS:5C formatted with the same file specification.  Then, the   
Name command formats file control blocks for file1 and file2 at CS:5C and       
CS:6C, respectively.  The file control block for MYPROG is overwritten.  The    
parameter area at CS:81 contains all characters entered after the N,            
including all delimiters, and CS:80 contains the count of those characters      
(hex 0C).                                                                       
                                                                                
O (OUTPUT) COMMAND                                                              
                                                                                
PURPOSE                                                                         
                                                                                
Sends the byte to the specified output port.                                    
                                                                                
FORMAT                                                                          
                                                                                
O portaddress byte                                                              
                                                                                
PARAMETERS                                                                      
                                                                                
portaddress A 1-4 character hexadecimal value specifying an 8- or 16-bit port   
address.                                                                        
                                                                                
EXAMPLE                                                                         
                                                                                
To send the byte value 4F to output port 2F8, enter:                            
                                                                                
O 2F8 4F                                                                        
                                                                                
P (PROCEED) COMMAND                                                             
                                                                                
PURPOSE                                                                         
                                                                                
Causes the execution of a subroutine call, a loop instruction, an interrupt,    
or a repeat string instruction to stop at the next instruction.                 
                                                                                
FORMAT                                                                          
                                                                                
P[=address][value]                                                              
                                                                                
PARAMETERS                                                                      
                                                                                
address     Any of the following formats:                                       
                                                                                
    A segment register plus an offset, such as CS:0100.                         
    A segment address plus an offset, such as 4BA:0100.                         
    An offset only, such as 100.  In this case, the default                     
segment is used.                                                                
                                                                                
value       A 1-4 character hexadecimal value, specifying the number of         
instructions to execute.                                                        
                                                                                
COMMENTS                                                                        
                                                                                
When at a subroutine call, a loop instruction, an interrupt, or a repeat        
string instruction, issue the Proceed command to execute the instruction        
(perform the entire function), and return control at the next instruction.      
The Proceed command has the same syntax as the Trace command.  Specifying P0    
is the same as specifying T0.                                                   
                                                                                
EXAMPLE                                                                         
                                                                                
If the following instructions are executed:                                     
                                                                                
0100    CALL    1000                                                            
0103    JC      2000                                                            
.                                                                               
.                                                                               
.                                                                               
1000    XOR     AX,AX                                                           
.                                                                               
.                                                                               
.                                                                               
1XXX    RET                                                                     
                                                                                
And if CS:IP was pointing to the CALL 1000 instruction, typing P causes the     
execution of the subroutine and returns control to DEBUG at the JC              
instruction.                                                                    
                                                                                
Q (QUIT) COMMAND                                                                
                                                                                
PURPOSE                                                                         
                                                                                
Ends the DEBUG program.                                                         
                                                                                
FORMAT                                                                          
                                                                                
Q                                                                               
                                                                                
COMMENTS                                                                        
                                                                                
The file that you are working on in memory is not saved by the Quit command.    
You must use the Write command to save the file.                                
                                                                                
DEBUG returns to the command processor which then issues the normal command     
prompt.                                                                         
                                                                                
EXAMPLE                                                                         
                                                                                
-Q                                                                              
A>                                                                              
                                                                                
R (REGISTER) COMMAND                                                            
                                                                                
PURPOSE                                                                         
                                                                                
The Register command has the following three functions:                         
                                                                                
    Displays the hexadecimal contents of a single register with the option of   
changing those contents                                                         
                                                                                
    Displays the hexadecimal contents of all the registers, plus the            
alphabetic flag settings, and the next instruction to be executed               
                                                                                
    Displays the eight 2-letter alphabetic flag settings with the option of     
changing any or all of them.                                                    
                                                                                
FORMAT                                                                          
                                                                                
R [registername]                                                                
                                                                                
PARAMETERS                                                                      
                                                                                
registername  The valid names are:                                              
                                                                                
AX    SP    CS   IP                                                             
BX    BP    DS   F                                                              
CX    SI    ES                                                                  
DX    DI    SS                                                                  
                                                                                
IP refers to the instruction pointer, and F refers to the flags                 
register.                                                                       
                                                                                
COMMENTS                                                                        
                                                                                
When the DEBUG program starts, the registers and flags are set to certain       
values for the program being debugged.  (Refer to "The DEBUG Work Space" on     
page 10-3.)                                                                     
                                                                                
DISPLAY A SINGLE REGISTER                                                       
                                                                                
To display the contents of a single register, enter the register name:          
                                                                                
R AX                                                                            
                                                                                
The system might respond with:                                                  
                                                                                
AX F1E4                                                                         
:                                                                               
                                                                                
Now you can take one of two actions: press the Enter key to leave the           
contents unchanged, or change the contents of the AX register by entering a     
1-4 character hexadecimal value, such as hex FFF.                               
                                                                                
AX F1E4                                                                         
:FFF                                                                            
                                                                                
Now, pressing the Enter key changes the contents of the AX register to hex      
0FFF.                                                                           
                                                                                
DISPLAY ALL REGISTERS AND FLAGS                                                 
                                                                                
To display the contents of all registers and flags and the next instruction     
to be executed, type:                                                           
                                                                                
R                                                                               
                                                                                
The system responds:                                                            
                                                                                
AX=0E00  BX=00FF  CX=0007  DX=01FF  SP=039D  BP=0000  SI=005C  DI=0000          
DS=3D5B  ES=3D5B  SS=3D5B  CS=3D5B  IP=011A   NV UP EI PL NZ NA PO NC           
3D5B:011A CD21          INT     21                                              
                                                                                
The first four lines display the hexadecimal contents of the registers and      
the eight alphabetic flag settings.  The last line indicates the location of    
the next instruction to be executed and its hexadecimal and unassembled         
formats.  This is the instruction pointed to by CS:IP.                          
                                                                                
A system with an 80-column display shows:                                       
                                                                                
1st line - 8 registers                                                          
2nd line - 5 registers and 8 flag settings                                      
3rd line - next instruction information                                         
                                                                                
A system with a 40-column display shows:                                        
                                                                                
1st line - 4 registers                                                          
2nd line - 4 registers                                                          
3rd line - 4 registers                                                          
4th line - 1 register and 8 flag settings                                       
5th line - next instruction information                                         
                                                                                
Display All Flags                                                               
                                                                                
There are eight flags, each with two-letter codes to indicate either a set      
condition or a clear condition.  The flags appear in displays in the order      
shown in the following table:                                                   
                                                                                
+---------------------------------------------------------------------------+   
| FLAG NAME                                                  SET     CLEAR  |   
+---------------------------------------------------------------------------+   
| Overflow (yes/no)                                          OV      NV     |   
+---------------------------------------------------------------------------+   
| Direction (decrease/increase)                              DN      UP     |   
+---------------------------------------------------------------------------+   
| Interrupt (enable/disable)                                 EI      DI     |   
+---------------------------------------------------------------------------+   
| Sign (negative/positive)                                   NG      PL     |   
+---------------------------------------------------------------------------+   
| Zero (yes/no)                                              ZR      NZ     |   
+---------------------------------------------------------------------------+   
| Auxiliary carry (yes/no)                                   AC      NA     |   
+---------------------------------------------------------------------------+   
| Parity (even/odd)                                          PE      PO     |   
+---------------------------------------------------------------------------+   
| Carry (yes/no)                                             CY      NC     |   
+---------------------------------------------------------------------------+   
                                                                                
To display all flags, enter:                                                    
                                                                                
R F                                                                             
                                                                                
If all the flags are in a set condition, the response is:                       
                                                                                
OV DN EI NG ZR AC PE CY -                                                       
                                                                                
Now you can either press the Enter key to leave the settings unchanged or       
change any or all of the settings.                                              
                                                                                
To change a flag, just enter its opposite code.  The opposite codes can be      
entered in any order with or without intervening spaces.  For example, to       
change the first, third, fifth, and seventh flags, enter:                       
                                                                                
OV DN EI NG ZR AC PE CY - PONZDINV                                              
                                                                                
The changes in this example are entered in reverse order.                       
                                                                                
Press the Enter key and the flags are modified as specified, the prompt         
appears, and you can enter the next command.                                    
                                                                                
If you want to see if the new codes are in effect, enter:                       
                                                                                
R F                                                                             
                                                                                
The response is:                                                                
                                                                                
NV DN DI NG NZ AC PO CY -                                                       
                                                                                
The first, third, fifth, and seventh flags are changed as requested.  The       
second, fourth, sixth, and eighth flags are unchanged.  A single flag can be    
changed only once for each R F command.                                         
                                                                                
S (SEARCH) COMMAND                                                              
                                                                                
PURPOSE                                                                         
                                                                                
Searches the range for the character(s) in the list.                            
                                                                                
FORMAT                                                                          
                                                                                
S range list                                                                    
                                                                                
range       Either of these two formats:                                        
                                                                                
    An address followed by an offset, such as CS:100 110.                       
    An address followed by L value, where value is the number of                
hexadecimal bytes to be processed.  For example, CS:100 L 10.                   
                                                                                
The limit for range is hexadecimal 10000 or decimal 64KB.  To                   
specify a range of 64KB within 4 hexadecimal characters, enter                  
0000 or 0 for value.                                                            
                                                                                
list        A string of byte values.  If you include a character string,        
enclose the characters in single or double quotation marks.  To                 
specify a quotation mark as a character within the string when it               
is also used to delimit the string, type it twice.                              
                                                                                
'These ''quotes'' are correct.'                                                 
'This one''s okay, too.'                                                        
                                                                                
COMMENTS                                                                        
                                                                                
All matches are indicated by displaying the addresses where matches are         
found.                                                                          
                                                                                
A display of the prompt without an address means that no match was found.       
                                                                                
NOTE:  If you enter only an offset for the starting address of the range, the   
S command assumes the segment contained in the DS register.                     
                                                                                
EXAMPLE                                                                         
                                                                                
If you want to search the range of addresses from CS:100 through CS:110 for     
hex 41, type:                                                                   
                                                                                
S CS:100 110 41                                                                 
                                                                                
If two matches are found the response might be:                                 
                                                                                
04BA:0104                                                                       
04BA:010D                                                                       
                                                                                
If you want to search the same range of addresses for a match with the 4-byte   
list (41 'AB' E), enter:                                                        
                                                                                
S CS:100 L 11 41 'AB' E                                                         
                                                                                
The starting addresses of all matches are listed.  If no match is found, no     
address is displayed.                                                           
                                                                                
T (TRACE) COMMAND                                                               
                                                                                
PURPOSE                                                                         
                                                                                
Executes one or more instructions starting with the instruction at CS:IP, or    
at =address, if it is specified.  The = must be entered.  One instruction is    
assumed, but you can specify more than one with value.  This command displays   
the contents of all registers and flags after each instruction executes.  For   
a description of the display format, refer to the Register command.             
                                                                                
FORMAT                                                                          
                                                                                
T [=address][value]                                                             
                                                                                
PARAMETERS                                                                      
                                                                                
address     Any of the following formats:                                       
                                                                                
    A segment register plus an offset, such as CS:0100.                         
    A segment address plus an offset, such as 4BA:0100.                         
    An offset only, such as 100.  In this case, the default                     
segment is used.                                                                
                                                                                
value       A 1-4 character hexadecimal value, specifying the number of         
instructions to execute.                                                        
                                                                                
COMMENTS                                                                        
                                                                                
The display caused by the Trace command continues until value instructions      
are executed.  Therefore, when tracing multiple instructions, remember you      
can suspend the scrolling at any time by pressing the Ctrl and the NumLock      
keys together, or the Pause key.  Resume scrolling by entering any other        
character.                                                                      
                                                                                
NOTES:                                                                          
                                                                                
1.  The Trace command disables all hardware interrupts before executing the     
user instruction, and then re-enables the interrupts when the trap              
interrupt occurs following the execution of the instruction.                    
                                                                                
2.  TRACE should not be used with any steps that change the contents of the     
8259 interrupt mask (ports 20 and 21).                                          
                                                                                
3.  If you trace an INT3 instruction, the breakpoint is set at the INT3         
location.                                                                       
                                                                                
EXAMPLE                                                                         
                                                                                
T                                                                               
                                                                                
If the IP register contains 011A, and that location contains B40E (MOV          
AH,0EH), this may be displayed:                                                 
                                                                                
AX=0E00  BX=00FF  CX=0007  DX=01FF  SP=039D  BP=0000  SI=005C  DI=0000          
DS=3D5B  ES=3D5B  SS=3D5B  CS=3D5B  IP=011C   NV UP EI PL NZ NA PO NC           
3D5B:011C CD21          INT     21                                              
                                                                                
This displays the results after the instruction at 011A is executed, and        
indicates the next instruction to be executed is the INT 21 at location         
04BA:011C.                                                                      
                                                                                
T 10                                                                            
                                                                                
Sixteen instructions are executed (starting at CS:IP).  The contents of all     
registers and flags are displayed after each instruction.  The display stops    
after the 16th instruction has been executed.  Displays may scroll off the      
screen unless you suspend the display by simultaneously pressing the Ctrl and   
NumLock keys, or the Pause key.                                                 
                                                                                
U (UNASSEMBLE) COMMAND                                                          
                                                                                
PURPOSE                                                                         
                                                                                
Unassembles instructions (that is, translates the contents of memory into       
assembler-like statements) and displays their addresses and hexadecimal         
values, together with assembler-like statements.  For example, a display        
might look like this:                                                           
                                                                                
04BA:0100  206472   AND [SI+72],AH                                              
04BA:0103  FC       CLD                                                         
04BA:0104  7665     JBE 016B                                                    
                                                                                
FORMAT                                                                          
                                                                                
U [address]                                                                     
                                                                                
or                                                                              
                                                                                
U [range]                                                                       
                                                                                
PARAMETERS                                                                      
                                                                                
address     Any of the following formats:                                       
                                                                                
    A segment register plus an offset, such as CS:0100.                         
    A segment address plus an offset, such as 4BA:0100.                         
    An offset only, such as 100.  In this case, the default                     
segment is used.                                                                
                                                                                
range       Either of these two formats:                                        
                                                                                
    An address followed by an offset, such as CS:100 110.                       
    An address followed by L value, where value is the number of                
hexadecimal bytes to be processed.  For example, CS:100 L 10.                   
                                                                                
The limit for range is hexadecimal 10000 or decimal 64KB.  To                   
specify a range of 64KB within 4 hexadecimal characters, enter                  
0000 or 0 for value.                                                            
                                                                                
COMMENTS                                                                        
                                                                                
The number of bytes to be unassembled depends on your system display format     
(40 or 80 columns), and which option you use with the Unassemble command.       
                                                                                
NOTES:                                                                          
                                                                                
1.  In all cases, the number of bytes unassembled and displayed may be          
slightly more than either the amount requested or the default amount.           
This happens because the length of the instructions vary.  Therefore,           
unassembling the last instruction may result in more bytes than expected.       
                                                                                
2.  Make sure that the address parameters refer to locations containing valid   
8086/8088 instruction codes.  If you specify an address that does not           
contain the first byte of a valid instruction, the display will be              
incorrect.                                                                      
                                                                                
3.  If you enter only an offset for the starting address, the U command         
assumes the segment contained in the CS register.                               
                                                                                
The Unassemble command has the following two format options:                    
                                                                                
OPTION 1                                                                        
                                                                                
Use this option to either unassemble instructions without specifying an         
address, or to unassemble instructions beginning with a specified address.      
For example:                                                                    
                                                                                
U                                                                               
                                                                                
or                                                                              
                                                                                
U address                                                                       
                                                                                
Sixteen bytes are unassembled with a 40-column display.  Thirty-two bytes are   
unassembled in 80-column mode.                                                  
                                                                                
Instructions are unassembled beginning with the specified address.              
                                                                                
If you do not specify an address, the U command assumes the starting address    
is the location following the last instruction unassembled by a previous U      
command.  Thus, it is possible to unassemble consecutive locations, producing   
continuous unassembled displays, by entering consecutive U commands without     
parameters.                                                                     
                                                                                
If no previous U command is entered, the location is offset hex 0100 into the   
segment originally initialized in the segment registers by DEBUG.               
                                                                                
OPTION 2                                                                        
                                                                                
Use this option to unassemble instructions in a specified address range.  For   
example:                                                                        
                                                                                
U range                                                                         
                                                                                
All instructions in the specified address range are unassembled, regardless     
of the system display format.                                                   
                                                                                
NOTE:  If you specify an ending address, enter it with only an offset value.    
                                                                                
For example:                                                                    
                                                                                
U 04ba:0100 108                                                                 
                                                                                
The display response may be:                                                    
                                                                                
04BA:0100  206472    AND [SI+72],AH                                             
04BA:0103  FC        CLD                                                        
04BA:0104  7665      JBE 016B                                                   
04BA:0106  207370    AND [BP+DI+70],DH                                          
                                                                                
The same display appears if you enter:                                          
                                                                                
U 04BA:100 L 7                                                                  
                                                                                
or                                                                              
                                                                                
U 04BA:100 L 8                                                                  
                                                                                
or                                                                              
                                                                                
U 04BA:100 L 9                                                                  
                                                                                
W (WRITE) COMMAND                                                               
                                                                                
PURPOSE                                                                         
                                                                                
Writes the data being debugged to disk.                                         
                                                                                
FORMAT                                                                          
                                                                                
W [address [drive sector sector]]                                               
                                                                                
PARAMETERS                                                                      
                                                                                
address     Any of the following formats:                                       
                                                                                
    A segment register plus an offset, such as CS:0100.                         
    A segment address plus an offset, such as 4BA:0100.                         
    An offset only, such as 100.  In this case, the default                     
segment is used.                                                                
                                                                                
drive       A decimal number that indicates a particular drive.  For example,   
drive A is 0, drive B is 1, and so on.                                          
                                                                                
sector      1-3 character hexadecimal values that specify the starting          
relative sector number and the number of sectors to be loaded or                
written.                                                                        
                                                                                
NOTE:  Relative sector numbers are obtained by counting the                     
sectors on the disk surface.  The first sector on the disk                      
is at track 0, sector 1, head 0, and is relative sector 0.                      
The numbering continues for each sector on that track and                       
head, then continues with the first sector on the next                          
head of the same track.  When all sectors on all heads of                       
the track have been counted, numbering continues with the                       
first sector on head 0 of the next track.                                       
                                                                                
COMMENTS                                                                        
                                                                                
No more than hex 80 sectors can be written with a single Write command.  A      
sector contains 512 bytes.                                                      
                                                                                
DEBUG displays a message if a disk write error occurs.  You can retry the       
write operation by pressing the F3 key to re-display the Write command, then    
press the Enter key.                                                            
                                                                                
The Write command has two format options.                                       
                                                                                
OPTION 1                                                                        
                                                                                
Use this option to write data to disk beginning at a specified address. For     
example:                                                                        
                                                                                
W address drive sector sector                                                   
                                                                                
The data beginning at the specified address is written to the disk in the       
indicated drive.  The data is written starting at the specified starting        
relative sector (first sector) and continues until the requested number of      
sectors are filled (second sector).                                             
                                                                                
NOTES:                                                                          
                                                                                
1.  Be extremely careful when you write data to absolute sectors because an     
erroneous sector specification destroys whatever was on the disk at that        
location.                                                                       
                                                                                
2.  If only an offset is entered for the beginning address, the W command       
assumes the segment is contained in the CS register.                            
                                                                                
3.  Remember, the starting sector and the sector count are both specified in    
hexadecimal.                                                                    
                                                                                
4.  Option 1 cannot be used if the specified drive is a network drive.          
                                                                                
For example:                                                                    
                                                                                
W 1FD 1 100 A                                                                   
                                                                                
The data beginning at CS:01FD is written to the diskette in drive B, starting   
at relative sector hex 100 (256) and continuing for hex 0A (10) sectors.        
                                                                                
OPTION 2                                                                        
                                                                                
This option permits you to use the WRITE command without specifying             
parameters or specifying only the address parameter.  For example:              
                                                                                
W                                                                               
                                                                                
or                                                                              
                                                                                
W address                                                                       
                                                                                
When issued as shown above, the Write command writes the file (whose file       
specification is at CS:80) to disk.                                             
                                                                                
This condition is met by specifying the file when starting the DEBUG program,   
or by using the Name command.                                                   
                                                                                
NOTE:  If DEBUG was started with a file specification and subsequent Name       
commands were used, you may need to enter a new Name command for the            
proper file specification before issuing the Write command.                     
                                                                                
In addition, the BX and CX registers must be set to the number of bytes to be   
written.  They may have been set properly by the DEBUG or Load commands, but    
were changed by a Go or Trace command.  You must be certain the BX and CX       
registers contain the correct values.                                           
                                                                                
The file beginning at CS:100, or at the location specified by address, is       
written to the diskette in the drive included in the file specification or      
the default drive if no drive was specified.                                    
                                                                                
The debugged file is written over the original file that was loaded into        
memory, or into a new file if the file name in the FCB didn't previously        
exist.                                                                          
                                                                                
NOTE:  An error message is issued if you try to write a file with an            
extension of .EXE or .HEX. These files are written in a specific                
format that DEBUG cannot support.                                               
                                                                                
If you find it necessary to modify a file with an extension of .EXE or .HEX,    
and the exact locations to be modified are known, use the following             
procedure:                                                                      
                                                                                
1.  RENAME the file to an extension other than .EXE or .HEX.                    
                                                                                
2.  Load the file into memory using the DEBUG or Load command.                  
                                                                                
3.  Modify the file as needed in memory, but do not try to execute it with      
the Go or Trace commands.  Unpredictable results would occur.                   
                                                                                
4.  Write the file back using the Write command.                                
                                                                                
5.  RENAME the file to its correct name.                                        
                                                                                
XA (EMS ALLOCATE) COMMAND                                                       
                                                                                
PURPOSE                                                                         
                                                                                
Allocates a specified number of expanded memory pages to a handle.              
                                                                                
FORMAT                                                                          
                                                                                
XA count                                                                        
                                                                                
COMMENTS                                                                        
                                                                                
The count indicates the number of 16K pages to allocate.  If the amount of      
expanded memory identified by count is available, a message is displayed,       
indicating that a handle has been created.  The XS (EMS Status) command can     
be used to display the number of expanded memory pages that are available.      
                                                                                
EXAMPLE                                                                         
                                                                                
To allocate two EMS pages, enter:                                               
                                                                                
XA 2                                                                            
                                                                                
If two pages of memory are available, a message like this is displayed:         
                                                                                
Handle created = 000E                                                           
                                                                                
XD (EMS DEALLOCATE) COMMAND                                                     
                                                                                
PURPOSE                                                                         
                                                                                
Deallocates a handle.                                                           
                                                                                
FORMAT                                                                          
                                                                                
XD handle                                                                       
                                                                                
COMMENTS                                                                        
                                                                                
The handle identifies the number of the handle to be deallocated.  If the       
number is valid, a message is displayed, indicating the handle has been         
deallocated.  The XS (EMS Status) command can be used to display the handles    
currently being used.                                                           
                                                                                
EXAMPLE                                                                         
                                                                                
To deallocate a handle when you have only one allocated, you can enter:         
                                                                                
XD 000E                                                                         
                                                                                
If the handle deallocation is successful, you receive a message like this:      
                                                                                
Handle 000E deallocated.                                                        
                                                                                
XM (EMS MAP) COMMAND                                                            
                                                                                
PURPOSE                                                                         
                                                                                
Maps an EMS logical page to an EMS physical page from an EMS handle.            
                                                                                
FORMAT                                                                          
                                                                                
XM lpage ppage handle                                                           
                                                                                
COMMENTS                                                                        
                                                                                
The lpage specifies the number of the handle's logical page that is to be       
mapped.  The ppage is the number of the physical page to be mapped to.  The     
handle is the EMS allocated label used to reference a group of logical pages.   
If syntax items are valid, a message is displayed indicating that the logical   
page has been mapped to the physical page.                                      
                                                                                
EXAMPLE                                                                         
                                                                                
To map a logical page to a physical page using handle 0001, you can enter:      
                                                                                
XM 1 0 1                                                                        
                                                                                
If the mapping is successful, you receive this message:                         
                                                                                
Logical page 01 mapped to physical page 00.                                     
                                                                                
XS (EMS STATUS) COMMAND                                                         
                                                                                
PURPOSE                                                                         
                                                                                
Displays the status of expanded memory.                                         
                                                                                
FORMAT                                                                          
                                                                                
XS                                                                              
                                                                                
COMMENTS                                                                        
                                                                                
The following expanded memory information is displayed:                         
                                                                                
Handle %1 has %2 pages allocated                                                
                                                                                
.                                                                               
.                                                                               
.                                                                               
Physical page %1 = Frame segment %2                                             
                                                                                
.                                                                               
.                                                                               
.                                                                               
%1 of a total %2 EMS pages have been allocated                                  
%1 of a total %2 EMS handles have been allocated                                
                                                                                
EXAMPLE                                                                         
                                                                                
A line is displayed for each handle allocated with its associated logical       
page count.                                                                     
                                                                                
DEBUG ERROR MESSAGES                                                            
                                                                                
The following error messages are produced by the DEBUG utility:                 
                                                                                
ACCESS DENIED                     The result of attempting to Write (W) to a    
read-only file.                                                                 
                                                                                
DISK ERROR READING DRIVE %1       An invalid parameter was entered on the       
Load (L) command or an error occurred on                                        
issuing the Load (L) command.                                                   
                                                                                
DISK ERROR WRITING DRIVE %1       An invalid parameter was entered on the       
Write (W) command or an error occurred on                                       
issuing the Write (W) command.                                                  
                                                                                
EMS HARDWARE/SOFTWARE FAILURE     The result of an EMS command.  Tells the      
user EMS is not functioning properly.                                           
                                                                                
EMS NOT INSTALLED                 The result of an EMS command.  Tells user     
EMS is not installed.                                                           
                                                                                
&HAT. ERROR                       Points to the offending operand in an error   
condition.                                                                      
                                                                                
ERROR IN EXE OR HEX FILE          The EXE or HEX file are in error.             
                                                                                
EXE AND HEX FILES CANNOT BE WRITTEN A file in EXE or HEX format cannot be       
written to a disk.                                                              
                                                                                
EXEC FAILURE                      The execution of the requested file failed.   
                                                                                
FILE CREATION ERROR               The result of attempting to Write (W) to a    
system or hidden file.                                                          
                                                                                
FILE NOT FOUND                    Issued from the Load (L) command when a       
file is not found for loading.                                                  
                                                                                
FREE PAGES EXCEEDED               The result of an EMS command.  Tells the      
user that the request has exceeded the                                          
amount of free EMS pages available.                                             
                                                                                
HANDLE NOT FOUND                  The result of an EMS command.  Tells the      
user an EMS handle was not found.                                               
                                                                                
INCORRECT DOS VERSION             Incorrect version of DEBUG for the DOS        
version running.                                                                
                                                                                
INSUFFICIENT MEMORY               Not enough memory to Load (L) the specified   
file.                                                                           
                                                                                
INSUFFICIENT SPACE ON DISK        Out of disk space for a Write (W) command.    
                                                                                
INVALID DRIVE SPECIFICATION       The drive referenced by the Name (N) and      
Load (L) command is invalid; that is, it                                        
does not exist.                                                                 
                                                                                
LOGICAL PAGE OUT OF RANGE         The result of an EMS command.  Tells the      
user that the logical page requested is not                                     
in the range of possible pages.                                                 
                                                                                
MISSING OR INVALID EMS PARAMETER  The result of an EMS command.  Tells the      
user a missing or invalid parameter was                                         
entered.                                                                        
                                                                                
NO FREE HANDLES                   The result of an EMS command.  Tells the      
user that there are no more EMS handles                                         
available for allocation.                                                       
                                                                                
PARAMETER ERROR                   The result of an EMS command.  Tells the      
user that an EMS parameter is in error.                                         
                                                                                
PHYSICAL PAGE OUT OF RANGE        The result of an EMS command.  Tells the      
user that the physical page requested is                                        
not in the range of possible pages.                                             
                                                                                
PROGRAM TERMINATED NORMALLY       An Interrupt 20H has been encountered,        
signalling program termination.                                                 
                                                                                
TOTAL PAGES EXCEEDED              The result of an EMS command.  Tells the      
user that the total EMS pages have been                                         
exceeded.                                                                       
                                                                                
WRITE (W) ERROR, NO DESTINATION DEFINED Attempt to Write (W) to a file that     
has not yet been Named (N).                                                     
                                                                                
WRITE PROTECT ERROR WRITING DRIVE %1 A Write (W) to a write-protected disk      
caused an error.                                                                
                                                                                
WRITING %1 BYTES                  Reports the number of bytes written to a      
file when the Write (W) command is issued.                                      

Chapter 11

CHAPTER 11.  WRITING AN INSTALLABLE DEVICE DRIVER 26
Types of Device Drivers 39
Character Device Drivers 48
Block Device Drivers 55
How DOS 4.00 Installs Device Drivers 84
The Basic Parts of a Device Driver 134
The Device Driver Header 148
The Strategy Routine 274
The Interrupt Routines 283
How DOS 4.00 Passes a Request 300
Responding to Requests 392
Initialization Request 456
Media Check Request 511
Build BPB Request 630
Input and Output Requests 815
Nondestructive Input No Wait Request 879
Character Input and Output Status Requests 901
Character Input and Output Flush Requests 935
Open and Close Requests 951
Removable Media Request 983
Generic IOCTL Request 1008
Get Logical Device Request 1037
Set Logical Device Request 1055
CLOCK$ Device Driver Example 1073
  
CHAPTER 11.  WRITING AN INSTALLABLE DEVICE DRIVER                               
                                                                                
This chapter provides guide and system architecture information to support      
successful creation of an installable device driver.                            
                                                                                
We recommend you use the information provided in this chapter in conjunction    
with the information provided in the VDISK.ASM file on the utilities            
diskette.  This file contains the fully documented source code for the VDISK    
device driver, which emulates a fixed disk in RAM.                              
                                                                                
NOTE:  The example on the utilities diskette does not reflect the current       
level of VDISK.SYS..                                                            
                                                                                
TYPES OF DEVICE DRIVERS                                                         
                                                                                
A device driver is a memory image file or an .EXE file that contains the code   
needed to implement a device.  DOS 4.00 allows two types of device drivers to   
be installed:                                                                   
                                                                                
    Character device drivers                                                    
    Block device drivers.                                                       
                                                                                
CHARACTER DEVICE DRIVERS                                                        
                                                                                
Character device drivers perform character I/O in a serial manner and have      
names such as CON, AUX, CLOCK$.  You can open handles or FCBs to perform        
input and output to character devices.  Because character device drivers have   
only one name, they support only one device.                                    
                                                                                
BLOCK DEVICE DRIVERS                                                            
                                                                                
Block device drivers are the fixed disk or diskette drives on the system.       
They perform random I/O in pieces called blocks, which are usually the          
physical sector size of the disk.  Block devices are not named as character     
devices are and you cannot open them.  Instead they are mapped using the        
drive letters such as A, B, and C.                                              
                                                                                
A single block device driver can be responsible for one or more disk or         
diskette drives.  For example, the first block device driver in the device      
chain may define four units such as A, B, C, and D.  The second device driver   
may define three units:  E, F, and G.  The limit is 26 devices with the         
letters A through Z assigned to drives.  The position of the driver in the      
chain determines the way in which the drive units and drive letters             
correspond.                                                                     
                                                                                
Support for Media Greater than 32MB                                             
                                                                                
A block device driver that runs in the DOS 4.00 environment can be written to   
process 32-bit sector numbers.  This ability provides support for fixed disk    
media greater than 32MB.  This support is described in these sections:          
                                                                                
    "The Device Driver Header" on page 11-3                                     
    "Build BPB Request" on page 11-14                                           
    "Input and Output Requests" on page 11-18.                                  
                                                                                
DOS 4.00 also allows users to install their own device drivers that support     
media greater than 32MB.                                                        
                                                                                
HOW DOS 4.00 INSTALLS DEVICE DRIVERS                                            
                                                                                
DOS 4.00 installs device drivers dynamically at startup time by reading and     
processing the DEVICE command in CONFIG.SYS.  For example, if you have          
written a device driver called DRIVER1, include the following command in        
CONFIG.SYS:                                                                     
                                                                                
device=driver1                                                                  
                                                                                
The DOS 4.00 device interface links device drivers together in a chain,         
permitting you to add device drivers for optional devices.                      
                                                                                
Each device must be initialized.  The device driver's initialization            
interrupt routine is called once when the device is installed.  The             
initialization routine returns the location in memory of the ending address     
of the device driver.  After setting the ending address field, a character      
device driver sets the status word and returns.                                 
                                                                                
DOS 4.00 processes installed character device drivers before handling default   
devices.  To have DOS 4.00 install a new CON device (for example, in the        
device driver header's Name/Unit field) name the device CON and set the         
standard input device and standard output device bits in the attribute field.   
Because DOS 4.00 installs drivers anywhere in memory, care must be taken in     
any references to locations not in the segment.  Your driver will not always    
be loaded at the same memory location each time.                                
                                                                                
Block devices are installed in the same manner as character devices, above.     
Block devices return additional information such as the number of units.        
This number identifies the devices' logical names.  For example, if the         
current maximum logical device letter is F at the time of the install call,     
and the block device driver initialization routine returns three logical        
units, the logical names of the devices are G, H, and I.  The mapping is        
determined by the position of the driver in the device list and the number of   
units associated with the device.  The number of units returned by INIT         
overrides the value in the name/unit field of the device header.                
                                                                                
A block device also returns a pointer to a BIOS parameter block (BPB) array.    
This is a pointer to an array of n word pointers, where n is the number of      
units defined.  If all the units are the same, the array is able to point to    
the same BIOS parameter block, thus saving space.  The array must be            
protected below the ending address pointer set by the return.                   
                                                                                
The BPB contains information pertinent to the devices such as the sector size   
and the number of sectors per allocation unit.  The sector size in the BPB      
cannot be greater than the maximum allowed size set by DOS 4.00                 
initialization.                                                                 
                                                                                
A block device returns the media descriptor byte passed to devices to report    
which parameters DOS 4.00 is using for a particular drive unit.                 
                                                                                
THE BASIC PARTS OF A DEVICE DRIVER                                              
                                                                                
A device driver is a memory image file or an .EXE file containing the code      
needed to implement a device.  All DOS 4.00 installable device drivers have     
these things in common:                                                         
                                                                                
    A device driver header, which identifies the device to DOS 4.00 and         
defines the strategy and interrupt entry points.  Since a device driver         
is simply loaded and does not use a program segment prefix, the device          
header must be located at physical location 0 of the device driver (ORG 0       
or no ORG statement).                                                           
    A strategy routine, which saves a pointer to the Request Header.            
    The interrupt routines, which perform the requested operation.              
                                                                                
THE DEVICE DRIVER HEADER                                                        
                                                                                
A device driver requires a device header containing the following:              
                                                                                
+---------------------------------------------------------------------------+   
| FIELD                                                          LENGTH     |   
+---------------------------------------------------------------------------+   
| Pointer to next device header                                  DWORD      |   
+---------------------------------------------------------------------------+   
| Attribute                                                      WORD       |   
+---------------------------------------------------------------------------+   
| Pointer to device strategy routine                             WORD       |   
+---------------------------------------------------------------------------+   
| Pointer to device interrupt routine                            WORD       |   
+---------------------------------------------------------------------------+   
| Name/unit field                                                8 BYTES    |   
+---------------------------------------------------------------------------+   
                                                                                
Pointer to Next Device Header                                                   
                                                                                
The device driver header is a pointer to the device header of the next device   
driver.  It is a doubleword field set by DOS 4.00 at the time the device        
driver is loaded.  The first word is an offset and the second word is the       
segment.                                                                        
                                                                                
If you are loading only one device driver, set the device header field to -1    
before loading the device.  If you are loading more than one device driver,     
set the first word of the device header field to the offset of the next         
device driver's header.  Set the device header field of the last device         
driver to -1.                                                                   
                                                                                
Attribute Field                                                                 
                                                                                
The attribute field identifies the device to DOS 4.00.                          
                                                                                
Bit 15                                                                          
                                                                                
Bit 15 identifies whether the device is a block device or a character device.   
If bit 15 is set to 0, this indicates a block device.  Setting bit 15 to 1      
indicates a character device.  Note how the setting of bit 15 affects the       
interpretation of the setting of the bits below.                                
                                                                                
Bit 14                                                                          
                                                                                
Bit 14, for both character and block devices, tells DOS 4.00 whether the        
device driver can handle control strings through IOCtl 44H, AL=2 through        
AL=5.  Set bit 14 to 1 if control strings can be processed.  IOCtl              
subfunctions permit the device driver to interpret the information passed to    
it, such as setting a baud rate or changing form lengths, without performing    
standard reads and writes.  Set bit 14 to 0 if control strings cannot be        
processed.  DOS 4.00 will return an error if an IOCtl is issued to send or      
receive control strings and bit 14 is set to 0.                                 
                                                                                
Bit 13                                                                          
                                                                                
Bit 13 is used for both block and character devices.  For block devices, set    
bit 13 to 0 if the media is an IBM format.  Set bit 13 to 1 if the media is a   
non-IBM format.  For character devices, set bit 13 to 0 if the driver           
supports output-until-busy.  Set bit 13 to 1 if it does not.                    
                                                                                
With the support of output-until-busy, the device driver will send characters   
to the device if the device is ready.  If the device is not ready, the device   
driver will immediately return an error.                                        
                                                                                
Bit 12                                                                          
                                                                                
Bit 12 is reserved.                                                             
                                                                                
Bit 11                                                                          
                                                                                
Set bit 11 if the device driver can handle removable media.  This bit is        
called the open/close removable media bit.                                      
                                                                                
Bits 10 through 7                                                               
                                                                                
Bits 10 through 7 are reserved.                                                 
                                                                                
Bit 6                                                                           
                                                                                
Bit 6 is the generic IOCtl bit for both character and block device drivers.     
If this bit is set to 1, the device driver supports generic IOCtl function      
calls.  Setting this bit to 1 also indicates support of the Get/Set Logical     
Drive function for a block device driver.                                       
                                                                                
Bits 5 and 4                                                                    
                                                                                
Bits 5 and 4 are reserved.                                                      
                                                                                
Bit 3                                                                           
                                                                                
Set bit 3 to 1 if the character device is a clock device; set bit 3 to 0 if     
it is not.                                                                      
                                                                                
Bit 2                                                                           
                                                                                
Set bit 2 to 1 if the character device is the NUL device; set bit 2 to 0 if     
it is not.  Setting the bit tells DOS 4.00 whether the NUL device is being      
used.  The NUL device cannot be reassigned.                                     
                                                                                
Bit 1                                                                           
                                                                                
If bit 15 is set to 0 for a character device, set bit 1 to 1 to indicate that   
the character device is the current standard output device.  If bit 15 is set   
to 1 for a block device, set bit 1 to 1 to indicate support for 32-bit sector   
numbers; otherwise 16-bit sector number support is assumed.                     
                                                                                
Bit 0                                                                           
                                                                                
Set bit 0 to 1 if the character device is the current standard input device;    
set bit 0 to 0 if it is not the current standard input device.                  
                                                                                
Pointers to Strategy Routine and Interrupt Routines                             
                                                                                
When DOS 4.00 passes a request to a device driver, it calls the device driver   
twice.  These two fields point to the first and second entry points:  the       
strategy routine and the interrupt routine.  The fields are word values, so     
they must be in the same segment as the device header.                          
                                                                                
Name/Unit Field                                                                 
                                                                                
These 8-byte fields identify a character device by name or a block device by    
unit.  A character device name is left-justified followed by spaces, if         
necessary.  For block devices, although DOS 4.00 automatically fills in this    
field with the value of number of units returned by INIT call, you may choose   
to place the number of units in the first place.                                
                                                                                
THE STRATEGY ROUTINE                                                            
                                                                                
DOS 4.00 calls a device driver at the strategy routine at first, passing in a   
request packet the information describing what DOS 4.00 wants the device        
driver to do.                                                                   
                                                                                
The strategy routine does not perform the request but queues the request or     
saves a pointer to the request packet.                                          
                                                                                
THE INTERRUPT ROUTINES                                                          
                                                                                
DOS 4.00 calls the device driver's interrupt routine with no parameters         
immediately after the strategy routine returns.  An interrupt routine's         
function is to perform the operation based on the queued request, process any   
data in the request packet, and set up information being returned to DOS        
4.00.                                                                           
                                                                                
It is the responsibility of the device driver to preserve the system state.     
For example, the device driver must save all registers on entry and restore     
them on exit.  The stack maintained by DOS 4.00 is used to save all             
registers.  If more stack space is needed, it is the device driver's            
responsibility to allocate and maintain an additional stack.                    
                                                                                
All calls to device drivers are FAR calls.  FAR returns should be executed to   
return to DOS 4.00.                                                             
                                                                                
HOW DOS 4.00 PASSES A REQUEST                                                   
                                                                                
DOS 4.00 passes a pointer in ES:BX to the request packet.  The packet           
consists of a request header that contains information common to all            
requests, followed by data pertinent to the request being made.                 
                                                                                
The structure of the request header is shown below.                             
                                                                                
+---------------------------------------------------------------------------+   
| FIELD                                                          LENGTH     |   
+---------------------------------------------------------------------------+   
| Length of the request header and subsequent data               BYTE       |   
+---------------------------------------------------------------------------+   
| Unit code for block devices only                               BYTE       |   
+---------------------------------------------------------------------------+   
| Command code                                                   BYTE       |   
+---------------------------------------------------------------------------+   
| Status                                                         WORD       |   
+---------------------------------------------------------------------------+   
| Reserved                                                       8-BYTE     |   
+---------------------------------------------------------------------------+   
| Data                                                           VARIABLE   |   
+---------------------------------------------------------------------------+   
                                                                                
Length Field                                                                    
                                                                                
The length field identifies the length of the request header and subsequent     
data in bytes.                                                                  
                                                                                
Unit Code Field                                                                 
                                                                                
The unit code field identifies the requesting unit in a block device driver.    
If a block device driver has three units defined, for example, the possible     
values for the unit code field are 0, 1, or 2.                                  
                                                                                
Command Code Field                                                              
                                                                                
The command code identifies the request.  See "Responding to Requests" on       
page 11-8 for a list of command code values and request descriptions.           
                                                                                
Status Field                                                                    
                                                                                
The status word field is zero on entry and is set by the driver interrupt       
routine on return.                                                              
                                                                                
Bit 15                                                                          
                                                                                
Bit 15 is the error bit.  If bit 15 is set to 1, the low order 8 bits of the    
status word (7-0) indicate the error code.                                      
                                                                                
Bits 14 - 10                                                                    
                                                                                
Bits 14 through 10 are reserved.                                                
                                                                                
Bit 9                                                                           
                                                                                
Bit 9 is the busy bit.  As a response to status request call, character         
device drivers can set the busy bit to indicate whether or not a device is      
ready to perform input and output requests.  Block device drivers can set the   
busy bit to indicate removable or nonremovable media.  See "Character Input     
and Output Status Requests" on page 11-21 and "Removable Media Request" on      
page 11-23 for more information about the calls.                                
                                                                                
Bit 8                                                                           
                                                                                
Bit 8 is the done bit.  If set, the operation is complete.  The driver sets     
the done bit to 1 when it exits.                                                
                                                                                
Bits 7 - 0                                                                      
                                                                                
Bits 7 through 0 are the low order 8 bits of the status word.  If bit 15 is     
set, bits 7 through 0 contain the error codes.  The error codes and errors      
are:                                                                            
                                                                                
CODES     MEANING                                                               
00        Write protect violation                                               
01        Unknown unit                                                          
02        Device not ready                                                      
03        Unknown command                                                       
04        CRC error                                                             
05        Bad drive request structure length                                    
06        Seek error                                                            
07        Unknown media                                                         
08        Sector not found                                                      
09        Printer out of paper                                                  
0A        Write fault                                                           
0B        Read fault                                                            
0C        General failure                                                       
0D        Reserved                                                              
0E        Reserved                                                              
0F        Invalid disk change                                                   
                                                                                
RESPONDING TO REQUESTS                                                          
                                                                                
Each request packet that is passed to the device driver contains a command      
code value in the request header to tell the driver which function to           
perform.  The table below contains the DOS 4.00 device interface command code   
values and the functions to be performed when these values are passed with      
data.  Note that some of these functions are specific to either a block         
device or a character device.                                                   
                                                                                
Following this table are detailed descriptions of request data structures and   
what the interrupt routines are expected to do.  Some of these descriptions     
pertain to more than one command code.                                          
                                                                                
+---------------------------------------------------------------------------+   
| COMMAND  REQUEST                                               DEVICE     |   
| CODE     DESCRIPTION                                           TYPE       |   
+---------------------------------------------------------------------------+   
|  0       Initialization                                        Both       |   
+---------------------------------------------------------------------------+   
|  1       Media check                                           Block      |   
+---------------------------------------------------------------------------+   
|  2       Build BPB                                             Block      |   
+---------------------------------------------------------------------------+   
|  3       IOCtl input (called only if bit 14 of attribute is    Both       |   
|          set to 1)                                                        |   
+---------------------------------------------------------------------------+   
|  4       Input (read)                                          Both       |   
+---------------------------------------------------------------------------+   
|  5       Nondestructive input no wait                          Character  |   
+---------------------------------------------------------------------------+   
|  6       Input status                                          Character  |   
+---------------------------------------------------------------------------+   
|  7       Input flush                                           Character  |   
+---------------------------------------------------------------------------+   
|  8       Output (write)                                        Both       |   
+---------------------------------------------------------------------------+   
|  9       Output (write with verify)                            Block      |   
+---------------------------------------------------------------------------+   
| 10       Output status                                         Character  |   
+---------------------------------------------------------------------------+   
| 11       Output flush                                          Character  |   
+---------------------------------------------------------------------------+   
| 12       IOCtl output (called only if bit 14 of attribute is   Both       |   
|          set to 1                                                         |   
+---------------------------------------------------------------------------+   
| 13       Device open (called only if bit 11 of attribute is    Both       |   
|          set to 1)                                                        |   
+---------------------------------------------------------------------------+   
| 14       Device close (called only if bit 11 of attribute is   Both       |   
|          set to 1)                                                        |   
+---------------------------------------------------------------------------+   
| 15       Removable media (called only if bit 11 of attribute   Block      |   
|          is set to 1)                                                     |   
+---------------------------------------------------------------------------+   
| 19       Generic IOCtl Request (called                         Both       |   
|          only if bit 6 of attribute is set to 1)                          |   
+---------------------------------------------------------------------------+   
| 23       Get logical device (called only if bit 6 of           Block      |   
|          attribute is set to 1)                                           |   
+---------------------------------------------------------------------------+   
| 24       Set logical device (called only if bit 6 of           Block      |   
|          attribute is set to 1)                                           |   
+---------------------------------------------------------------------------+   
                                                                                
INITIALIZATION REQUEST                                                          
                                                                                
COMMAND CODE = 0                                                                
                                                                                
+---------------------------------------------------------------------------+   
| FIELD                                                          LENGTH     |   
+---------------------------------------------------------------------------+   
| Request header                                                 13-BYTE    |   
+---------------------------------------------------------------------------+   
|  Number of units (not set by character devices)                BYTE       |   
+---------------------------------------------------------------------------+   
| Ending address of resident program code                        DWORD      |   
+---------------------------------------------------------------------------+   
| Pointer to BPB array (not set by character devices) pointer    DWORD      |   
| to remainder of arguments                                                 |   
+---------------------------------------------------------------------------+   
| Drive number                                                   BYTE       |   
+---------------------------------------------------------------------------+   
| CONFIG.SYS Error Message control flag                          WORD       |   
+---------------------------------------------------------------------------+   
                                                                                
The driver must do the following:                                               
                                                                                
    Set the number of units (block devices only).                               
    Set up the pointer to the BPB array (block devices only).                   
    Perform any initialization code (to modems, printers, and others).          
    Set the ending address of the resident program code.                        
    Set the status word in the request header.                                  
                                                                                
To obtain information passed from CONFIG.SYS to a device driver at              
initialization time, the BPB pointer field points to a buffer containing the    
information passed in CONFIG.SYS following the =.  This string may end with     
either a carriage return (0DH) or a linefeed (0AH).  This information is        
read-only.  Only system calls 01H-0CH and 30H can be issued by the              
initialization code of the driver.                                              
                                                                                
The last byte parameter contains the drive letter for the first unit of a       
block driver.  For example, 0=A, 1=B and so forth.                              
                                                                                
If an initialization routine determines that it cannot set up the device and    
wants to terminate without using any memory, use the following procedure:       
                                                                                
    Set the number of units to 0.                                               
    Set the ending address offset to 0.                                         
    Set the ending address segment to the code segment (CS).                    
                                                                                
If there are multiple device drivers in a single memory image file, the         
ending address returned by the last initialization called is the one DOS 4.00   
uses.  IBM recommends that all device drivers in a single memory image file     
return the same ending address.                                                 
                                                                                
If initialization of your device driver fails, and you want the system to       
display the ERROR IN CONFIG.SYS LINE # error message, set the CONFIG.SYS        
error message control flag to a non-zero value.                                 
                                                                                
MEDIA CHECK REQUEST                                                             
                                                                                
COMMAND CODE = 1                                                                
                                                                                
+---------------------------------------------------------------------------+   
| FIELD                                                          LENGTH     |   
+---------------------------------------------------------------------------+   
| Request header                                                 13-BYTE    |   
+---------------------------------------------------------------------------+   
| Media descriptor from DOS 4.00                                 BYTE       |   
+---------------------------------------------------------------------------+   
| Return                                                         BYTE       |   
+---------------------------------------------------------------------------+   
| A pointer to the previous volume ID (if bit 11 = 1 and disk    DWORD      |   
| change is returned)                                                       |   
+---------------------------------------------------------------------------+   
                                                                                
When the command code field is 1, DOS 4.00 calls Media Check for a drive unit   
and passes its current Media Descriptor byte.  See "Media Descriptor Byte."     
Media Check returns one of the following:                                       
                                                                                
    Media not changed                                                           
    Media changed                                                               
    Not sure                                                                    
    Error code.                                                                 
                                                                                
The driver must perform the following:                                          
                                                                                
    Set the status word in the request header.                                  
    Set the return byte to:                                                     
                                                                                
-1   for "media changed"                                                        
0  for "not sure"                                                               
1  for "media not changed."                                                     
                                                                                
The driver uses the following method to determine how to set the return byte:   
                                                                                
    If the media is nonremovable (a fixed disk), set the return byte to 1.      
    If less than 2 seconds since last successful access, set the return byte    
to 1.                                                                           
    If changeline not available, set the return byte to 0.                      
    If changeline is available but not active, set the return byte to 1.        
    If the media byte in the new BPB does not match the old media byte, set     
the return byte to -1.                                                          
    If the current volume ID matches the previous volume ID, or if the serial   
number matches the previous serial number, set the return byte to 0.            
                                                                                
Media Descriptor Byte                                                           
                                                                                
Currently the media descriptor byte has been defined for some media types.      
This byte should be identical to the media byte if the device has the non-IBM   
format bit off.  These predefined values are:                                   
                                                                                
Media descriptor                                                                
byte -->        1  1  1  1  1  x  x  x                                          
bits -->         7  6  5  4  3  2  1  0                                         
                                                                                
BIT   MEANING                                                                   
0   1=2-sided         0=not 2-sided                                             
1   1=8 sector        0=not 8 sector                                            
2   1=removable       0=not removable                                           
3-7   must be set to 1                                                          
                                                                                
NOTE:  An exception to the above is that the media descriptor byte value of     
F0, which is used to indicate any media types not defined, and F9,              
which is used for 5.25-inch media with 2 sides and 15 sectors/tracks.           
                                                                                
Examples of current DOS 4.00 media descriptor bytes:                            
                                                                                
+---------------------------------------------------------------------------+   
| DISK                                         #      SECTORS/   MEDIA      |   
| TYPE                                         SIDES  TRACK      DESCRIPTOR |   
+---------------------------------------------------------------------------+   
| Fixed disk                                   --     --         F8H        |   
+---------------------------------------------------------------------------+   
| 5.25 inch                                    2      15         F9H        |   
+---------------------------------------------------------------------------+   
| 5.25 inch                                    1       9         FCH        |   
+---------------------------------------------------------------------------+   
| 5.25 inch                                    2       9         FDH        |   
+---------------------------------------------------------------------------+   
| 5.25 inch                                    1       8         FEH        |   
+---------------------------------------------------------------------------+   
| 5.25 inch                                    2       8         FFH        |   
+---------------------------------------------------------------------------+   
| 8 inch                                       1      26         FEH        |   
+---------------------------------------------------------------------------+   
| 8 inch                                       2      26         FDH        |   
+---------------------------------------------------------------------------+   
| 8 inch                                       2       8         FEH        |   
+---------------------------------------------------------------------------+   
| 3.5 inch                                     2       9         F9H        |   
+---------------------------------------------------------------------------+   
| 3.5 inch                                     2      18         F0H        |   
+---------------------------------------------------------------------------+   
                                                                                
To determine whether you are using a single-sided or a double-sided diskette,   
attempt to read the second side.  If an error occurs, you may assume the        
diskette is single-sided.  Media descriptor F0H may be used for those media     
types not described earlier.  Programs should not use the media descriptor      
values to distinguish media.  DOS 4.00 internal routines use information in     
the BIOS parameter block (BPB) to determine the media type of IBM-formatted     
diskettes.  These media descriptor bytes do not necessarily indicate a unique   
media type.                                                                     
                                                                                
For 8-inch diskettes:                                                           
                                                                                
    FEH (IBM 3740 Format) -- Single-sided, single-density, 128 bytes per        
sector, soft-sectored, 4 sectors per allocation unit, 1 reserved sector,        
2 FATs, 68 directory entries, 77*26 sectors.                                    
                                                                                
    FDH (IBM 3740 Format) -- Double-sided, single-density, 128 bytes per        
sector, soft-sectored, 4 sectors per allocation unit, 4 reserved sectors,       
2 FATs, 68 directory entries, 77*26*2 sectors.                                  
                                                                                
    FEH -- Double-sided, double-density, 1024 bytes per sector, soft            
sectored, 1 sector per allocation unit, 1 reserved sector, 2 FATs, 192          
directory entries, 77*8*2 sectors.                                              
                                                                                
BUILD BPB REQUEST                                                               
                                                                                
COMMAND CODE = 2                                                                
                                                                                
+---------------------------------------------------------------------------+   
| FIELD                                                          LENGTH     |   
+---------------------------------------------------------------------------+   
| Request header                                                 13-BYTE    |   
+---------------------------------------------------------------------------+   
| Media descriptor from DOS 4.00                                 BYTE       |   
+---------------------------------------------------------------------------+   
| Transfer address (buffer address)                              DWORD      |   
+---------------------------------------------------------------------------+   
| Pointer to BPB table                                           DWORD      |   
+---------------------------------------------------------------------------+   
                                                                                
DOS 4.00 calls Build BPB (BIOS Parameter Block) under the following two         
conditions:                                                                     
                                                                                
    If "Media Changed" is returned                                              
    If "Not Sure" is returned, there are no used buffers.  Used buffers are     
buffers with changed data not yet written to the disk.                          
                                                                                
The driver must do the following:                                               
                                                                                
    Set the pointer to the BPB                                                  
    Set the status word in the request header.                                  
                                                                                
The device driver must determine the media type that is in the unit to return   
the pointer to the BPB table.  In previous versions of IBMBIO, the FAT ID       
byte determined the structure and layout of the media.  The FAT ID byte has     
only eight possible values (F8 through FF), so, as new media types are          
invented, the available values will soon be exhausted.  With the varying        
media layouts, DOS 4.00 needs to be aware of the location of the FATs  and      
directories before it asks to read them.                                        
                                                                                
The following paragraphs explain the method DOS 4.00 uses to determine the      
media type.                                                                     
                                                                                
The information relating to the BPB for a particular media is kept in the       
boot sector for the media.  The format of the boot sector is as follows:        
                                                                                
+---------------------------------------------------------------------------+   
| FIELD                                                          LENGTH     |   
+---------------------------------------------------------------------------+   
| A 2-byte short JMP instruction (EBH), followed by a NOP        WORD       |   
| instruction (90H).                                                        |   
+---------------------------------------------------------------------------+   
| Product name and version                                       8 BYTES    |   
+---------------------------------------------------------------------------+   
| Bytes per sector; must be power of 2                           WORD       |   
+---------------------------------------------------------------------------+   
| Sectors per allocation unit; must be power of 2                BYTE       |   
+---------------------------------------------------------------------------+   
                                                                                
+---------------------------------------------------------------------------+   
| FIELD                                                          LENGTH     |   
+---------------------------------------------------------------------------+   
| Reserved sectors starting at logical sector 0                  WORD       |   
+---------------------------------------------------------------------------+   
| Number of FATs                                                 BYTE       |   
+---------------------------------------------------------------------------+   
| Maximum number of root directory entries                       WORD       |   
+---------------------------------------------------------------------------+   
| Total number of sectors in media including the boot sector,    WORD       |   
| FAT areas, and directories                                                |   
+---------------------------------------------------------------------------+   
| Media descriptor                                               BYTE       |   
+---------------------------------------------------------------------------+   
| Number of sectors occupied by a FAT                            WORD       |   
+---------------------------------------------------------------------------+   
| Sectors per track                                              WORD       |   
+---------------------------------------------------------------------------+   
| Number of heads                                                WORD       |   
+---------------------------------------------------------------------------+   
| Number of hidden sectors                                       WORD       |   
+---------------------------------------------------------------------------+   
                                                                                
The BPB information contained in the boot sector starts with the Bytes per      
Sector entry.  The last three words are intended to help the device driver      
identify the media.  The number of heads is useful for supporting different     
multihead drives with the same storage capacity but a different number of       
surfaces.  The number of hidden sectors is useful for supporting drive          
partitioning schemes.                                                           
                                                                                
For drivers that support volume identification and disk change, this call       
causes a new volume identification to be read from the disk.  This call         
indicates that the disk has changed in a permissible manner.                    
                                                                                
To handle the partition that is bigger than 32MB, or one that starts beyond     
or crosses the 32MB boundary, DOS 4.00 defines an extended BPB structure.       
Depending on the size of the media, you can use either the existing BPB or      
the extended one, which contains an additional DWORD field to indicate the      
size of the partition in sectors.                                               
                                                                                
Bit 1 of the attribute field in the block device driver header indicates        
whether the device can process 32-bit sector numbers.  Set bit 1 to indicate    
32-bit support.                                                                 
                                                                                
+---------------------------------------------------------------------------+   
| FIELD                                                          LENGTH     |   
+---------------------------------------------------------------------------+   
| Bytes per sector                                               WORD       |   
+---------------------------------------------------------------------------+   
| Sectors per allocation unit                                    BYTE       |   
+---------------------------------------------------------------------------+   
| Reserved sectors starting at logical sector 0                  WORD       |   
+---------------------------------------------------------------------------+   
                                                                                
+---------------------------------------------------------------------------+   
| FIELD                                                          LENGTH     |   
+---------------------------------------------------------------------------+   
| Number of FATs -- 0 if not a FAT system                        BYTE       |   
+---------------------------------------------------------------------------+   
| Maximum number of root directory entries                       WORD       |   
+---------------------------------------------------------------------------+   
| Total number of sectors in the media.  This field is used to   WORD       |   
| define a partition that is less than 32MB.  Setting this                  |   
| field to 0 indicates to use the total (32-bit) number of                  |   
| sectors in the media below.                                               |   
+---------------------------------------------------------------------------+   
| Media descriptor                                               BYTE       |   
+---------------------------------------------------------------------------+   
| Number of sectors occupied by a FAT                            WORD       |   
+---------------------------------------------------------------------------+   
| Sectors per track                                              WORD       |   
+---------------------------------------------------------------------------+   
| Number of heads                                                WORD       |   
+---------------------------------------------------------------------------+   
| Number of hidden sectors                                       DWORD      |   
+---------------------------------------------------------------------------+   
| Total (32-bit) number of sectors in the media. This field is   DWORD      |   
| used to define a partition that is greater than 32MB, or one              |   
| that crosses the 32MB boundary.                                           |   
+---------------------------------------------------------------------------+   
                                                                                
The extended BPB is a superset of the traditional BPB structure.  To achieve    
the maximum compatibility between this structure and that of the traditional    
BPB, DOS 4.00 uses the following rule:                                          
                                                                                
    If (the number of hidden sectors plus the total number of sectors in the    
media) is greater than 64KB, use the 32-bit total number of sectors in          
the media entry (DWORD).                                                        
                                                                                
    Otherwise, use the TOTAL NUMBER OF SECTORS IN THE MEDIA entry (WORD).       
                                                                                
A boot record exists at the beginning of each disk partition and each           
extended DOS 4.00 partition volume.  DOS 4.00 automatically creates the         
extended boot record.  The format of the extended boot record is:               
                                                                                
+---------------------------------------------------------------------------+   
| FIELD                                                          LENGTH     |   
+---------------------------------------------------------------------------+   
| A 2-byte short JMP instruction (EBH) followed by a NOP         WORD       |   
| instruction (90H).                                                        |   
+---------------------------------------------------------------------------+   
| Product name and version                                       8 BYTES    |   
+---------------------------------------------------------------------------+   
| Extended BPB                                                   25 BYTES   |   
+---------------------------------------------------------------------------+   
| Physical drive number                                          BYTE       |   
+---------------------------------------------------------------------------+   
| Reserved                                                       BYTE       |   
+---------------------------------------------------------------------------+   
| Extended boot record signature                                 BYTE       |   
+---------------------------------------------------------------------------+   
| Volume serial number                                           DWORD      |   
+---------------------------------------------------------------------------+   
| Volume label                                                   11 BYTES   |   
+---------------------------------------------------------------------------+   
| Reserved                                                       8 BYTES    |   
+---------------------------------------------------------------------------+   
                                                                                
NOTE:  The value of Extended boot record signature is 29H.  The value of the    
physical drive number is always 0H or 80H.                                      
                                                                                
On all requests to extended drivers with a sector number in their request       
headers, the sector number is a DWORD.  The standard DOS 4.00 block device      
drivers set the attribute bit 1 for 32-bit support.                             
                                                                                
For each call to a device driver, DOS 4.00 checks to see if the starting        
sector number passed in the request can be supported by the device driver.      
If this value is greater than 64K for an old-style device driver, DOS 4.00      
returns an UNKNOWN MEDIA (07H) device driver error.                             
                                                                                
INPUT AND OUTPUT REQUESTS                                                       
                                                                                
COMMAND CODES = 3, 4, 8, 9, 12                                                  
                                                                                
+---------------------------------------------------------------------------+   
| FIELD                                                          LENGTH     |   
+---------------------------------------------------------------------------+   
| Request header                                                 13-BYTE    |   
+---------------------------------------------------------------------------+   
| Media descriptor byte                                          BYTE       |   
+---------------------------------------------------------------------------+   
| Transfer address (buffer address)                              DWORD      |   
+---------------------------------------------------------------------------+   
| Byte/sector count                                              WORD       |   
+---------------------------------------------------------------------------+   
| Starting sector number (If -1, use DWORD starting sector       WORD       |   
| number.  This entry has no meaning for a character device.)               |   
+---------------------------------------------------------------------------+   
| For DOS 3.0 to DOS 4.00, pointer to the volume identification  DWORD      |   
| if error code 0FH is returned                                             |   
+---------------------------------------------------------------------------+   
| Starting 32-bit sector number.  (Use this entry to the block   DWORD      |   
| device driver with the attribute bit 1 set.)                              |   
+---------------------------------------------------------------------------+   
                                                                                
The DOS 4.00 INPUT/OUTPUT request structure can process 32-bit sector           
numbers, providing support for media of more than 4 billion sectors.            
                                                                                
The driver must do the following:                                               
                                                                                
    Set the status word in the request header                                   
    Perform the requested function                                              
    Set the actual number of sectors or bytes transferred.                      
                                                                                
No error checking is performed on an IOCtl call.  However, the driver must      
set the return sector or byte count to the actual number of bytes               
transferred.                                                                    
                                                                                
Under certain circumstances the block device driver may be asked to do a        
WRITE operation of 64KB that seems to be a wraparound of the transfer address   
in the device driver request packet.  This occurs because an optimization is    
added to the WRITE code in DOS 4.00.  It will only happen on WRITEs that are    
within a sector size of 64KB on files that are being extended past the          
current end of file.  The block device driver is allowed to ignore the          
balance of the WRITE that wraps around.  For example, a WRITE of 10000H bytes   
of sectors with a transfer address of XXXX:1 ignores the last two bytes.        
                                                                                
Remember that a program using DOS 4.00 function calls cannot request an input   
or output operation of more than FFFFH bytes because a wrap around in the       
transfer buffer segment must occur.  You can ignore bytes that would have       
wrapped around in the transfer segment.                                         
                                                                                
If the driver returns an error code of 0FH (Invalid Disk Change), it must       
provide a DWORD pointer to an ASCIIZ string identifying the correct volume ID   
and the system prompts the user to reinsert the disk.                           
                                                                                
The reference count of open files on the disk maintained by OPEN and CLOSE      
calls allows the driver to determine when to return error 0FH.  If there are    
no open files (reference count=0) and the disk has been changed, the I/O is     
valid, and error 0FH is not returned.  If there are open files (reference       
count > 0) and the disk has been changed, an error 0FH situation may exist.     
DOS 4.00 IBMDOS.COM will request an OPEN or CLOSE function only if SHARE is     
loaded.                                                                         
                                                                                
NONDESTRUCTIVE INPUT NO WAIT REQUEST                                            
                                                                                
COMMAND CODE = 5                                                                
                                                                                
+---------------------------------------------------------------------------+   
| FIELD                                                          LENGTH     |   
+---------------------------------------------------------------------------+   
| Request header                                                 13-BYTE    |   
+---------------------------------------------------------------------------+   
| Byte read from device                                          BYTE       |   
+---------------------------------------------------------------------------+   
                                                                                
The driver must do the following:                                               
                                                                                
    Return a byte from the device.                                              
    Set the status word in the request header.                                  
                                                                                
If the character device returns busy bit = 0, meaning there are characters in   
buffer, the next character that would be read is returned.  This character is   
not removed from the input buffer, (that is, nondestructive input).  This       
call allows DOS 4.00 to look ahead one input character.                         
                                                                                
CHARACTER INPUT AND OUTPUT STATUS REQUESTS                                      
                                                                                
COMMAND CODES = 6, 10                                                           
                                                                                
+---------------------------------------------------------------------------+   
| FIELD                                                          LENGTH     |   
+---------------------------------------------------------------------------+   
| Request header                                                 13-BYTE    |   
+---------------------------------------------------------------------------+   
                                                                                
The driver must do the following:                                               
                                                                                
    Perform the requested function.                                             
    Set the busy bit.                                                           
    Set the status word in the request header.                                  
                                                                                
The busy bit is set differently for output and input.                           
                                                                                
Output on Character Devices                                                     
                                                                                
If the busy bit is 1 on return, a write request would wait for completion of    
a current request.  If the busy bit is 0, no request is waiting or running.     
Therefore, a write request would start immediately.                             
                                                                                
Input on Character Devices with a Buffer                                        
                                                                                
If the busy bit is 1 on return, a read request goes to the physical device.     
If the busy bit is 0, characters are in the device buffer and a read returns    
quickly.  This also indicates that the user has typed something.  DOS 4.00      
assumes that all character devices have a type-ahead input buffer.  Devices     
that do not have this buffer should always return busy = 0 so that DOS 4.00     
does not loop endlessly, waiting for information to be put in a buffer that     
does not exist.                                                                 
                                                                                
CHARACTER INPUT AND OUTPUT FLUSH REQUESTS                                       
                                                                                
COMMAND CODES = 7, 11                                                           
                                                                                
+---------------------------------------------------------------------------+   
| FIELD                                                          LENGTH     |   
+---------------------------------------------------------------------------+   
| Request header                                                 13-BYTE    |   
+---------------------------------------------------------------------------+   
                                                                                
This call tells the driver to flush (terminate) all pending requests of which   
it has knowledge.  Its primary use is to flush the input queue on character     
devices.                                                                        
                                                                                
The driver must set the status word in the Request Header upon return.          
                                                                                
OPEN AND CLOSE REQUESTS                                                         
                                                                                
COMMAND CODES = 13, 14                                                          
                                                                                
+---------------------------------------------------------------------------+   
| FIELD                                                          LENGTH     |   
+---------------------------------------------------------------------------+   
| Request header                                                 13-BYTE    |   
+---------------------------------------------------------------------------+   
                                                                                
These calls are designed to give the device information about current file      
activity on the device if bit 11 of the attribute word is set.                  
                                                                                
On block devices, these calls can be used to manage local buffering.  The       
device can keep a reference count.  Every OPEN increases the reference count.   
Every CLOSE decreases the device reference count.  When the reference count     
is 0, there are no open files on the device.  Therefore, the device should      
flush buffers inside the device to which it has written because the user can    
change the media on a removable media drive.  If the media has been changed,    
reset the reference count to 0 without flushing the buffers.                    
                                                                                
These calls are more useful on character devices.  The OPEN call can send a     
device an initialization string.  On a printer, the call can send a string to   
set the font or the page size so the printer is always in a known state at      
the start of an I/O stream.                                                     
                                                                                
Similarly, the CLOSE call can send a post string, such as a form feed, at the   
end of an I/O stream.                                                           
                                                                                
Using IOCtl to set the preliminary and ending strings provides a flexible       
mechanism for serial I/O device stream control.                                 
                                                                                
REMOVABLE MEDIA REQUEST                                                         
                                                                                
COMMAND CODE = 15                                                               
                                                                                
+---------------------------------------------------------------------------+   
| FIELD                                                          LENGTH     |   
+---------------------------------------------------------------------------+   
| Request header                                                 13-BYTE    |   
+---------------------------------------------------------------------------+   
                                                                                
To use this call, set bit 11 of the attribute field to 1.  Block devices can    
use this call only by way of a subfunction of the IOCtl function call (44H).    
                                                                                
This call is useful because it notifies a utility if it is dealing with a       
removable or nonremovable media drive.  For example, the FORMAT utility needs   
to know whether a drive is removable or nonremovable because it displays        
different versions of some prompts.                                             
                                                                                
The information is returned in the busy bit of the status word.  If the busy    
bit is 1, the media is nonremovable.  If the busy bit is 0, the media is        
removable.                                                                      
                                                                                
No error bit checking is performed.  It is assumed that this call always        
succeeds.                                                                       
                                                                                
GENERIC IOCTL REQUEST                                                           
                                                                                
COMMAND CODE = 19                                                               
                                                                                
+---------------------------------------------------------------------------+   
| FIELD                                                          LENGTH     |   
+---------------------------------------------------------------------------+   
| Request header                                                 13-BYTE    |   
+---------------------------------------------------------------------------+   
| Major function                                                 BYTE       |   
+---------------------------------------------------------------------------+   
| Minor function                                                 BYTE       |   
+---------------------------------------------------------------------------+   
| Contents of SI                                                 WORD       |   
+---------------------------------------------------------------------------+   
| Contents of DI                                                 WORD       |   
+---------------------------------------------------------------------------+   
| Pointer to Generic IOCTL request packet                        DWORD      |   
+---------------------------------------------------------------------------+   
                                                                                
The driver must:                                                                
                                                                                
    Support the functions described under Generic IOCtl request                 
    Maintain its own track table (TrackLayout).                                 
                                                                                
See Appendix C, "I/O Control for Devices (IOCtl)" on page C-1 for a             
description of the functions provided by generic IOCtl requests.                
                                                                                
                                                                                
GET LOGICAL DEVICE REQUEST                                                      
                                                                                
COMMAND CODE = 23                                                               
                                                                                
+---------------------------------------------------------------------------+   
| FIELD                                                          LENGTH     |   
+---------------------------------------------------------------------------+   
| Request header                                                 13-BYTE    |   
+---------------------------------------------------------------------------+   
| Input (unit code)                                              BYTE       |   
+---------------------------------------------------------------------------+   
| Command code                                                   BYTE       |   
+---------------------------------------------------------------------------+   
| Status                                                         WORD       |   
+---------------------------------------------------------------------------+   
| Reserved                                                       DWORD      |   
+---------------------------------------------------------------------------+   
                                                                                
SET LOGICAL DEVICE REQUEST                                                      
                                                                                
COMMAND CODE = 24                                                               
                                                                                
+---------------------------------------------------------------------------+   
| FIELD                                                          LENGTH     |   
+---------------------------------------------------------------------------+   
| Request header                                                 13-BYTE    |   
+---------------------------------------------------------------------------+   
| Input (unit code)                                              BYTE       |   
+---------------------------------------------------------------------------+   
| Command code                                                   BYTE       |   
+---------------------------------------------------------------------------+   
| Status                                                         WORD       |   
+---------------------------------------------------------------------------+   
| Reserved                                                       DWORD      |   
+---------------------------------------------------------------------------+   
                                                                                
CLOCK$ DEVICE DRIVER EXAMPLE                                                    
                                                                                
This feature is a "Real-Time Clock" Board.  To allow this board to be           
integrated into the system for TIME and DATE, a special device driver is        
specified by an attribute word, the CLOCK$ device.  This device driver          
defines and performs functions like any other character device driver.  Most    
functions will be set done bit, reset error bit, return.  When a read or        
write to this device occurs, 6 bytes are transferred.  The first 2 bytes are    
a word, which is the count of days since 1-1-80.  The third byte is minutes;    
the fourth is hours; the fifth is hundredths of seconds; and the sixth is       
seconds.  Reading the CLOCK$ device gets the date and time; writing to it       
sets the date and time.                                                         
                                                                                
PART 3.  APPENDIXES                                                             
                                                                               

Appendix A

APPENDIX A. DOS 4.00 INTERRUPTS 13 20H Program Terminate 21 21H Function Request 41 22H Terminate Address 46 23H Ctrl-Break Exit Address 53 24H Critical Error Handler Vector 83 25H/26H Absolute Disk Read/Write 287 27H Terminate but Stay Resident 353 28H-2EH Reserved for DOS 4.00 392 2FH Multiplex Interrupt 396 30H-3FH Reserved for DOS 4.00 695

APPENDIX A. DOS 4.00 INTERRUPTS

This chapter contains information to support use of the DOS 4.00 interrupts.

DOS 4.00 reserves interrupt types 20H to 3FH for its use. Absolute memory locations 80H to FFH are reserved by DOS 4.00. All interrupt values are in hexadecimal.

20H PROGRAM TERMINATE

Issue interrupt 20H to exit from a program. This vector transfers to the logic in DOS 4.00 to restore the terminate address, the Ctrl-Break address, and the critical error exit address to the values they had on entry to the program. All file buffers are flushed and all handles are closed. You should close all files changed in length (see function call 10H and 3EH) before issuing this interrupt. If the changed file is not closed, its length, date, and time are not recorded correctly in the directory.

For a program to pass a completion code or an error code when terminating, it must use either function call 4CH (Terminate a Process) or 31H (Terminate Process and Stay Resident). These two methods are preferred over using interrupt 20H, and the codes returned by them can be interrogated in batch processing. See function call 4CH for information on the ERRORLEVEL subcommand of batch processing.

IMPORTANT: Before you issue interrupt 20H, your program must ensure that the CS register contains the segment address of its program segment prefix.

21H FUNCTION REQUEST

Refer to each function call issued within 21H in Appendix B, "DOS 4.00 Function Calls" on page B-1.

22H TERMINATE ADDRESS

Control transfers to the address at this interrupt location when the program terminates. This address is copied into the program's Program Segment Prefix at the time the segment is created. You should not issue this interrupt; the EXEC function call does this for you.

23H CTRL-BREAK EXIT ADDRESS

If the user presses the Ctrl and Break keys during standard input, standard output, standard printer, or asynchronous communications adapter operations, an interrupt 23H is executed. If BREAK is on, the interrupt 23H is checked on most function calls (except calls 06H and 07H). If the user-written Ctrl-Break routine saves all registers, it may end with a return-from-interrupt instruction (IRET) to continue program execution.

If the user-written interrupt program returns with a long return, the carry flag is used to determine whether the program will be ended. If the carry flag is set, the program is ended, otherwise execution continues (as with a return by IRET).

If the user-written Ctrl-Break interrupt uses function calls 09H or 0AH, then &hat.C, carriage-return and linefeed are output. If execution is continued with an IRET, I/O continues from the start of the line.

When the interrupt occurs, all registers are set to the value they had when the original function call to DOS 4.00 was made. There are no restrictions on what the Ctrl-Break handler is allowed to do, including DOS 4.00 function calls, as long as the registers are unchanged if IRET is used.

When the program creates a new segment and loads in a second program it changes the Ctrl-Break address. The termination of the second program and return to the first causes the Ctrl-Break address to be restored to the value it had before execution of the second program. It is restored from the second program's Program Segment Prefix. You should not issue this interrupt.

24H CRITICAL ERROR HANDLER VECTOR

A critical error is returned when a DOS function cannot be performed. This error is frequently caused by a hardware condition, such as the printer being out of paper, a diskette drive door open, or a diskette out of space. When a critical error occurs within DOS 4.00, control is transferred with an interrupt 24H. On entry to the error handler, AH will have its bit 7=0 (high-order bit) if the error was a disk error (the most common occurrence), bit 7=1 if it was not.

BP:SI contains the address of a Device Header Control Block from which additional information can be retrieved. See page A-5.

The registers are set up for a retry operation, and an error code is in the lower half of the DI register with the upper half undefined. The error codes follow:

ERROR CODE MEANING 0 Attempt to write on write-protected diskette 1 Unknown unit 2 Drive not ready 3 Unknown command 4 Data error (CRC) 5 Bad request structure length 6 Seek error 7 Unknown media type 8 Sector not found 9 Printer out of paper A Write fault B Read fault C General failure

The user stack is in effect and contains the following from top to bottom:

IP DOS 4.00 registers from issuing INT 24H

CS FLAGS AX User registers at time of original BX INT 21H request CX DX SI DI BP DS ES IP From the original interrupt 21H CS from the user to DOS 4.00 FLAGS

The registers are set such that if an IRET is executed, DOS 4.00 responds according to (AL) as follows:

(AL) =0 ignore the error. =1 retry the operation. =2 terminate the program through interrupt 22H. =3 fail the system call in progress.

NOTE: Be careful when choosing ignore as a response because this causes DOS 4.00 to believe that an operation has completed successfully when it may not have.

To return control from the critical error handler to a user error routine, the following should occur:

Before an INT 24H occurs:

1. The user application initialization code should save the INT 24H vector and replace the vector with one pointing to the user error routine.

When the INT 24H occurs:

2. When the user error routine receives control, it should push the flag register onto the stack, and then execute a CALL FAR to the original INT 24H vector saved in step 1.

3. DOS 4.00 gives the appropriate prompt, and waits for the user input (Abort, Retry, Fail or Ignore). After the user input, DOS 4.00 returns control to the user error routine at the instruction following the CALL FAR.

4. The user error routine can now do any tasks necessary. To return to the original application at the point the error occurred, the error routine needs to execute an IRET instruction. Otherwise, the user error routine should remove the IP, CS, and Flag registers from the stack. Control can then be passed to the desired point.

Disk Errors

If it is a hard error on disk (AH bit 7=0), register AL contains the failing drive number (0 = drive A, and so on). AH bits 0-2 indicate the affected disk area and whether it was a read or write operation, as follows:

Bit 0=0 if read operation, 1 if write operation Bits 2-1 (affected disk area) 0 0 DOS 4.00 area 0 1 file allocation table 1 0 directory 1 1 data area

AH bits 3-5 indicate which responses are valid. They are:

Bit 3=0 if FAIL is not allowed =1 if FAIL is allowed Bit 4=0 if RETRY is not allowed =1 if RETRY is allowed Bit 5=0 if IGNORE is not allowed =1 if IGNORE is allowed

Handling of Invalid Responses

If IGNORE is specified (AL=0) and IGNORE is not allowed (bit 5=0), make the response FAIL (AL=3).

If RETRY is specified (AL=1) and RETRY is not allowed (bit 4=0), make the response FAIL (AL=3).

If FAIL is specified (AL=3) and FAIL is not allowed (bit 3=0), make the response END (AL=2).

Other Errors

If AH bit 7=1, the error occurred on a character device, or was the result of a bad memory image of the FAT. The device header passed in BP:SI can be examined to determine which case exists. If the attribute byte high-order bit indicates a block device, then the error was a bad FAT. Otherwise, the error is on a character device.

If a character device is involved, the contents of AL are unpredictable and the error code is in DI as above.

NOTES:

1. Retry five times before giving this routine control for disk errors. When the errors are in the FAT or a directory, retry three times.

2. For disk errors, this exit is taken only for errors occurring during an interrupt 21H function call. It is not used for errors during an interrupt 25H or 26H.

3. This routine is entered in a disabled state.

4. All registers must be preserved.

5. This interrupt handler should refrain from using DOS 4.00 function calls. If necessary, it may use calls 01H through 0CH, 30H, and 59H. Use of any other call destroys the DOS 4.00 stack and leaves DOS 4.00 in an unpredictable state.

6. The interrupt handler must not change the contents of the device header.

7. If the interrupt handler handles errors itself rather than returning to DOS 4.00, it should restore the application program's registers from the stack, remove all but the last three words on the stack, then issue an IRET. This will return to the program immediately after the INT 21H that experienced the error. Note that if this is done, DOS 4.00 will be in an unstable state until a function call higher than 0CH is issued; therefore, it is not recommended.

The recommended way to end a critical error is to use FAIL and then test the extended error code of the INT 21H.

8. IGNORE requests (AL=0) are converted to FAIL for critical errors that occur on FAT or DIR sectors.

9. Refer to "Responding to Errors" on page B-5 and "Extended Error Codes" on page B-5 for information on how to obtain additional error information.

10. For DOS 4.00, IGNORE requests (AL=0) are converted to FAIL requests for certain critical errors (50-79).

The device header pointed to by BP:SI is formatted as follows:

+---------------------------------------------------------------------------+ | DWORD Pointer to next device (FFFFH if last device) | +---------------------------------------------------------------------------+ | WORD Attributes: | | Bit 15 = 1 if character device | | = 0 if block device | | If bit 15 is 1: | | Bit 0 = 1 if current standard input | | Bit 1 = 1 if current standard output | | Bit 2 = 1 if current NULL device | | Bit 3 = 1 if current CLOCK device | | Bit 14 is the IOCtl bit | +---------------------------------------------------------------------------+ | WORD pointer to device driver strategy entry point. | +---------------------------------------------------------------------------+ | WORD pointer to device driver interrupt entry point. | +---------------------------------------------------------------------------+ | 8-BYTE character device named field for block devices. The first byte is | | the number of units. | +---------------------------------------------------------------------------+

To tell if the error occurred on a block or character device, look at bit 15 in the attribute field (WORD at BP:SI+4).

If the name of the character device is desired, look at the eight bytes starting at BP:SI+10.

25H/26H ABSOLUTE DISK READ/WRITE

Interrupt vectors 25H and 26H transfer control to the device driver. They have been extended to allow direct access to media greater than 32MB in size. Their use of the CX register is what distinguishes them from the conventional 25H and 26H interrupts. Note that if the conventional format parameters are used in an attempt to access media greater than 32MB, an error code of 0207H is returned in AX.

The request for extended 25H or 26H is:

MOV AL,DRIVE  ; Drive number to process

0 = A
1 = B
2 = C . . .

LDS BX,PACKET  ; Parameter list MOV CX,-1  ; Indicates extended format INT 25H or 26H  ; Issue request to DOS 4.00 POP AX  ; Discard stack word JC ERROR  ; Error code returned in AX

PACKET LABEL BYTE  ; Control packet DD RBA  ; RBA of first sector

(0 origin)

DW COUNT  ; Number of sectors to I/O DD BUFFER  ; Data buffer

On return, the original flags are still on the stack (put there by the INT instruction). This is necessary because return information is passed back in the current flags. Be sure to pop the stack to prevent uncontrolled growth.

WARNING: IF DISK I/O HANDLED BY THIS INTERRUPT EXCEEDS THE LIMIT IMPOSED BY THE 64KB DIRECT MEMORY ACCESS BOUNDARY, UNPREDICTABLE RESULTS CAN OCCUR. WE RECOMMEND YOU CAREFULLY CHECK THE SECTOR SIZE AND THE NUMBER OF SECTORS TO BE READ OR WRITTEN TO BEFORE ISSUING THIS CALL.

The number of sectors specified is transferred between the given drive and the transfer address. Logical sector numbers (LSN) are obtained by numbering each sector sequentially starting from track 0, head 0, sector 1 (logical sector 0) and continuing along the same head, then to the next head until the last sector on the last head of the track is counted. Thus, logical sector 1 is track 0, head 0, sector 2; logical sector 2 is track 0, head 0, sector 3; and so on. Numbering continues with sector 1 on head 0 of the next track. Note that although the sectors are sequentially numbered (for example, sectors 2 and 3 on track 0 in the example above), they may not be physically adjacent on the disk, because of interleaving. Note that the mapping is different from that used by DOS version 1.10 for dual-sided diskettes.

All registers except the segment registers are destroyed by this call. If the transfer was successful, the carry flag (CF) is 0. If the transfer was not successful CF=1 and (AX) indicate the error as follows. (AL) is the DOS 4.00 error code that is the same as the error code returned in the low byte of DI when an interrupt 24H is issued, and (AH) contains:

80H Attachment failed to respond 40H SEEK operation failed 08H Bad CRC on diskette read 04H Requested sector not found 03H Write attempt on write-protected diskette 02H Error other than types listed above

WARNING: BEFORE ISSUING THIS INTERRUPT TO REMOVABLE MEDIA, THE MEDIA IN THE DRIVE MUST BE ESTABLISHED CORRECTLY. THIS CAN BE ACCOMPLISHED BY ISSUING EITHER AN INT 21H GENERIC IOCTL (AH=44H), WITH A REQUEST TO RETURN THE BPB THAT BUILD BPB RETURNS; OR AN INT 21H GET CURRENT DIRECTORY (AH=47H).

27H TERMINATE BUT STAY RESIDENT

This vector is used by programs that are to remain resident when COMMAND.COM regains control.

DOS 4.00 function call 31H is the preferred method to cause a program to remain resident, because this allows return information to be passed. It allows a program larger than 64KB to remain resident. After initializing itself, the program must set DX to its last address plus one, relative to the program's initial DS or ES value (the offset at which other programs can be loaded), and then execute an interrupt 27H. DOS 4.00 then considers the program as an extension of itself, so the program is not overlaid when other programs are executed. This concept is useful for loading programs such as user-written interrupt handlers that must remain resident.

NOTES:

1. This interrupt must not be used by .EXE programs that are loaded into the high end of memory.

2. This interrupt restores the interrupt 22H, 23H, and 24H vectors in the same manner as interrupt 20H. Therefore, it cannot be used to install permanently resident Ctrl-Break or critical error handler routines.

3. The maximum size of memory that can be made resident by this method is 64KB.

4. Memory can be used more efficiently if the block containing a copy of the environment is deallocated before terminating. This can be done by loading ES with the segment contained in 2C of the PSP, and issuing function call 49H (Free Allocated Memory).

5. DOS 4.00 function call 4CH allows the terminating program to pass a completion (or error) code to DOS 4.00, which can be interpreted within batch processing (see function call 31H).

6. Terminate-but-stay-resident function calls do not automatically close files.

28H-2EH RESERVED FOR DOS 4.00

These interrupts are reserved for DOS 4.00 use.

2FH MULTIPLEX INTERRUPT

Interrupt 2FH is the multiplex interrupt. A general interface is defined between two processes. The specific application using interrupt 2FH defines specific functions and parameters.

Every multiplex interrupt handler is assigned a specific multiplex number. The multiplex number is specified in the AH register. The specific function that the handler is to perform is specified in the AL register. Other parameters are placed in the other registers, as needed. The handlers are chained into the interrupt 2FH interrupt vector. There is no defined method for assigning a multiplex number to a handler. You must pick one. To avoid

a conflict if two applications choose the same multiplex number, the multiplex numbers used by an application should be patchable.

The multiplex numbers AH=00H through BFH are reserved for DOS 4.00. Applications should use multiplex numbers C0H through FFH.

NOTE: When in the chain for interrupt 2FH, if your code calls DOS 4.00 or if you execute with interrupts enabled, your code must be reentrant and recursive.

Function Codes

AH=1

AH=1 is the resident part of PRINT.

The following table contains the function codes that you can specify in AL to request the resident portion of print to perform a specific function:

+---------------------------------------------------------------------------+ | FUNCTION | | CODES DESCRIPTION | +---------------------------------------------------------------------------+ | 0 Get installed state | +---------------------------------------------------------------------------+ | 1 Submit file | +---------------------------------------------------------------------------+ | 2 Cancel file | +---------------------------------------------------------------------------+ | 3 Cancel all files | +---------------------------------------------------------------------------+ | 4 Status | +---------------------------------------------------------------------------+ | 5 End of status | +---------------------------------------------------------------------------+

Print Error Codes

The following table contains the error codes that are returned from the resident portion of print.

+---------------------------------------------------------------------------+ | ERROR | | CODES DESCRIPTION | +---------------------------------------------------------------------------+ | 1 Invalid function | +---------------------------------------------------------------------------+ | 2 File not found | +---------------------------------------------------------------------------+ | 3 Path not found | +---------------------------------------------------------------------------+ | 4 Too many open files | +---------------------------------------------------------------------------+ | 5 Access denied | +---------------------------------------------------------------------------+ | 6 Queue full | +---------------------------------------------------------------------------+ | 9 Busy | +---------------------------------------------------------------------------+ | 12 Name too long | +---------------------------------------------------------------------------+ | 15 Invalid drive | +---------------------------------------------------------------------------+

AL=0 Get Installed State

This call must be defined by all interrupt 2FH handlers. It is used by the caller of the handler to determine if the handler is present. On entry, AL=0, AH=1. On return, AL contains the installed state as follows:

AL=0 Not installed, permissible to install

AL=1 Not installed, not permissible to install

AL=FF Installed

AL=1 Submit File

On entry, AL=1, AH=1, and DS:DX points to the submit packet. A submit packet contains the level (BYTE) and a pointer to the ASCIIZ string (DWORD in offset segment form). The level value for DOS 4.00 is 0. The ASCIIZ string must contain the drive, path, and filename of the file you want to print. The filename cannot contain global filename characters. AL=2 Cancel File

On entry, AL=2, AH=1, and DS:DX points to the ASCIIZ string for the print file you want to cancel. Global filename characters are allowed in the filename. AL=3 Cancel all Files

On entry, AL=3 and AH=1.

AL=4 Status

This call holds the jobs in the print queue so that you can scan the queue. Issuing any other code releases the jobs. On entry, AL=4, AH=1. On return, DX contains the error count. The error count is the number of consecutive failures PRINT had while trying to output the last character. If there are no failures, the number is 0. DS:SI points to the print queue. The print queue consists of a series of filename entries. Each entry is 64 bytes long. The first entry in the queue is the file currently being printed. The end of the queue is marked by a queue entry having a null as the first character.

AL=5 End of Status: Issue this call to release the queue from call 4. On entry, AL=5 and AH=1. On return, AX contains the error codes. For information on the error codes returned, refer to "Print Error Codes" on page A-10.

AL=F8-FF Reserved by DOS 4.00

AH=6

AH=6 is the resident part of ASSIGN. The Get Installed State function (AL=0) is supported.

AH=8H, 10H, 12H, 13H Reserved by DOS 4.00

AH=B7H

AH=B7H is the resident part of APPEND. The Get Installed State function (AL=0) is supported.

AL=2 Get APPEND version This call is for distinguishing between the PC LAN APPEND and the DOS 4.00 APPEND. On return, if AX=FFFFH then the DOS 4.00 APPEND is loaded.

AL=4 Get APPEND Path Pointer (DOS 4.00 APPEND only) On return ES:DI points to the currently active APPEND path.

AL=6 Get APPEND Function State (DOS 4.00 APPEND only)

BX is returned with bits set indicating if APPEND is currently enabled and what functions are in effect.

+--------+------------------------------------------------------------------+ | BIT | FUNCTION IN EFFECT IF BIT IS ON | +--------+------------------------------------------------------------------+ | 0 | APPEND enabled | +--------+------------------------------------------------------------------+ | 13 | /PATH | +--------+------------------------------------------------------------------+ | 14 | /E | +--------+------------------------------------------------------------------+ | 15 | /X | +--------+------------------------------------------------------------------+

NOTE: The functions in effect do not change whether or not APPEND is disabled.

AL=7 Set function state (DOS 4.00 APPEND only)

On input BX is the new setting for all functions. The suggested procedure is to get the current function state, turn on or turn off the desired bits, then use this call to set the function state.

AL=11H Set Return Found Name State (DOS 4.00 APPEND only)

On request AL=17, a process system state flag is set. If this flag is set, then on the next ASCIIZ 3DH, 43H or 6CH function call within Interrupt 21H that APPEND processes, APPEND returns the name it finds to the application filename buffer. This name may be different from the one the application offered. The application must provide enough space for the found name. After APPEND has processed an

Interrupt 21H, it resets the Return Found Name state. An example of this process follows:

MOV AH,0B7H  ; Indicate APPEND MOV AL,0  ; Get installed state INT 2FH CMP AL,0  ; APPEND installed? JE NOT_INSTALLED

MOV AH,0B7H  ; Indicate APPEND MOV AL,2  ; Get APPEND version INT 2FH CMP AX,-1  ; DOS version? PC_LAN_APPEND  ; AX<> -1 means PC LAN JNE APPEND

The following functions are valid only if DOS 4.00 APPEND

MOV AH,0B7H  ; Indicate APPEND MOV AL,4  ; Get APPEND path pointer INT 2FH

ES
DI = address of APPEND path
(Buffer is 128 characters long)

MOV AH,0B7H  ; Indicate APPEND MOV AL,6  ; Get function state INT 2FH

BX = function state
8000H = /X is on
4000H = /E is on
2000H = /PATH is on
0001H = APPEND enabled
If off, similar to null
APPEND path
Set on by any non-status
occurrence of APPEND

MOV AH,0B7H  ; Indicate APPEND MOV AL,7  ; Set function state MOV BX,state  ; New state INT 2FH

MOV AH,0B7H  ; Indicate APPEND MOV AL,11H  ; Set Return Found

Name state

INT 2FH

Example 2FH Handler

MYNUM DB X ; X = The specific AH

multiplex number.

INT 2F NEXT DD  ? ; Chain location INT 2F:

ASSUME DS:NOTHING,ES:NOTHING,SS:NOTHING

CMP AH,MYNUM JE MINE JMP INT 2F NEXT  ; Chain to next

2FH Handler

MINE:

CMP AL,0F8H JB DO FUNC IRET  ; IRET on reserved

functions

DO FUNC:

OR AL,AL JNE NON INSTALL  ; Non Get Installed

State request

MOV AL,0FFH  ; Say I'm here IRET  ; All done

NON INSTALL: . . .

Installing the Handler

The following example contains the functions necessary to install a handler:

MOV AH,MYNUM XOR AL,AL INT 2FH  ; Ask if already

installed

OR AL,AL JZ OK INSTALL

BAD INSTALL:  ; Handler already installed .sp 3 OK INSTALL:  ; Install my

handler

MOV AL,2FH MOV AH,GET INTERRUPT VECTOR INT 21H  ; Get multiplex

vector

MOV WORD PTR INT 2F NEXT+2,ES MOV WORD PTR INT 2F NEXT,BX MOV DX,OFFSET INT 2F MOV AL,2FH MOV AH,SET INTERRUPT VECTOR INT 21H  ; Set multiplex

vector

. . .

30H-3FH RESERVED FOR DOS 4.00

These interrupts are reserved for DOS 4.00 use.

Appendix B

APPENDIX B.  DOS 4.00 FUNCTION CALLS 100
Using DOS 4.00 Function Calls 219
Program Code Fragments 230
.COM Programs 236
DOS 4.00 Registers 256
Responding to Errors 340
Extended Error Codes 355
00H --  Program Terminate 518
01H --  Console Input with Echo 546
02H --  Display Output 581
03H --  Auxiliary Input 603
04H --  Auxiliary Output 631
05H --  Printer Output 655
06H --  Direct Console I/O 672
07H --  Direct Console Input Without Echo 720
08H --  Console Input Without Echo 753
09H --  Display String 786
0AH --  Buffered Keyboard Input 815
0BH --  Check Standard Input Status 848
0CH --  Clear Keyboard Buffer and Invoke a Keyboard Function 868
0DH --  Disk Reset 885
0EH --  Select Disk 904
0FH --  Open File 937
10H --  Close File 985
11H --  Search for First Entry 1022
12H --  Search for Next Entry 1099
13H --  Delete File 1149
14H --  Sequential Read 1193
15H --  Sequential Write 1240
16H --  Create File 1286
17H --  Rename File 1330
19H --  Current Disk 1380
1AH --  Set Disk Transfer Address 1400
1BH --  Allocation Table Information 1425
1CH --  Allocation Table Information for Specific Device 1453
21H --  Random Read 1489
22H --  Random Write 1541
23H --  File Size 1599
24H --  Set Relative Record Field 1642
25H --  Set Interrupt Vector 1668
26H --  Create New Program Segment 1695
27H --  Random Block Read 1725
28H --  Random Block Write 1787
29H --  Parse Filename 1847
2AH --  Get Date 1918
2BH --  Set Date 1946
2CH --  Get Time 1979
2DH --  Set Time 2015
2EH --  Set/Reset Verify Switch 2051
2FH --  Get Disk Transfer Address (DTA) 2085
30H --  Get DOS Version Number 2108
31H --  Terminate Process and Remain Resident 2138
33H --  Get/Set System Value 2175
35H --  Get Interrupt Vector 2235
36H --  Get Disk Free Space 2262
38H --  Get or Set Country Dependent Information 2318
39H --  Create Subdirectory (MKDIR) 2431
3AH --  Remove Subdirectory (RMDIR) 2466
3BH --  Change the Current Directory (CHDIR) 2498
3CH --  Create a File (CREAT) 2530
3DH --  Open a File 2577
3EH --  Close a File Handle 2812
3FH --  Read from a File or Device 2840
40H --  Write to a File or Device 2886
41H --  Delete a File from a Specified Directory (UNLINK) 2938
42H --  Move File Read Write Pointer (LSEEK) 2971
43H --  Change File Mode (CHMOD) 3035
44H --  I/O Control for Devices 3092
45H --  Duplicate a File Handle (DUP) 3103
46H --  Force a Duplicate of a Handle (FORCDUP) 3137
47H --  Get Current Directory 3170
48H --  Allocate Memory 3202
49H --  Free Allocated Memory 3237
4AH --  Modify Allocated Memory Blocks (SETBLOCK) 3263
4BH --  Load or Execute a Program (EXEC) 3303
4CH --  Terminate a Process (EXIT) 3442
4DH --  Get Return Code of a Subprocess (WAIT) 3466
4EH --  Find First Matching File (FIND FIRST) 3494
4FH --  Find Next Matching File (FIND NEXT) 3567
54H --  Get Verify Setting 3606
56H --  Rename a File 3628
57H --  Get/Set File's Date and Time 3669
59H --  Get Extended Error 3714
5AH --  Create Unique File 3785
5BH --  Create New File 3839
5CH --  Lock/Unlock File Access 3878
5E00H --  Get Machine Name 3972
5E02H --  Set Printer Setup 4006
5E03H --  Get Printer Setup 4051
5F02H --  Get Redirection List Entry 4096
5F03H --  Redirect Device 4169
5F04H --  Cancel Redirection 4259
62H --  Get Program Segment Prefix Address 4305
65H --  Get Extended Country Information 4328
66H --  Get/Set Global Code Page 4451
67H --  Set Handle Count 4494
68H --  Commit File 4539
6CH --  Extended Open/Create 4562
  
APPENDIX B.  DOS 4.00 FUNCTION CALLS                                            
                                                                                
NUMBER      FUNCTION NAME                                                       
00H         Program terminate                                                   
01H         Console input with echo                                             
02H         Display output                                                      
03H         Auxiliary input                                                     
04H         Auxiliary output                                                    
05H         Printer output                                                      
06H         Direct console I/O                                                  
07H         Direct console input without echo                                   
08H         Console input without echo                                          
09H         Display string                                                      
0AH         Buffered keyboard input                                             
0BH         Check standard input status                                         
0CH         Clear keyboard buffer, invoke a keyboard function                   
0DH         Disk reset                                                          
0EH         Select disk                                                         
0FH         Open file                                                           
10H         Close file                                                          
11H         Search for first entry                                              
12H         Search for next entry                                               
13H         Delete file                                                         
14H         Sequential read                                                     
15H         Sequential write                                                    
16H         Create file                                                         
17H         Rename file                                                         
18H         Reserved by DOS 4.00                                                
19H         Current disk                                                        
1AH         Set disk transfer address                                           
1BH         Allocation table information                                        
1CH         Allocation table information for specific device                    
1DH         Reserved by DOS 4.00                                                
1EH         Reserved by DOS 4.00                                                
1FH         Reserved by DOS 4.00                                                
20H         Reserved by DOS 4.00                                                
21H         Random read                                                         
22H         Random write                                                        
23H         File size                                                           
24H         Set relative record field                                           
25H         Set interrupt vector                                                
26H         Create new program segment                                          
27H         Random block read                                                   
28H         Random block write                                                  
29H         Parse filename                                                      
2AH         Get date                                                            
2BH         Set date                                                            
2CH         Get time                                                            
2DH         Set time                                                            
2EH         Set/reset verify switch                                             
2FH         Get disk transfer address                                           
30H         Get DOS 4.00 version number                                         
31H         Terminate process and remain resident                               
                                                                                
32H         Reserved by DOS 4.00                                                
33H         Get/Set system value                                                
34H         Reserved by DOS 4.00                                                
35H         Get interrupt vector                                                
36H         Get disk free space                                                 
37H         Reserved by DOS 4.00                                                
38H         Get/set country dependent information                               
39H         Create subdirectory (MKDIR)                                         
3AH         Remove subdirectory (RMDIR)                                         
3BH         Change current directory (CHDIR)                                    
3CH         Create a file (CREAT)                                               
3DH         Open a file                                                         
3EH         Close a file handle                                                 
3FH         Read from a file or device                                          
40H         Write to a file or device                                           
41H         Delete a file from a specified directory (UNLINK)                   
42H         Move file read/write pointer  (LSEEK)                               
43H         Change file mode (CHMOD)                                            
44H         I/O control for devices (IOCtl)                                     
45H         Duplicate a file handle (DUP)                                       
46H         Force a duplicate of a file handle (FORCDUP)                        
47H         Get current directory                                               
48H         Allocate memory                                                     
49H         Free allocated memory                                               
4AH         Modify allocated memory blocks (SETBLOCK)                           
4BH         Load or execute a program (EXEC)                                    
4CH         Terminate a process (EXIT)                                          
4DH         Get return code of a subprocess (WAIT)                              
4EH         Find first matching file (FIND FIRST)                               
4FH         Find next matching file (FIND NEXT)                                 
50H         Reserved by DOS 4.00                                                
51H         Reserved by DOS 4.00                                                
52H         Reserved by DOS 4.00                                                
53H         Reserved by DOS 4.00                                                
54H         Get verify setting                                                  
55H         Reserved by DOS 4.00                                                
56H         Rename a file                                                       
57H         Get/set a file's date and time                                      
58H         Reserved for DOS 4.00                                               
59H         Get extended error                                                  
5AH         Create unique file                                                  
5BH         Create new file                                                     
5CH         Lock/unlock file access                                             
5DH         Reserved by DOS 4.00                                                
5E00H       Get machine name                                                    
5E02H       Set printer setup                                                   
5E03H       Get printer setup                                                   
5F02H       Get redirection list entry                                          
5F03H       Redirect device                                                     
5F04H       Cancel redirection                                                  
60H         Reserved by DOS 4.00                                                
61H         Reserved by DOS 4.00                                                
62H         Get PSP address                                                     
63H         Reserved by DOS 4.00                                                
64H         Reserved by DOS 4.00                                                
65H         Get extended country information                                    
66H         Get/set global code page                                            
                                                                                
67H         Set handle count                                                    
68H         Commit file                                                         
69H         Reserved by DOS 4.00                                                
6AH         Reserved                                                            
6BH         Reserved by DOS 4.00                                                
6CH         Extended open/create                                                
                                                                                
USING DOS 4.00 FUNCTION CALLS                                                   
                                                                                
Most function calls require input to be passed to them in registers. After      
setting the appropriate register values, issue the function calls in either     
of the following ways:                                                          
                                                                                
    The preferred method is to place the function number in AH and issue        
interrupt 21H.                                                                  
    Place the function number in AH and execute a call to offset 50H in your    
program segment prefix.                                                         
                                                                                
PROGRAM CODE FRAGMENTS                                                          
                                                                                
In each of the function call descriptions in this chapter, the input, output    
and method of use are described using a small program code fragment.  These     
fragments are written in IBM PC Assembler Language.                             
                                                                                
.COM PROGRAMS                                                                   
                                                                                
The descriptions assume that the program is an .EXE, not a .COM program.  If    
a .COM program is desired, do not include either of the following               
instructions:                                                                   
                                                                                
MOV ES,SEG --                                                                   
or                                                                              
MOV DS,SEG --                                                                   
                                                                                
NOTES:                                                                          
                                                                                
1.  Some FCB function calls do not permit invalid characters (0DH-29H).         
                                                                                
2.  Device names cannot end in a colon.                                         
                                                                                
3.  The contents of the AX register can be altered by any of the function       
calls.  Even though no error code is returned in AX, it is possible that        
AX has been changed.                                                            
                                                                                
DOS 4.00 REGISTERS                                                              
                                                                                
DOS 4.00 uses the following registers, pointers, and flags when executing       
interrupts and function calls:                                                  
                                                                                
+---------------------------------------------------------------------------+   
| REGISTER    GENERAL                                                       |   
| DEFINITION  REGISTERS                                                     |   
+---------------------------------------------------------------------------+   
| AX          Accumulator (16-bit)                                          |   
| AH          Accumulator high-order byte (8-bit)                           |   
| AL          Accumulator low-order byte (8-bit)                            |   
+---------------------------------------------------------------------------+   
| BX          Base (16-bit)                                                 |   
| BH          Base high-order byte (8-bit)                                  |   
| BL          Base low-order byte (8-bit)                                   |   
+---------------------------------------------------------------------------+   
| CX          Count (16-bit)                                                |   
| CH          Count high-order byte (8-bit)                                 |   
| CL          Count low-order byte (8-bit)                                  |   
+---------------------------------------------------------------------------+   
| DX          Data (16-bit)                                                 |   
| DH          Data high-order (8-bit)                                       |   
| DL          Data low-order (8-bit)                                        |   
+---------------------------------------------------------------------------+   
| Flags       OF,DF,IF,TF,SF,ZF,AF,PF,CF                                    |   
+---------------------------------------------------------------------------+   
                                                                                
+---------------------------------------------------------------------------+   
| REGISTER                                                                  |   
| DEFINITION  POINTERS                                                      |   
+---------------------------------------------------------------------------+   
| SP          Stack pointer (16-bit)                                        |   
+---------------------------------------------------------------------------+   
| BP          Base pointer (16-bit)                                         |   
+---------------------------------------------------------------------------+   
| IP          Instruction pointer (16-bit)                                  |   
+---------------------------------------------------------------------------+   
                                                                                
+---------------------------------------------------------------------------+   
| REGISTER    SEGMENT                                                       |   
| DEFINITION  REGISTERS                                                     |   
+---------------------------------------------------------------------------+   
| CS          Code segment (16-bit)                                         |   
+---------------------------------------------------------------------------+   
| DS          Data segment (16-bit)                                         |   
+---------------------------------------------------------------------------+   
| SS          Stack segment (16-bit)                                        |   
+---------------------------------------------------------------------------+   
| ES          Extra segment (16-bit)                                        |   
+---------------------------------------------------------------------------+   
                                                                                
+---------------------------------------------------------------------------+   
| REGISTER    INDEX                                                         |   
| DEFINITION  REGISTERS                                                     |   
+---------------------------------------------------------------------------+   
| DI          Destination index (16-bit)                                    |   
+---------------------------------------------------------------------------+   
| SI          Source index (16-bit)                                         |   
+---------------------------------------------------------------------------+   
                                                                                
Register Numbering Convention                                                   
                                                                                
Each register is 16 bits long and is divided into a high and low byte.  Each    
byte is 8 bits long.  The bits are numbered from right to left.  The low byte   
contains bits 0 through 7 and the high byte contains bits 8 through 15.  The    
chart below shows the hexadecimal values assigned to each bit.                  
                                                                                
+--------------------------------------------------------------------------+    
|             High Byte                      Low Byte                      |    
+--------------------------------------------------------------------------+    
| Bit         15 14 13 12 11 10 9 8           7  6  5  4  3  2  1 0        |    
+--------------------------------------------------------------------------+    
| Hex value    8   4   2  1   8  4  2 1       8  4  2  1  8  4  2 1        |    
+--------------------------------------------------------------------------+    
                                                                                
DOS 4.00 Internal Stack                                                         
                                                                                
When DOS 4.00 gains control, it switches to an internal stack.  User            
registers are preserved unless information is passed back to the requester as   
indicated in the specific requests.  The user stack needs to be sufficient to   
accommodate the interrupt system.  It is recommended that the user stack be     
200H in addition to what the user needs.                                        
                                                                                
RESPONDING TO ERRORS                                                            
                                                                                
Handle function calls report an error by setting the carry flag and returning   
the error code in AX.  FCB function calls report an error by returning FFH in   
AL.                                                                             
                                                                                
Extended error support (59H) provides a common set of error codes and           
specific error information such as error classification, location, and          
recommended action.  In most critical cases, applications can analyze the       
error code and take specific action.  Recommended actions are intended for      
programs that do not understand the error codes.  Programs can take advantage   
of extended error support both from interrupt 24H critical error handlers and   
after issuing interrupt 21H function calls.  Do not code to specific error      
codes.                                                                          
                                                                                
EXTENDED ERROR CODES                                                            
                                                                                
HEXADECIMAL   DECIMAL                                                           
CODE          CODE        MEANING                                               
01H           1           Invalid function number                               
02H           2           File not found                                        
03H           3           Path not found                                        
04H           4           Too many open files (no handles left)                 
05H           5           Access denied                                         
06H           6           Invalid handle                                        
07H           7           Memory control blocks destroyed                       
08H           8           Insufficient memory                                   
09H           9           Invalid memory block address                          
0AH           10          Invalid environment                                   
0BH           11          Invalid format                                        
0CH           12          Invalid access code                                   
0DH           13          Invalid data                                          
0EH           14          Reserved                                              
0FH           15          Invalid drive was specified                           
10H           16          Attempt to remove the current directory               
11H           17          Not same device                                       
12H           18          No more files                                         
13H           19          Attempt to write on write-protected diskette          
14H           20          Unknown unit                                          
15H           21          Drive not ready                                       
16H           22          Unknown command                                       
17H           23          Cyclic redundancy check (CRC) -- part of diskette     
is bad                                                                          
18H           24          Bad request structure length                          
19H           25          Seek error                                            
1AH           26          Unknown media type                                    
1BH           27          Sector not found                                      
1CH           28          Printer out of paper                                  
1DH           29          Write fault                                           
1EH           30          Read fault                                            
1FH           31          General failure                                       
20H           32          Sharing violation                                     
21H           33          Lock violation                                        
22H           34          Invalid disk change                                   
23H           35          FCB unavailable                                       
24H           36          Sharing buffer overflow                               
25H           37          Reserved by DOS 4.00                                  
26H           38          Unable to complete file operation                     
27H-31H       39-49       Reserved by DOS 4.00                                  
50H           80          File exists                                           
51H           81          Reserved                                              
52H           82          Cannot make directory entry                           
53H           83          Fail on INT 24                                        
54H           84          Too many redirections                                 
55H           85          Duplicate redirection                                 
56H           86          Invalid password                                      
57H           87          Invalid parameter                                     
58H           88          Network data fault                                    
59H           89          Function not supported by network                     
                                                                                
HEXADECIMAL   DECIMAL                                                           
CODE          CODE        MEANING                                               
5AH           90          Required system component not installed               
                                                                                
Extended Error Codes                                                            
                                                                                
HEXADECIMAL   DECIMAL                                                           
CODE          CODE        MEANING                                               
32H           50          Network request not supported                         
33H           51          Remote computer not listening                         
34H           52          Duplicate name on network                             
35H           53          Network path not found                                
36H           54          Network busy                                          
37H           55          Network device no longer exists                       
38H           56          NETBIOS command limit exceeded                        
39H           57          System error; NETBIOS error                           
3AH           58          Incorrect response from network                       
3BH           59          Unexpected network error                              
3CH           60          Incompatible remote adapter                           
3DH           61          Print queue full                                      
3EH           62          Not enough space for print file                       
3FH           63          Print file was cancelled                              
40H           64          Network name was deleted                              
41H           65          Access denied                                         
42H           66          Network device type incorrect                         
43H           67          Network name not found                                
44H           68          Network name limit exceeded                           
45H           69          NETBIOS session limit exceeded                        
46H           70          Sharing temporarily paused                            
47H           71          Network request not accepted                          
48H           72          Print or disk redirection is paused                   
49H-4FH       73-79       Reserved                                              
                                                                                
Error Classes                                                                   
                                                                                
HEXADECIMAL   DECIMAL                                                           
VALUE         VALUE       DESCRIPTION                                           
01H           1           OUT OF RESOURCE: Example:  out of space or            
channels.                                                                       
02H           2           TEMPORARY SITUATION: Something expected to            
disappear with time.  This is not an error                                      
condition, but a temporary situation such as a                                  
locked file.                                                                    
03H           3           AUTHORIZATION: Permission problem.                    
04H           4           INTERNAL: Internal error in system software.          
Probable problem with system software rather than a                             
user or system failure.                                                         
                                                                                
HEXADECIMAL   DECIMAL                                                           
VALUE         VALUE       DESCRIPTION                                           
05H           5           HARDWARE FAILURE: A serious problem not the fault     
of user program.                                                                
06H           6           SYSTEM FAILURE: Serious failure of system software    
not the fault of the user, such as missing or                                   
incorrect configuration files.                                                  
07H           7           APPLICATION PROGRAM ERROR: Inconsistent requests.     
08H           8           NOT FOUND: File or item not found.  Inconsistent      
requests.                                                                       
09H           9           BAD FORMAT: File or value in invalid format or        
type; unsuitable.                                                               
0AH           10          LOCKED: Locked file or item.                          
0BH           11          MEDIA: Media failure such as incorrect disk, CRC      
error, or defective media.                                                      
0CH           12          ALREADY EXISTS: Duplication error, such as            
declaring a machine name that already exists.                                   
0DH           13          UNKNOWN: Classification does not exist or is          
inappropriate.                                                                  
                                                                                
Actions                                                                         
                                                                                
HEXADECIMAL   DECIMAL                                                           
CODE          CODE        DESCRIPTION                                           
01H           1           RETRY:  Retry a few times, then prompt user to        
determine if the program should continue or be                                  
terminated.                                                                     
02H           2           DELAY RETRY: Retry several times after pause, then    
prompt user to determine if the program should                                  
continue or be terminated.                                                      
03H           3           USER: If the input was entered by a user, advise      
reentry.  The error, however, may have occurred in                              
the program itself, such as a bad drive letter or                               
bad filename specification.                                                     
04H           4           ABORT: Abort application with cleanup.  The           
application cannot proceed, but the system is in an                             
orderly state and it is safe to stop the                                        
application.                                                                    
05H           5           IMMEDIATE EXIT: Stop application immediately          
without clearing registers.  Do not use the                                     
application to close files or update indexes, but                               
exit as soon as possible.                                                       
06H           6           IGNORE: Ignore.                                       
07H           7           RETRY AFTER USER INTERVENTION: The user needs to      
perform some action such as removing a diskette and                             
inserting a different one.  Then retry the                                      
operation.                                                                      
                                                                                
Locus (Location)                                                                
                                                                                
HEXADECIMAL   DECIMAL                                                           
VALUE         VALUE       DESCRIPTION                                           
01H           1           UNKNOWN: Not specific; not appropriate.               
02H           2           BLOCK DEVICE: Related to random access mass disk      
storage.                                                                        
03H           3           NET: Related to the network.                          
04H           4           SERIAL DEVICE: Related to serial devices.             
05H           5           MEMORY: Related to random access memory.              
                                                                                
00H --                                                                          
PROGRAM TERMINATE                                                               
                                                                                
PURPOSE                                                                         
                                                                                
Stops the execution of a program.                                               
                                                                                
EXAMPLE                                                                         
                                                                                
MOV     AH,00H   ; Function Call -- Terminate Program                           
INT     21H      ; Issue request to DOS                                         
; No return                                                                     
                                                                                
COMMENTS                                                                        
                                                                                
The terminate, Ctrl-Break, and critical error exit addresses are restored to    
the values they had on entry to the terminating program, from the values        
saved in the program segment prefix.  All file buffers are flushed and the      
handles opened by the process are closed.  Any files that have changed in       
length and not closed are not recorded properly in the directory.  Control      
transfers to the terminate address.  This call performs exactly the same        
function as interrupt 20H.  It is the program's responsibility to ensure that   
the CS register contains the segment address of its program segment prefix      
control block before calling this function.                                     
                                                                                
Function 4CH -- Terminate a Process is the preferred method for ending a        
program.                                                                        
                                                                                
01H --                                                                          
CONSOLE INPUT WITH ECHO                                                         
                                                                                
PURPOSE                                                                         
                                                                                
Waits for a character to be read at the standard input device (unless one is    
ready), then echoes the character to the standard output device and returns     
the character in AL.                                                            
                                                                                
EXAMPLE                                                                         
                                                                                
MOV     AH,01H          ; Function Call -- keyboard input                       
INT     21H             ; Issue request to DOS                                  
MOV     Char,AL         ; Save character                                        
CMP     AL,0            ; Extended character ?                                  
JNE     Normal Char     ; No!                                                   
MOV     AH,01H          ; Function Call -- keyboard input                       
INT     21H             ; Issue request to DOS                                  
MOV     ExtChar,AL      ; Save extended character                               
Normal Char:                                                                    
                                                                                
                                                                                
Character     LABEL   WORD    ; Complete character                              
Char          DB      ?       ; Character buffer                                
ExtChar       DB      ?       ; Character buffer                                
                                                                                
COMMENTS                                                                        
                                                                                
The character is checked for a Ctrl-Break.  If Ctrl-Break is detected, an       
interrupt 23H is executed.                                                      
                                                                                
For function call 01H, extended ASCII codes require two function calls.  The    
first call returns 00H as an indicator that the next call will return an        
extended code.                                                                  
                                                                                
02H --                                                                          
DISPLAY OUTPUT                                                                  
                                                                                
PURPOSE                                                                         
                                                                                
Outputs the character in DL to the standard output device.                      
                                                                                
EXAMPLE                                                                         
                                                                                
MOV     AH,02H           ; Function Call -- Display Output                      
MOV     DL,Char          ; Get character to display                             
INT     21H              ; Issue request to DOS                                 
                                                                                
                                                                                
CHAR        DB         ?     ; Character buffer                                 
                                                                                
COMMENTS                                                                        
                                                                                
If the character in DL is a backspace (08), the cursor is moved left one        
position (nondestructive).  If a Ctrl-Break is detected after the output, an    
interrupt 23H is executed.                                                      
                                                                                
03H --                                                                          
AUXILIARY INPUT                                                                 
                                                                                
PURPOSE                                                                         
                                                                                
Waits for a character from the standard auxiliary device, then returns that     
character in AL.                                                                
                                                                                
EXAMPLE                                                                         
                                                                                
MOV     AH,03H          ; Function Call -- Auxiliary Input                      
INT     21H             ; Issue request to DOS                                  
MOV     Char,AL         ; Save character                                        
                                                                                
                                                                                
CHAR    DB      ?               ; Character buffer                              
                                                                                
COMMENTS                                                                        
                                                                                
Auxiliary (AUX) support is unbuffered and noninterrupt driven.                  
                                                                                
At startup, DOS 4.00 initializes the first auxiliary port to 2400 baud, no      
parity, one-stop bit, and 8-bit word.                                           
                                                                                
The auxiliary function calls (03H and 04H) do not return status or error        
codes.  For greater control, you should use the ROM BIOS routine (interrupt     
14H) or write an AUX device driver and use IOCtl.                               
                                                                                
04H --                                                                          
AUXILIARY OUTPUT                                                                
                                                                                
PURPOSE                                                                         
                                                                                
Outputs the character in DL to the standard auxiliary device.                   
                                                                                
EXAMPLE                                                                         
                                                                                
MOV     AH,04H          ; Function Call -- Auxiliary Output                     
                                                                                
MOV     DL,Char         ; Get character to output                               
INT     21H             ; Issue request to DOS                                  
; Nothing returned                                                              
                                                                                
                                                                                
CHAR    DB      ?              ; Character buffer                               
                                                                                
COMMENTS                                                                        
                                                                                
If the character in DL is a backspace (08), the cursor is moved left one        
position (nondestructive).  If a Ctrl-Break is detected after the output, an    
interrupt 23H is executed.                                                      
                                                                                
05H --                                                                          
PRINTER OUTPUT                                                                  
                                                                                
PURPOSE                                                                         
                                                                                
Outputs the character in DL to the standard printer device.                     
                                                                                
EXAMPLE                                                                         
                                                                                
MOV     AH,05H          ; Function Call -- Printer Output                       
MOV     DL,Char         ; Get character to output                               
INT     21H             ; Issue request to DOS                                  
; Nothing returned                                                              
                                                                                
                                                                                
CHAR    DB       ?              ; Character buffer                              
                                                                                
06H --                                                                          
DIRECT CONSOLE I/O                                                              
                                                                                
PURPOSE                                                                         
                                                                                
Gets a character from the standard input device if one is ready, or outputs a   
character to the standard output device.                                        
                                                                                
EXAMPLES                                                                        
                                                                                
In loop:                                                                        
MOV     AH,06H         ; Function Call -- Direct Console I/O                    
MOV     DL,-1          ; 0FFH for input                                         
INT     21H            ; Issue request to DOS                                   
JZ      In loop        ; No character yet on input                              
MOV     Char,AL        ; Save character                                         
CMP     AL,0           ; Extended Character ?                                   
JNE     Normal Char    ; No!                                                    
MOV     AH,07H         ; Function Call -- Keyboard Input                        
INT     21H            ; Issue request to DOS                                   
MOV     ExtChar,AL     ; Save extended character                                
Normal Char:                                                                    
                                                                                
or                                                                              
                                                                                
MOV     AH,06H         ; Function Call -- Direct Console I/O                    
MOV     DL,Char        ; Get character to display (not 0FFH)                    
INT     21H            ; Issue request to DOS                                   
                                                                                
                                                                                
Character    LABEL  WORD ; Complete character                                   
Char          DB      ?  ; Character buffer                                     
ExtChar       DB      ?  ; Character buffer                                     
                                                                                
COMMENTS                                                                        
                                                                                
If DL is FFH, AL returns with the 0 flag clear and an input character from      
the standard input device, if one is ready.  If a character is not ready, the   
0 flag will be set.                                                             
                                                                                
If DL is not FFH, DL is assumed to have a valid character that is output to     
the standard output device. This function does not check for Ctrl-Break, or     
Ctrl-PrtSc.                                                                     
                                                                                
For function call 06H, extended ASCII codes require two function calls.  The    
first call returns 00H as an indicator that the next call will return an        
extended code.                                                                  
                                                                                
07H --                                                                          
DIRECT CONSOLE INPUT WITHOUT ECHO                                               
                                                                                
PURPOSE                                                                         
                                                                                
Waits for a character to be read at the standard input device (unless one is    
ready), then returns the character in AL.                                       
                                                                                
EXAMPLE                                                                         
                                                                                
MOV   AH,07H       ; Function Call -- Direct Console Input (no echo)            
INT   21H          ; Issue request to DOS                                       
MOV   Char,AL      ; Save character                                             
CMP   AL,0         ; Extended character ?                                       
JNE   Normal Char  ; No!                                                        
MOV   AH,07H       ; Function Call -- Direct Console Input (no echo)            
INT   21H          ; Issue request to DOS                                       
MOV   ExtChar,AL   ; Save extended character                                    
Normal Char:                                                                    
                                                                                
                                                                                
Character     LABEL   WORD    ; Complete character                              
Char          DB      ?       ; Character buffer                                
ExtChar       DB      ?       ; Character buffer                                
                                                                                
COMMENTS                                                                        
                                                                                
As with function call 06H, no checks are made on the character.                 
                                                                                
For function call 07H, extended ASCII codes require two function calls.  The    
first call returns 00H as an indicator that the next call will return an        
extended code.                                                                  
                                                                                
08H --                                                                          
CONSOLE INPUT WITHOUT ECHO                                                      
                                                                                
PURPOSE                                                                         
                                                                                
Waits for a character to be read at the standard input device (unless one is    
ready) and returns the character in AL.                                         
                                                                                
EXAMPLE                                                                         
                                                                                
MOV  AH,08H     ; Function Call -- Console Input (no echo)                      
INT  21H        ; Issue request to DOS                                          
MOV  Char,AL    ; Save character                                                
CMP  AL,0       ; Extended character ?                                          
JNE  Normal Char; No!                                                           
MOV  AH,08H     ; Function Call -- Console Input (no echo)                      
INT  21H        ; Issue request to DOS                                          
MOV  ExtChar,AL ; Save extended character                                       
Normal Char:                                                                    
                                                                                
Character     LABEL   WORD    ; Complete character                              
Char          DB      ?       ; Character buffer                                
ExtChar       DB      ?       ; Character buffer                                
                                                                                
COMMENTS                                                                        
                                                                                
The character is checked for Ctrl-Break.  If Ctrl-Break is detected, an         
interrupt 23H is executed.                                                      
                                                                                
For function call 08H, extended ASCII codes require two function calls.  The    
first call returns 00H as an indicator that the next call will return an        
extended code.                                                                  
                                                                                
09H --                                                                          
DISPLAY STRING                                                                  
                                                                                
PURPOSE                                                                         
                                                                                
Sends the characters in the string to the standard output device.               
                                                                                
EXAMPLE                                                                         
                                                                                
MOV  AX,SEG String                                                              
MOV  DS,AX             ;Set DS:DX to string                                     
MOV  DX,OFFSET String                                                           
MOV  AH,09H            ;Function Call - Display String                          
INT  21H               ;Issue request to DOS                                    
                                                                                
                                                                                
String        DB      "This string ends at the first Dollar"                    
DB      0DH,0AH                                                                 
DB      "$"                                                                     
                                                                                
COMMENTS                                                                        
                                                                                
The character string in memory must be terminated by a $ (24H).  Each           
character in the string is output to the standard output device in the same     
form as function call 02H.                                                      
                                                                                
ASCII codes 0DH and 0AH represent carriage return and line feed,                
respectively.                                                                   
                                                                                
0AH --                                                                          
BUFFERED KEYBOARD INPUT                                                         
                                                                                
PURPOSE                                                                         
                                                                                
Reads characters from the standard input device and places them in the buffer   
beginning at the third byte.                                                    
                                                                                
EXAMPLE                                                                         
                                                                                
MOV  AX,SEG Buffer                                                              
MOV  DS,AX               ;Set DS:DX to return Buffer                            
MOV  DX,OFFSET Buffer                                                           
MOV  AH,0AH              ;Function Call-Buffered                                
;Keyboard Input                                                                 
INT 21H                  ;Issue request to DOS                                  
                                                                                
                                                                                
Buffer   DB   128        ; Max length of input                                  
CurLen   DB   ?          ; Number of characters input                           
; (excludes Return (0DH))                                                       
CurText  DB   128 DUP(?) ; Up to 128 characters allowed                         
                                                                                
COMMENTS                                                                        
                                                                                
The first byte of the input buffer specifies the number of characters the       
buffer can hold.  This value cannot be 0.  Reading the standard input device    
and filling the buffer continues until Enter is read.  If the buffer fills to   
one less than the maximum number of characters it can hold, each additional     
character read is ignored and causes the bell to ring, until Enter is read.     
The second byte of the buffer is set to the number of characters received,      
excluding the carriage return (0DH), which is always the last character.        
                                                                                
0BH --                                                                          
CHECK STANDARD INPUT STATUS                                                     
                                                                                
PURPOSE                                                                         
                                                                                
Checks if there is a character available from the standard input device.        
                                                                                
EXAMPLE                                                                         
                                                                                
In LOOP:                                                                        
MOV     AH,0BH    ; Function Call -- Check Input                                
INT     21H       ; Issue request to DOS                                        
CMP     AL,-1     ; 0FFH indicates character available                          
JNE     In LOOP:                                                                
                                                                                
COMMENTS                                                                        
                                                                                
If a character is available from the STDIN device, AL is FFH.  Otherwise,  AL   
is undefined.  If a Ctrl-Break is detected, an interrupt 23H is executed.       
                                                                                
0CH --                                                                          
CLEAR KEYBOARD BUFFER AND INVOKE A KEYBOARD FUNCTION                            
                                                                                
PURPOSE                                                                         
                                                                                
Clears the standard input device of any characters, then executes the           
function call number in AL.                                                     
                                                                                
EXAMPLE                                                                         
                                                                                
MOV    AH,0CH        ; Function Call -- Clear keyboard &                        
; Invoke function                                                               
MOV    AL,Function   ; Function Call to execute                                 
; (only 01H, 06H, 07H, 08H, and 0AH are allowed).                               
INT    21H           ; Issue request to DOS                                     
; Output depends on Function Call selected                                      
                                                                                
0DH --                                                                          
DISK RESET                                                                      
                                                                                
PURPOSE                                                                         
                                                                                
Writes to the disk file buffers that have been modified.  All buffers are       
then made available for reuse.                                                  
                                                                                
EXAMPLE                                                                         
                                                                                
MOV     AH,0DH          ; Function Call -- Disk Reset                           
INT     21H             ; Issue request to DOS                                  
; No return                                                                     
                                                                                
COMMENTS                                                                        
                                                                                
It is necessary to close or commit all open files to correctly update the       
disk directory.                                                                 
                                                                                
0EH --                                                                          
SELECT DISK                                                                     
                                                                                
PURPOSE                                                                         
                                                                                
Selects the drive specified in DL (0=A, 1=B, etc.)  (if valid) as the default   
drive.                                                                          
                                                                                
EXAMPLE                                                                         
                                                                                
MOV     AH,0EH          ; Function Call -- Select Disk                          
MOV     DL,Drive        ; Drive to select       (0=A:, 1=B:, ...)               
INT     21H             ; Issue request to DOS                                  
MOV     LastDrive,AL    : Save max drive number (1=A:, 2=B:, ...)               
MOV     AH,19H          ; Function Call -- Get Current Disk                     
INT     21H             ; Issue request to DOS                                  
CMP     AL,DL           ; Selected drive = requested                            
JNE     Error           ; No, Error!                                            
                                                                                
                                                                                
Drive         DB        ; New Drive to select                                   
LastDrive     DB        ; Highest Valid Drive                                   
                                                                                
COMMENTS                                                                        
                                                                                
The total number of unique drive letters, including diskette and fixed disk     
drives, that can be referenced is returned in AL.  The value in AL is equal     
to the value of LASTDRIVE in CONFIG.SYS or the total number of installed        
devices, whichever is greater.  For DOS 4.00 5 is the minimum value returned    
in AL.  If the system has only one diskette drive, it is counted as two to be   
consistent with the philosophy of thinking of the system as having logical      
drives A and B.                                                                 
                                                                                
0FH --                                                                          
OPEN FILE                                                                       
                                                                                
PURPOSE                                                                         
                                                                                
Searches the current directory for the named file and AL returns FFH if it is   
not found.  If the named file is found, AL returns 00H and the FCB is filled    
as described below.                                                             
                                                                                
EXAMPLE                                                                         
                                                                                
MOV  AX,SEG FCB    ;Address FCB Parameter Block                                 
MOV  DS,AX                                                                      
MOV  DX,OFFSET FCB                                                              
MOV  AH,0FH        ;Function Call-FCB Open                                      
INT  21H           ;Issue request to DOS                                        
                                                                                
                                                                                
FCB           LABEL   BYTE                                                      
Drive         DB      0               ; Drive (0=Current, 1=A, 2=B, ...)        
FName         DB      "FILENAME"      ; File Name      (blank padded)           
Ext           DB      "EXT"           ; File Extension (blank padded)           
DB      25 DUP(0)       ; Filled in by DOS 4.00                                 
                                                                                
COMMENTS                                                                        
                                                                                
AL is 00H if the file is opened.                                                
                                                                                
AL is FFH if the file was not opened.                                           
                                                                                
Use Function Call 59H (Get Extended Error) to determine the actual error        
condition.                                                                      
                                                                                
If the drive code was 0 (default drive), it is changed to the actual drive      
used (1=A, 2=B, and so on).  This allows changing the default drive without     
interfering with subsequent operations on this file.  The current block field   
(FCB bytes C-D) is set to 0.  The size of the record to be worked with (FCB     
bytes E-F) is set to the system default of 80H.  The size of the file and the   
date are set in the FCB from information obtained from the directory.  You      
can change the default value for the                                            
                                                                                
record size (FCB bytes E-F) or set the random record size and/or current        
record field.  Perform these actions after the open, but before any disk        
operations.                                                                     
                                                                                
The file is opened in compatibility mode.  For information on compatibility     
mode, refer to function call 3DH.                                               
                                                                                
10H --                                                                          
CLOSE FILE                                                                      
                                                                                
PURPOSE                                                                         
                                                                                
Closes a file.                                                                  
                                                                                
EXAMPLE                                                                         
                                                                                
MOV  AX,SEG FCB         ; Address FCB Parameter Block                           
MOV  DS,AX                                                                      
MOV  DX,OFFSET FCB                                                              
MOV  AH,10H             ; Function Call-FCB Close                               
INT  21H                ; Issue request to DOS                                  
CMP  AL,0               ; File Closed?                                          
JNE  Error              ; No, Error!                                            
                                                                                
                                                                                
FCB           LABEL   BYTE                                                      
; Contents set by previous operations                                           
                                                                                
COMMENTS                                                                        
                                                                                
AL is 00H if the file is closed.                                                
                                                                                
AL is FFH if the file was not closed.                                           
                                                                                
Use Function Call 59H (Get Extended Error) to determine the actual error        
condition.                                                                      
                                                                                
This function call must be executed on open files after file writes, and we     
highly recommend that it be used on all files.  If the file is not found in     
its correct position in the current directory, it is assumed the disk was       
changed and AL returns FFH.  Otherwise, the directory is updated to reflect     
the status in the FCB, the buffers for that file are flushed, and AL returns    
00H.                                                                            
                                                                                
11H --                                                                          
SEARCH FOR FIRST ENTRY                                                          
                                                                                
PURPOSE                                                                         
                                                                                
Searches the current directory for the first matching filename.                 
                                                                                
EXAMPLE                                                                         
                                                                                
MOV  AX,SEG DTA    ; Address Buffer for found file                              
MOV  DS,AX         ; information                                                
MOV  DX,OFFSET DTA                                                              
MOV  AH,1AH        ; Function Call-Set DTA address                              
INT  21H           ; Issue request to DOS                                       
MOV  AX,SEG FCB    ; Address FCB parameter block                                
MOV  DS,AX                                                                      
MOV  DX,OFFSET FCB                                                              
MOV  AH,11H        ; Function Call-FCB search first                             
INT  21H           ; Issue request to DOS                                       
CMP  AL,0          ; File found?                                                
JNE  Error         ; No, Error!                                                 
                                                                                
                                                                                
FCB     LABEL   BYTE                                                            
Fdrive  DB      0               ; Drive (0=Current, 1=A, 2=B, ...)              
Fname   DB      "FILENAME"      ; File name      (blank padded, may use ?)      
Fext    DB      "EXT"           ; File extension (blank padded, may use ?)      
DB      25 DUP(0)       ; Filled in by DOS                                      
                                                                                
DTA     LABEL   BYTE                                                            
Ddrive  DB      ?               ; Drive                                         
Dname   DB      "????????"      ; File Name      (blank padded)                 
Dext    DB      "???"           ; File Extension (blank padded)                 
DB      25 DUP(0)       ; Filled in by DOS 4.00                                 
                                                                                
COMMENTS                                                                        
                                                                                
AL is 00H if the file is found.                                                 
                                                                                
AL is FFH if the file was not found.                                            
                                                                                
Use Function Call 59H (Get Extended Error) to determine the actual error        
condition.                                                                      
                                                                                
The current disk directory is searched for the first matching filename.  If     
none are found, AL returns FFH.  Global filename characters are allowed in      
the filename and extension.  If a matching filename is found, AL returns 00H    
and the locations at the disk transfer address are set as follows:              
                                                                                
    If the FCB provided for searching was an extended FCB, the first byte at    
the disk transfer address is set to FFH followed by 5 bytes of 0, then          
the attribute byte from the search FCB, then the drive number used (1=A,        
2=B, etc.), then the 32 bytes of the directory entry.  Thus, the disk           
transfer address contains a valid unopened extended FCB with the same           
search attributes as the search FCB.                                            
                                                                                
    If the FCB provided for searching was a standard FCB, then the first byte   
is set to the drive number used (1=A, 2=B), and the next 32 bytes contain       
the matching directory entry.  Thus, the disk transfer address contains a       
valid unopened normal FCB.                                                      
                                                                                
NOTES:                                                                          
                                                                                
If an extended FCB is used, the following search pattern is used:               
                                                                                
1.  If the attribute is 0, only normal file entries are found. Entries for      
volume label, sub-directories, hidden and system files, are not returned.       
                                                                                
2.  If the attribute field is set for hidden or system files, or directory      
entries, it is an inclusive search. All normal file entries, plus all           
entries matching the specified attributes, are returned.  To look at all        
directory entries except the volume label, the attribute byte may be set        
to hidden + system + directory (all 3 bits on).                                 
                                                                                
3.  If the attribute field is set for the volume label, it is considered an     
exclusive search, and only the volume label entry is returned.                  
                                                                                
12H --                                                                          
SEARCH FOR NEXT ENTRY                                                           
                                                                                
PURPOSE                                                                         
                                                                                
Searches the current directory for the next matching filename.                  
                                                                                
EXAMPLE                                                                         
                                                                                
MOV        AX,SEG DTA    ; Address Buffer for found file                        
MOV        DS,AX         ; Information                                          
MOV        DX,OFFSET DTA                                                        
MOV        AH,1AH        ; Function Call-Set DTA address                        
INT        21H           ; Issue request to DOS                                 
MOV        AX,SEG FCB    ; Address FCB Parameter Block                          
MOV        DS,AX                                                                
MOV        DX,OFFSET FCB                                                        
MOV        AH,12H        ; Function Call-FCB Search Next                        
INT        21H           ; Issue request to DOS                                 
CMP        AL,0          ; File found?                                          
JNE      Error           ; No, Error!                                           
                                                                                
                                                                                
FCB           LABEL   BYTE                                                      
; As set by FCB Search First                                                    
                                                                                
DTA           LABEL   BYTE                                                      
Drive         DB      ?               ; Drive                                   
Fname         DB      "????????"      ; File Name      (blank padded)           
Ext           DB      "???"           ; File Extension (blank padded)           
DB      25 DUP(0)       ; Filled in by DOS 4.00                                 
                                                                                
COMMENTS                                                                        
                                                                                
AL is 00H if the file is found.                                                 
                                                                                
AL is FFH if the file was not found.                                            
                                                                                
Use Function Call 59H (Get Extended Error) to determine the actual error        
condition.                                                                      
                                                                                
After a matching filename has been found using function call 11H, function      
12H may be called to find the next match to an ambiguous request.               
                                                                                
The DTA contains information from the previous Search First or Search Next.     
All of the FCB, except for the name/extension field, is used to keep            
information necessary for continuing the search, so no disk operations may be   
performed if this FCB is between a previous function 11H or 12H call and this   
one.                                                                            
                                                                                
13H --                                                                          
DELETE FILE                                                                     
                                                                                
PURPOSE                                                                         
                                                                                
Deletes all current directory entries that match the specified filename.  The   
specified filename cannot be read-only.                                         
                                                                                
EXAMPLE                                                                         
                                                                                
MOV  AX,SEG FCB       ;Address FCB Parameter Block                              
MOV  DS,AX                                                                      
MOV  DX,OFFSET FCB                                                              
MOV  AH,13H           ;Function Call-FCB Delete File                            
INT  21H              ;Issue request to DOS                                     
CMP  AL,0             ;File(s) Deleted?                                         
JNE  Error            ;No, Error!                                               
                                                                                
                                                                                
FCB           LABEL   BYTE                                                      
Drive         DB      0               ; Drive                                   
FName         DB      Filename        ; File Name      (blank padded, may       
Ext           DB      Ext             ; File Extension (blank padded, may       
DB      25 DUP(0)       ; Filled in by DOS                                      
                                                                                
COMMENTS                                                                        
                                                                                
AL is 00H if the file is found.                                                 
                                                                                
AL is FFH if the file was not found.                                            
                                                                                
Use Function Call 59H (Get Extended Error) to determine the actual error        
condition.                                                                      
                                                                                
All matching current directory entries are deleted.  The global filename        
character "?" is allowed in the filename or extension.  If no directory         
entries match, AL returns FFH; otherwise AL returns 00H.                        
                                                                                
If the file is specified in read-only mode, the file is not deleted.            
                                                                                
NOTE:  Close open files before deleting them.                                   
                                                                                
Network Access Rights:  Requires Create access rights.                          
                                                                                
14H --                                                                          
SEQUENTIAL READ                                                                 
                                                                                
PURPOSE                                                                         
                                                                                
Loads the record addressed by the current block (FCB bytes C-D) and the         
current record (FCB byte 1F) at the disk transfer address (DTA), then the       
record address is increased.                                                    
                                                                                
EXAMPLE                                                                         
                                                                                
MOV  AX,SEG DTA            ;Address Data buffer                                 
MOV  DS,AX                                                                      
MOV  DX,OFFSET DTA                                                              
MOV  AH,1AH                ;Function call Set DTA Address                       
INT  21H                   ;Issue request to DOS                                
MOV  AX,SEG FCB            ;Address FCB Parameter Block                         
MOV  DS,AX                                                                      
MOV  DX,OFFSET FCB                                                              
MOV  AH,14H                ;Function Call-FCB Sequential Read                   
INT  21H                   ;Issue request to DOS                                
CMP  AL,0                  ;Data Read?                                          
JNE  Error                 ;No, Error!                                          
                                                                                
                                                                                
FCB           LABEL   BYTE                                                      
; Set by previous open                                                          
DTA           LABEL   BYTE                                                      
DB      ?Dup(0)   ;I/O buffer                                                   
                                                                                
COMMENTS                                                                        
                                                                                
AL is 00H if the read was successful.                                           
                                                                                
AL is 01H if the file was at End of File (EOF).                                 
                                                                                
AL is 02H if the read would have caused a wrap or overflow because the DTA      
was too small (the read was not completed).                                     
                                                                                
AL is 03H if EOF (a partial record was read and filled out with 0's).           
                                                                                
Use Function Call 59H (Get Extended Error) to determine the actual error        
condition.  The length of the record is determined by the FCB record size       
field.                                                                          
                                                                                
Network Access Rights:  Requires Read access rights.                            
                                                                                
15H --                                                                          
SEQUENTIAL WRITE                                                                
                                                                                
PURPOSE                                                                         
                                                                                
Writes the record addressed by the current block and record fields (size        
determined by the FCB record size field) from the disk transfer address.  If    
records are less than the sector size, the record is buffered for an eventual   
write when a sector's worth of data is accumulated.  Then the record address    
is increased.                                                                   
                                                                                
EXAMPLE                                                                         
                                                                                
MOV  AX,SEG DTA            ;Address Data buffer                                 
MOV  DS,AX                                                                      
MOV  DX,OFFSET DTA                                                              
MOV  AH,1AH                ;Function Set DTA Address                            
INT  21H                   ;Issue request to DOS                                
MOV  AX,SEG FCB            ;Address FCB Parameter Block                         
MOV  DS,AX                                                                      
MOV  DX,OFFSET FCB                                                              
MOV  AH,15H                ;Function Call-FCB Sequential Write                  
INT  21H                   ;Issue request to DOS                                
CMP  AL,0                  ;Data Written?                                       
JNE  Error                 ;No, Error!                                          
                                                                                
FCB    LABEL   BYTE                                                             
; Set by previous open                                                          
DTA    LABEL   BYTE                                                             
DB      ?Dup(0)      ;I/O buffer                                                
                                                                                
COMMENTS                                                                        
                                                                                
AL is 00H if the write was successful.                                          
                                                                                
AL is 01H if the disk or diskette is full (write cancelled).                    
                                                                                
AL is 02H if the write would have caused a wrap or overflow because the DTA     
was too small (write cancelled).                                                
                                                                                
Use Function Call 59H (Get Extended Error) to determine the actual error        
condition.  If the file is specified in read-only mode, the sequential write    
is not performed and 01H is returned in AL.                                     
                                                                                
Network Access Rights:  Requires Write access rights.                           
                                                                                
16H --                                                                          
CREATE FILE                                                                     
                                                                                
PURPOSE                                                                         
                                                                                
Creates a new file.                                                             
                                                                                
EXAMPLE                                                                         
                                                                                
MOV  AX,SEG FCB    ;Address FCB parameter block                                 
MOV  DS,AX                                                                      
MOV  DX,OFFSET FCB                                                              
MOV  AH,16H        ;Function Call-FCB create file                               
INT  21H           ;Issue request to DOS                                        
CMP  AL,0          ;File created and opened?                                    
JNE  Error         ;No, Error!                                                  
                                                                                
                                                                                
FCB            LABEL   BYTE                                                     
Fdrive         DB      0            ; Drive (0=Current, 1=A, 2=B, ...)          
Fname          DB      "FILENAME"   ; File name      (blank padded)             
Fext           DB      "EXT"        ; File extension (blank padded)             
DB      25 DUP(0)    ; Filled in by DOS                                         
                                                                                
COMMENTS                                                                        
                                                                                
AL is 00H if the file is created and opened.                                    
                                                                                
AL is FFH if the file was not created (normally a full directory or disk        
full).                                                                          
                                                                                
Use Function Call 59H (Get Extended Error) to determine the actual error        
condition.                                                                      
                                                                                
If a matching entry is found it is reused.  If no match is found, the           
directory is searched for an empty entry.  If a match is found, the entry is    
initialized to a 0-length file, the file is opened (see function call 0FH),     
and AL returns 00H.                                                             
                                                                                
The file may be marked hidden during its creation by using an extended FCB      
containing the appropriate attribute byte.                                      
                                                                                
Network Access Rights:  Requires Create access rights.                          
                                                                                
17H --                                                                          
RENAME FILE                                                                     
                                                                                
PURPOSE                                                                         
                                                                                
Changes every matching occurrence of the first filename in the current          
directory of the specified drive to the second, with the restriction that two   
files cannot have the same name and extension.                                  
                                                                                
EXAMPLE                                                                         
                                                                                
MOV  AX,SEG FCB       ;Address FCB Parameter Block                              
MOV  DS,AX                                                                      
MOV  DX,OFFSET FCB                                                              
MOV  AH,17H           ;Function Call-FCB Rename File                            
INT  21H              ;Issue request to DOS                                     
CMP  AL,0             ;File(s) Renamed?                                         
JNE  Error            ;No, Error!                                               
                                                                                
                                                                                
FCB      LABEL   BYTE                                                           
Fdrive   DB      0           ; Drive (0=Current, 1=A, 2=B, ...)                 
Fname    DB      "FILENAME"  ; File Name          (blank padded, may use ?      
Fext     DB      "EXT"       ; File Extension     (blank padded, may use ?      
DB      5 DUP(0)    ; Reserved                                                  
NewName  DB      "FILENAME"  ; New File Name      (blank padded, may use ?      
NewExt   DB      "EXT"       ; New File Extension (blank padded, may use ?      
DB      7 DUP(0)    ; Reserved                                                  
                                                                                
COMMENTS                                                                        
                                                                                
AL is 00H if the file or files were renamed.                                    
                                                                                
AL is FFH if a file in the current directory did not match or the new name      
already exists.                                                                 
                                                                                
Use Function Call 59H (Get Extended Error) to determine the actual error        
condition.                                                                      
                                                                                
The modified FCB has a drive code and filename in the usual position, and a     
second filename on the sixth byte after the first (DS:DX+11H) in what is        
normally a reserved area.                                                       
                                                                                
If "?"s appear in the second name, the corresponding positions in the           
original name are unchanged.                                                    
                                                                                
If the file is specified in read-only mode, the file is not renamed.            
                                                                                
Network Access Rights:  Requires Create access rights.                          
                                                                                
19H --                                                                          
CURRENT DISK                                                                    
                                                                                
PURPOSE                                                                         
                                                                                
Determines the current default drive.                                           
                                                                                
EXAMPLE                                                                         
                                                                                
MOV     AH,19H   ; Function Call -- Get Current Disk                            
INT     21H      ; Issue request to DOS                                         
MOV     Disk,AL  ; Save Current Disk                                            
                                                                                
Disk   DB     ?        ; Current Disk code (0=A:, 1=B:, ...)                    
                                                                                
COMMENTS                                                                        
                                                                                
AL returns with the code of the current default drive (0=A, 1=B, and            
others.).                                                                       
                                                                                
1AH --                                                                          
SET DISK TRANSFER ADDRESS                                                       
                                                                                
PURPOSE                                                                         
                                                                                
Sets the disk transfer address to DS:DX.                                        
                                                                                
EXAMPLE                                                                         
                                                                                
MOV  AX,SEG DTA       ;Address Buffer                                           
MOV  DS,AX                                                                      
MOV  DX,OFFSET DTA                                                              
MOV  AH,1AH           ;Function Call-Set DTA address                            
INT  21H              ;Issue request to DOS                                     
                                                                                
DTA    LABEL   BYTE    ; Data Buffer                                            
                                                                                
COMMENTS                                                                        
                                                                                
The area defined by this call is from the address in DS:DX to the end of the    
segment in DS.  DOS 4.00 does not allow disk transfers to wrap around within    
the segment, or overflow into the next segment.  If you do not set the DTA,     
the default DTA is offset 80H in the program segment prefix.  To get the DTA,   
issue function call 2FH.                                                        
                                                                                
1BH --                                                                          
ALLOCATION TABLE INFORMATION                                                    
                                                                                
PURPOSE                                                                         
                                                                                
Returns information about the allocation table for the default drive.           
                                                                                
EXAMPLE                                                                         
                                                                                
MOV  AH,1BH                  ; Function Call -- Allocation Table                
; Information                                                                   
INT  21H                     ; Issue request to DOS                             
MOV  NumAllocUnits,DX        ; Save Number of Allocation Units                  
MOV  NumSectsAllocUnit,AL    ; Save Number of Sectors/Allocation Unit           
MOV  SectSize,CX             ; Save of Sector Size                              
MOV  WORD PTR MediaType@+0,BX; Save Pointer to Media Type Byte                  
MOV  WORD PTR MediaType@+2,DS                                                   
                                                                                
                                                                                
NumAllocUnits     DW      ?       ; Number of Allocation Units on Current       
NumSectsAllocUnit DB      ?       ; Number Sectors in an Allocation Unit        
SectSize          DW      ?       ; Sector Size                                 
MediaType@        DD      ?       ; Pointer to Media Type byte                  
                                                                                
COMMENTS                                                                        
                                                                                
Refer to function call 36H (Get Disk Free Space).                               
                                                                                
1CH --                                                                          
ALLOCATION TABLE INFORMATION FOR SPECIFIC DEVICE                                
                                                                                
PURPOSE                                                                         
                                                                                
Returns allocation table information for a specific device.                     
                                                                                
EXAMPLE                                                                         
                                                                                
MOV   AH,1CH                   ; Function Call --                               
; Allocation Table Information                                                  
MOV   DL,Drive                 ; Drive requested  (0=current,                   
; 1=A:, ...)                                                                    
INT   21H                      ; Issue request to DOS                           
MOV   NumAllocUnits,DX         ; Save Number of Allocation Units                
MOV   NumSectsAllocUnit,AL     ; Save Number of Sectors/Allocation Unit         
MOV   SectSize,CX              ; Save of Sector Size                            
MOV   WORD PTR MediaType@+0,BX ; Save Pointer to Media Type Byte                
MOV   WORD PTR MediaType@+2,DS                                                  
                                                                                
Drive             DB            ; drive number to get info for                  
NumAllocUnits     DW    ?       ; Number of Allocation Units                    
; on specified drive                                                            
NumSectsAllocUnit DB    ?       ; Number Sectors in an                          
; Allocation Unit                                                               
SectSize          DW    ?       ; Sector Size                                   
MediaType@        DD    ?       ; Pointer to Media Type byte                    
                                                                                
COMMENTS                                                                        
                                                                                
This call is the same as call 1BH, except that on entry DL contains the         
number of the drive that contains the needed information (0 = default, 1 = A,   
and so forth.).  For more information on DOS 4.00 disk allocation, refer to     
"The Disk Directory" on page 2-3.  Also, refer to function call 36H (Get Disk   
Free Space).                                                                    
                                                                                
21H --                                                                          
RANDOM READ                                                                     
                                                                                
PURPOSE                                                                         
                                                                                
Reads the record addressed by the current block and current record fields       
into memory at the current disk transfer address.                               
                                                                                
EXAMPLE                                                                         
                                                                                
;Set up FCB                                                                     
MOV  AX,SEG DTA            ;Address Data buffer                                 
MOV  DS,AX                                                                      
MOV  DX,OFFSET DTA                                                              
MOV  AH,1AH                ;Function Set DTA Address                            
INT  21H                   ;Issue request to DOS                                
MOV  AX,SEG FCB            ;Address FCB Parameter Block                         
MOV  DS,AX                                                                      
MOV  DX,OFFSET FCB                                                              
MOV  AH,21H                ;Function Call-FCB Random Read                       
INT  21H                   ;Issue request to DOS                                
CMP   AL,0                 ;Data Read?                                          
JNE   Error                ;No, Error!                                          
                                                                                
                                                                                
FCB           LABEL   BYTE                                                      
; Set by previous open                                                          
; DTA label byte                                                                
                                                                                
COMMENTS                                                                        
                                                                                
AL is 00H if the read was successful.                                           
                                                                                
AL is 01H if the file was at End of File (EOF) (no data read).                  
                                                                                
AL is 02H if the read would have caused a wrap or overflow because the DTA      
was too small (the read was not completed).                                     
                                                                                
AL is 03H if EOF (a partial record was read and filled out with 0's).           
                                                                                
Use Function Call 59H (Get Extended Error) to determine the actual error        
condition.                                                                      
                                                                                
The current block and current record fields are set to agree with the random    
record field.  The record addressed by these fields is read into memory at      
the current disk transfer address.  For information on record size see          
Chapter 4, "Accessing Files Using File Control Blocks" on page 4-1.             
                                                                                
NOTE:  Function 24H must be called before using this function.                  
                                                                                
Network Access Rights:  Requires Read access rights.                            
                                                                                
22H --                                                                          
RANDOM WRITE                                                                    
                                                                                
PURPOSE                                                                         
                                                                                
Writes the record addressed by the current block and current record fields      
from the current disk transfer address.                                         
                                                                                
EXAMPLE                                                                         
                                                                                
;Set up FCB                                                                     
                                                                                
MOV  AX,SEG FCB       ;Address FCB parameter block                              
MOV  DS,AX                                                                      
MOV  DX,OFFSET FCB                                                              
MOV  AH,24H           ;Function Call--FCB Set                                   
;Relative record field                                                          
INT  21H              ;Issue request to DOS                                     
MOV  AX,SEG DTA       ;Address data buffer                                      
MOV  DS,AX                                                                      
MOV  DX,OFFSET DTA                                                              
MOV  AH,1AH           ;Function Set DTA address                                 
INT  21H              ;Issue request to DOS                                     
MOV  AX,SEG FCB       ;Address FCB parameter block                              
MOV  DS,AX                                                                      
MOV  DX,OFFSET FCB                                                              
MOV  AH,22H           ;Function Call-FCB random writers                         
INT  21H              ;Issue request to DOS                                     
CMP  AL,0             ;Data written?                                            
JNE  Error            ;No, error!                                               
                                                                                
                                                                                
FCB           LABEL   BYTE                                                      
; Set by previous open                                                          
DTA           LABEL   BYTE                                                      
                                                                                
COMMENTS                                                                        
                                                                                
AL is 00H if the write was successful.                                          
                                                                                
AL is 01H if the write or diskette is full (write cancelled).                   
                                                                                
AL is 02H if the read would have caused a wrap or overflow because the DTA      
was too small (write cancelled).                                                
                                                                                
Use Function Call 59H (Get Extended Error) to determine the actual error        
condition.                                                                      
                                                                                
The current block and current record fields are set to agree with the random    
record field.  Then the record addressed by these fields is written (or in      
the case of records not the same as sector sizes -- buffered) from the disk     
transfer address.                                                               
                                                                                
If the file is specified in read-only mode, the random write is not             
performed.                                                                      
                                                                                
Network Access Rights:  Requires Write access rights.                           
                                                                                
23H --                                                                          
FILE SIZE                                                                       
                                                                                
PURPOSE                                                                         
                                                                                
Searches the current directory for an entry that matches the specified file     
and sets the FCBs random record field to the number of records in the file.     
                                                                                
EXAMPLE                                                                         
                                                                                
MOV  AX,SEG FCB      ; Address FCB parameter block                              
MOV  DS,AX                                                                      
MOV  DX,OFFSET FCB                                                              
MOV  AH,23H          ; Function Call-FCB file size                              
INT  21H             ; Issue request to DOS                                     
CMP  AL,0            ; File found?                                              
JNE  Error           ; No, error!                                               
                                                                                
                                                                                
FCB           LABEL   BYTE                                                      
Drive         DB      0               ; Drive (0=Current, 1=A, 2=B, ...)        
Name          DB      "FILENAME"      ; File name      (blank padded)           
Ext           DB      "EXT"           ; File extension (blank padded)           
DB      25 DUP(0)       ; Filled in by DOS                                      
                                                                                
COMMENTS                                                                        
                                                                                
AL is 00H if the file exists.                                                   
                                                                                
AL is FFH if the file was not created (normally a full directory or disk        
full).                                                                          
                                                                                
Use Function Call 59H (Get Extended Error) to determine the actual error        
condition.                                                                      
                                                                                
The directory is searched for the matching entry.  If a matching entry is       
found, the random record field is set to the number of records in the file      
(in terms of the record size field rounded up).  If no matching entry is        
found, AL returns FFH.                                                          
                                                                                
NOTE:  If you do not set the FCB record size field before using this            
function, incorrect information is returned.                                    
                                                                                
24H --                                                                          
SET RELATIVE RECORD FIELD                                                       
                                                                                
PURPOSE                                                                         
                                                                                
Sets the random record field to the same file address as the current block      
and record fields.                                                              
                                                                                
EXAMPLE                                                                         
                                                                                
MOV  AX,SEG FCB    ;Address FCB parameter block                                 
MOV  DS,AX                                                                      
MOV  DX,OFFSET FCB                                                              
MOV  AH,24H        ;Function Call--FCB set                                      
;Relative record field                                                          
INT  21H           ;Issue request to DOS                                        
                                                                                
                                                                                
FCB           LABEL   BYTE                                                      
; Set by previous open                                                          
                                                                                
COMMENTS                                                                        
                                                                                
You must call this function before you perform random reads and writes, and     
random block reads and writes.                                                  
                                                                                
25H --                                                                          
SET INTERRUPT VECTOR                                                            
                                                                                
PURPOSE                                                                         
                                                                                
Sets the interrupt vector table for the interrupt number.                       
                                                                                
EXAMPLE                                                                         
                                                                                
MOV  AX,SEG Handler    ;Address new handler                                     
MOV  DS,AX                                                                      
MOV  DX,OFFSET Handler                                                          
MOV  AH,25H            ;Function Call - Set Interrupt                           
;Vector                                                                         
MOV  AL,Vector                                                                  
INT  21H               ;Issue request to DOS                                    
                                                                                
                                                                                
Vector     DB     ?    ;Number of vector to be replaced                         
Handler:               ;Code to process interrupt                               
                                                                                
COMMENTS                                                                        
                                                                                
The interrupt vector table for the interrupt number specified in AL is set to   
address contained in DS:DX.  Use function call 35H (Get Interrupt Vector) to    
obtain the contents of the interrupt vector.                                    
                                                                                
26H --                                                                          
CREATE NEW PROGRAM SEGMENT                                                      
                                                                                
PURPOSE                                                                         
                                                                                
Creates a new program segment.                                                  
                                                                                
EXAMPLE                                                                         
                                                                                
MOV     AH,26H               ; Function Call -- Create Program                  
; Segment                                                                       
MOV     DX,SEG PSP           ; Segment address to create new PSP                
INT     21H                  ; Issue request to DOS                             
                                                                                
                                                                                
PSP           LABEL   BYTE    ; Area to fill in                                 
DB      100H DUP(0)                                                             
                                                                                
COMMENTS                                                                        
                                                                                
The entire 100H area at location 0 in the current program segment is copied     
into location 0 in the new program segment.  The memory size information at     
location 6 in the new segment is updated and the current termination,           
Ctrl-Break exit and critical error addresses from interrupt vector table        
entries for interrupts 22H, 23H, and 24H are saved in the new program segment   
starting at 0AH.  They are restored from this area when the program ends.       
                                                                                
NOTE:  The EXEC function call 4BH provides a more complete service.             
Therefore, you should use the EXEC 4BH and avoid using this call.               
                                                                                
27H --                                                                          
RANDOM BLOCK READ                                                               
                                                                                
PURPOSE                                                                         
                                                                                
Reads the specified number of records (in terms of the record size field)       
from the file address specified by the random record field into the disk        
transfer address.                                                               
                                                                                
EXAMPLE                                                                         
                                                                                
;Set up disk transfer address                                                   
                                                                                
MOV  AX,SEG DTA          ;Address data buffer                                   
MOV  DS,AX                                                                      
MOV  DX,OFFSET DTA                                                              
MOV  AH,1AH              ;Function set DTA address                              
INT  21H                 ;Issue request to DOS                                  
                                                                                
MOV  AX,SEG FCB          ;Address FCB parameter block                           
MOV  DS,AX                                                                      
MOV  DX,OFFSET FCB                                                              
MOV  AH,24H              ;Function Call -- FCB Set                              
;Relative Record Field                                                          
INT  21H                 ;Issue request to DOS                                  
MOV  AX,SEG FCB          ;Address FCB parameter block                           
MOV  DS,AX                                                                      
MOV  DX,OFFSET FCB                                                              
MOV  CX,Records to read  ;number of records to read                             
MOV  AH,27H              ;Function Call - FCB random block read                 
INT  21H                 ;Issue request to DOS                                  
CMP  AL,0                ;Data read?                                            
JNE  Error               ;No, error!                                            
                                                                                
                                                                                
FCB              LABEL   BYTE                                                   
; Set by previous open                                                          
DTA              LABEL   BYTE                                                   
DB      ?Dup(0)    ;I/O buffer                                                  
Records to read  DW     ?                                                       
                                                                                
COMMENTS                                                                        
                                                                                
AL is 00H if the read was successful.                                           
                                                                                
AL is 01H if the file was at End of File (EOF) (no data read).                  
                                                                                
AL is 02H if the read would have caused a wrap or overflow because the DTA      
was too small (the read was not completed.).                                    
                                                                                
AL is 03H if EOF (a partial record was read and filled out with zeros).         
                                                                                
Use Function Call 59H (Get Extended Error) to determine the actual error        
condition.                                                                      
                                                                                
The random record field and the current block/record fields are set to          
address the next record (the first record not read).                            
                                                                                
NOTE:  Function 24H must be called before using this function.                  
                                                                                
Network Access Rights:  Requires Read access rights.                            
                                                                                
28H --                                                                          
RANDOM BLOCK WRITE                                                              
                                                                                
PURPOSE                                                                         
                                                                                
Writes the specified number of records from the disk transfer address into      
the file address specified by the random record field.                          
                                                                                
EXAMPLE                                                                         
                                                                                
;Set up disk transfer address                                                   
MOV  AX,SEG DTA            ;Address data buffer                                 
MOV  DS,AX                                                                      
MOV  DX,OFFSET DTA                                                              
MOV  AH,1AH                ;Function set DTA address                            
INT  21H                   ;Issue request to DOS                                
                                                                                
MOV  AX,SEG FCB            ;Address FCB parameter block                         
MOV  DS,AX                                                                      
MOV  DX,OFFSET FCB                                                              
MOV  AH,24H                ;Function Call--FCB set                              
;Relative record field                                                          
INT  21H                   ;Issue request to DOS                                
                                                                                
MOV  AX,SEG FCB            ;Address FCB parameter block                         
MOV  DS,AX                                                                      
MOV  DX,OFFSET FCB                                                              
MOV  CX,Records to write   ;Number of records to write                          
MOV  AH,28H                ;Function Call - FCB Random Block write              
INT  21H                   ;Issue request to DOS                                
CMP  AL,0                  ;Data written?                                       
JNE  Error                 ;No, error!                                          
                                                                                
                                                                                
DTA     LABEL     BYTE                                                          
                                                                                
DB        ?DUP(0)     ; I/O Buffer                                              
                                                                                
COMMENTS                                                                        
                                                                                
AL is 00H if the write was successful.                                          
                                                                                
AL is 01H if the disk or diskette is full (write cancelled).                    
                                                                                
AL is 02H if the write would have caused a wrap or overflow because the DTA     
was too small (write cancelled).                                                
                                                                                
Use Function Call 59H (Get Extended Error) to determine the actual error        
condition.                                                                      
                                                                                
If there is insufficient space on the disk, AL returns 01H and no records are   
written.  If CX is 0 upon entry, no records are written, but the file is set    
to the length specified by the random record field, whether longer or shorter   
than the current file size.  (Allocation units are released or allocated as     
appropriate.)                                                                   
                                                                                
NOTE:  Function call 24H must be called before using this function.             
                                                                                
Network Access Rights:  Requires Write access rights.                           
                                                                                
29H --                                                                          
PARSE FILENAME                                                                  
                                                                                
PURPOSE                                                                         
                                                                                
Parses the specified filename.                                                  
                                                                                
EXAMPLE                                                                         
                                                                                
MOV   AX,SEG CmdBuf                                                             
MOV   DS,AX            ;Address command string                                  
MOV   SI,OFFSET CmdBuf                                                          
MOV   AX,SEG FCB                                                                
MOV   ES,AX            ;Address FCB Parameter Block                             
MOV   DI,OFFSET FCB                                                             
MOV   AH,29H           ;Function Call - FCB Parse Filename                      
MOV   AL,OPTIONS       ;Set desired action                                      
INT   21H              ;Issue request to DOS                                    
                                                                                
CMP   AL,-1            ;Drive valid?                                            
JE    Error            ;No, Error!                                              
                                                                                
CmdBuf      LABEL    BYTE                                                       
DB       " a:file.ext      ",0DH                                                
                                                                                
FCB         LABEL    BYTE                                                       
; Created in a pre-open state based on input found.                             
Options   DB   ?   ;parsing options                                             
                                                                                
COMMENTS                                                                        
                                                                                
The contents of AL are used to determine the action to take, as shown below:    
                                                                                
<must = 0>                                                                      
bit:  7   6   5   4   3   2   1   0                                             
                                                                                
If bit 0 = 1, leading separators are scanned off the command line at DS:SI.     
Otherwise, no scan-off of leading separators takes place.                       
                                                                                
If bit 1 = 1, the drive ID byte in the FCB will be set (changed) only if a      
drive was specified in the command line being parsed.                           
                                                                                
If bit 2 = 1, the filename in the FCB will be changed only if the command       
line contains a filename.                                                       
                                                                                
If bit 3 = 1, the filename extension in the FCB will be changed only if the     
command line contains a filename extension.                                     
                                                                                
Filename separators include the following characters:                           
                                                                                
: . : , = + along with TAB and SPACE.  Filename terminators include all of      
these characters plus ,<, >, |, /, ', [, ], and any control characters.         
                                                                                
Output:                                                                         
                                                                                
AL is 00H if no global characters (? or *) were found in the Command String.    
                                                                                
AL is 01H if global characters (? or *) were found in the Command String.       
                                                                                
AL is FFH if the drive specified is invalid.                                    
                                                                                
The command line is parsed for a filename of the form d:filename.ext, and if    
found, a corresponding unopened FCB is created at ES:DI.  If no drive           
specifier is present, it is assumed to be all blanks.  If the character *       
appears in the filename or extension, it and all remaining characters in the    
name or extension are set to ?.                                                 
                                                                                
DS:SI returns pointing to the first character after the filename and ES:DI      
points to the first byte of the formatted FCB.  If no valid filename is         
present, ES:DI+1 contains a blank.                                              
                                                                                
2AH --                                                                          
GET DATE                                                                        
                                                                                
PURPOSE                                                                         
                                                                                
Returns the day of the week, the year, month and date.                          
                                                                                
EXAMPLE                                                                         
                                                                                
MOV     AH,2AH          ; Function Call -- Get Date                             
INT     21H             ; Issue request to DOS                                  
MOV     DayofWeek,AL    ; Save Day of the Week                                  
MOV     Year,CX         ; Save Year                                             
MOV     Month,DH        ; Save Month                                            
MOV     Day,DL          ; Save Day                                              
                                                                                
                                                                                
DayofWeek     DB      ?       ; 0=Sunday, ... 6=Saturday                        
Year          DW      ?       ; 1980 to 2099                                    
Month         DB      ?       ; 1 to 12                                         
Day           DB      ?       ; 1 to 31                                         
                                                                                
COMMENTS                                                                        
                                                                                
If the time-of-day clock rolls over to the next day, the date is adjusted       
accordingly, taking into account the number of days in each month and leap      
years.                                                                          
                                                                                
2BH --                                                                          
SET DATE                                                                        
                                                                                
PURPOSE                                                                         
                                                                                
Sets the date (also sets CMOS clock, if present).                               
                                                                                
EXAMPLE                                                                         
                                                                                
MOV     AH,2BH          ; Function Call -- Set Date                             
MOV     CX,Year         ; Set Year                                              
MOV     DH,Month        ; Set Month                                             
MOV     DL,Day          ; Set Day                                               
INT     21H             ; Issue request to DOS                                  
CMP     AL,0            ; Valid Date?                                           
JNE     Error           ; No!                                                   
                                                                                
Year          DW      ?       ; 1980 to 2099                                    
Month         DB      ?       ; 1 to 12                                         
Day           DB      ?       ; 1 to 31                                         
                                                                                
COMMENTS                                                                        
                                                                                
AL is 00H if the date is valid and the operation is successful.                 
                                                                                
AL is FFH if the date is not valid.                                             
                                                                                
On entry, CX:DX must have a valid date in the same format as returned by        
function call 2AH.                                                              
                                                                                
On return, AL returns 00H if the date is valid and the set operation is         
successful.  AL returns FFH if the date is not valid.                           
                                                                                
2CH --                                                                          
GET TIME                                                                        
                                                                                
PURPOSE                                                                         
                                                                                
Returns the time; hours, minutes, seconds and hundredths of seconds.            
                                                                                
EXAMPLE                                                                         
                                                                                
MOV     AH,2CH          ; Function Call  --                                     
; Get Time                                                                      
INT     21H             ; Issue request to DOS                                  
MOV     Hour,CH         ; Save Hour                                             
MOV     Minute,CL       ; Save Minute                                           
MOV     Second,DH       ; Save Second                                           
MOV     Hundredth,DL    ; Save Partial Second                                   
                                                                                
                                                                                
Hour          DB      ?       ; 0 to 23                                         
Minute        DB      ?       ; 0 to 59                                         
Second        DB      ?       ; 0 to 59                                         
Hundredth     DB      ?       ; 0 to 99                                         
                                                                                
COMMENTS                                                                        
                                                                                
On entry, AH contains 2CH.  On return, CX:DX contains the time-of-day.  Time    
is actually represented as four 8-bit binary quantities as follows:             
                                                                                
CH        Hours (0-23)                                                          
CL        Minutes (0-59)                                                        
DH        Seconds (0-59)                                                        
DL        1/100 seconds (0-99).                                                 
                                                                                
This format is readily converted to a printable form yet can also be used for   
calculations, such as subtracting one time value from another.                  
                                                                                
2DH --                                                                          
SET TIME                                                                        
                                                                                
PURPOSE                                                                         
                                                                                
Sets the time (also sets the CMOS clock, if present).                           
                                                                                
EXAMPLE                                                                         
                                                                                
MOV     AH,2DH          ; Function Call -- Set Time                             
MOV     CH,Hour         ; Sst Hour                                              
MOV     CL,Minute       ; Set Minute                                            
MOV     DH,Second       ; Set Second                                            
MOV     DL,Hundredth    ; Set Partial Second                                    
INT     21H             ; Issue request to DOS                                  
CMP     AL,0            ; Valid Time?                                           
JNE     Error           ; No!                                                   
                                                                                
                                                                                
Hour          DB      ?       ; 0 to 23                                         
Minute        DB      ?       ; 0 to 59                                         
Second        DB      ?       ; 0 to 59                                         
Hundredth     DB      ?       ; 0 to 99                                         
                                                                                
COMMENTS                                                                        
                                                                                
AL is 00H if the time is valid.                                                 
                                                                                
AL is FFH if the time is not valid.                                             
                                                                                
On entry, CX:DX has time in the same format as returned by function 2CH.  On    
return, if any component of the time is not valid, the set operation is         
cancelled and AL returns FFH.  If the time is valid, AL returns 00H.            
                                                                                
If your system has a CMOS realtime clock, it will be set.                       
                                                                                
2EH --                                                                          
SET/RESET VERIFY SWITCH                                                         
                                                                                
PURPOSE                                                                         
                                                                                
Sets the verify switch.                                                         
                                                                                
EXAMPLE                                                                         
                                                                                
; To set VERIFY=OFF                                                             
                                                                                
MOV     AH,2EH          ; Function Call -- Set                                  
; VERIFY                                                                        
MOV     AL,0            ; Set OFF                                               
INT     21H             ; Issue request to DOS                                  
                                                                                
; To set VERIFY=ON                                                              
                                                                                
MOV     AH,2EH          ; Function Call -- Set                                  
; VERIFY                                                                        
MOV     AL,1            ; Set ON                                                
INT     21H             ; Issue request to DOS                                  
                                                                                
COMMENTS                                                                        
                                                                                
On entry, AL must contain 01H to turn verify on, or 00H to turn verify off.     
When verify is on, DOS 4.00 performs a verify operation each time it performs   
a disk write to assure proper data recording.  Although disk recording errors   
are very rare, this function has been provided for applications in which you    
may wish to verify the proper recording of critical data.  You can obtain the   
current setting of the verify switch through function call 54H.                 
                                                                                
NOTE:  Verification is not supported on data written to a network disk.         
                                                                                
2FH --                                                                          
GET DISK TRANSFER ADDRESS (DTA)                                                 
                                                                                
PURPOSE                                                                         
                                                                                
Returns the current disk transfer address.                                      
                                                                                
EXAMPLE                                                                         
                                                                                
MOV     AH,2FH              ; Function Call -- Get                              
; DTA Address                                                                   
INT     21H                 ; Issue request to DOS                              
MOV     WORD PTR DTA@+0,BX  ; Save Address                                      
MOV     WORD PTR DTA@+2,ES                                                      
                                                                                
                                                                                
DTA@          DD      ?           ; DTA Buffer                                  
                                                                                
COMMENTS                                                                        
                                                                                
On entry, AH contains 2FH.  On return, ES:BX contains the current Disk          
Transfer Address.  You can set the DTA using function call 1AH.                 
                                                                                
30H --                                                                          
GET DOS VERSION NUMBER                                                          
                                                                                
PURPOSE                                                                         
                                                                                
Returns the DOS version number.                                                 
                                                                                
EXAMPLE                                                                         
                                                                                
PUSH    CX              ; CX and BX destroyed in call                           
PUSH    BX                                                                      
MOV     AH,30H          ; Function Call -- Get DOS 4.00                         
; Version                                                                       
INT     21H             ; Issue request to DOS                                  
MOV     MajorVersion,AL ; Save Version                                          
MOV     MinorVersion,AH                                                         
POP     BX                                                                      
POP     CX                                                                      
                                                                                
MajorVersion  DB      ?       ; X of X.YY                                       
MinorVersion  DB      ?       ; YY of X.YY                                      
                                                                                
COMMENTS                                                                        
                                                                                
On entry, AH contains 30H.  On return, BX and CX are set to 0.  AL contains     
the major version number.  AH contains the minor version number.                
                                                                                
If AL returns a major version number of 0, you can assume that the DOS          
version is pre-DOS 2.00.                                                        
                                                                                
31H --                                                                          
TERMINATE PROCESS AND REMAIN RESIDENT                                           
                                                                                
PURPOSE                                                                         
                                                                                
Terminates the current process and attempts to set the initial allocation       
block to the memory size in paragraphs.                                         
                                                                                
EXAMPLE                                                                         
                                                                                
MOV     AH,31H          ; Function Call -- Terminate                            
; and Keep Process                                                              
MOV     AL,RetCode      ; Set value of ERRORLEVEL                               
MOV     DX,MySize       ; Set my program and data size                          
INT     21H             ; Issue request to DOS                                  
INT     20H             ; Be safe if on DOS 4.00 Version 1.X                    
                                                                                
                                                                                
RetCode       DB      ?       ; Value to return to my EXEC'er                   
MySize        DW      ?       ; Size of my code and data                        
; (in paragraphs)                                                               
                                                                                
COMMENTS                                                                        
                                                                                
On entry, AL contains a binary return code.  DX contains the memory size        
value in paragraphs.  This function call does not free up any other             
allocation blocks belonging to that process.  Files opened by the process are   
not closed when the call is executed.  The return code passed in AL is          
retrievable by the parent through Wait (function call 4DH) and can be tested    
through the ERRORLEVEL batch subcommands.                                       
                                                                                
Memory is used efficiently if the block containing a copy of the environment    
is deallocated before terminating.  This can be done by loading ES with the     
segment contained in 2C of the PSP, and issuing function call 49H (Free         
Allocated Memory).  The five standard handles, 0000 through 0004, should be     
closed before exiting.                                                          
                                                                                
33H --                                                                          
GET/SET SYSTEM VALUE                                                            
                                                                                
PURPOSE                                                                         
                                                                                
Set or get the state of System Values such as BREAK (Ctrl-Break checking).      
                                                                                
EXAMPLE                                                                         
                                                                                
; To check BREAK state                                                          
                                                                                
MOV     AH,33H          ; Function Call -- Get/Set                              
; System value                                                                  
MOV     AL,0            ; Do Get BREAK                                          
INT     21H             ; Issue request to DOS                                  
MOV     BREAK,DL        ; Save state                                            
                                                                                
; To set BREAK=OFF                                                              
                                                                                
MOV     AH,33H          ; Function Call -- Get/Set                              
; System value                                                                  
MOV     AL,1            ; Do Set BREAK                                          
MOV     DL,0            ; Set OFF                                               
INT     21H             ; Issue request to DOS                                  
                                                                                
; To set BREAK=ON                                                               
                                                                                
MOV     AH,33H          ; Function Call -- Get/Set                              
; System Value                                                                  
MOV     AL,1            ; Do Set BREAK                                          
MOV     DL,1            ; Set ON                                                
INT     21H             ; Issue request to DOS                                  
                                                                                
; To get the Boot Drive                                                         
                                                                                
MOV     AH,33H          ; Function Call -- Get/Set                              
; System Value                                                                  
MOV     AL,5            ; Do Get Boot Drive                                     
INT     21H             ; Issue request to DOS                                  
MOV     Drive,DL        ; Save boot drive                                       
                                                                                
                                                                                
BREAK         DB      ?       ; Current BREAK state (0=OFF, 1=ON)               
Drive         DB      ?       ; DOS 4.00 boot drive                             
                                                                                
COMMENTS                                                                        
                                                                                
For BREAK:                                                                      
                                                                                
AL contains 00H to request the current state of Ctrl-Break checking.  On        
return DL contains the current state (00H = OFF, 01H = ON).                     
                                                                                
AL contains 01H to set the state.  DL contains the new state (00H = OFF, 01H    
= ON).                                                                          
                                                                                
For boot drive:                                                                 
                                                                                
AL contains 05H to request the boot drive.  On return DL contains the drive     
(A:=1, C:=3, ...).                                                              
                                                                                
35H --                                                                          
GET INTERRUPT VECTOR                                                            
                                                                                
PURPOSE                                                                         
                                                                                
To obtain the address in an interrupt vector.                                   
                                                                                
EXAMPLE                                                                         
                                                                                
MOV   AH,35H                 ; Function Call --                                 
; Set Interrupt Vector                                                          
MOV   AL,Vector              ; Vector to get (0 to 255)                         
INT   21H                    ; Issue request to DOS                             
MOV   WORD PTR OldVect+0,BX                                                     
MOV   WORD PTR OldVect+2,ES                                                     
                                                                                
                                                                                
OldVect       DD      ?            ; Previous vector contents                   
Vector        DB      ?            ; Vector number to get                       
                                                                                
COMMENTS                                                                        
                                                                                
On entry, AH contains 35H.  AL contains a hexadecimal interrupt number.  On     
return, ES:BX contains the CS:IP interrupt vector for the specified             
interrupt.  Use function call 25H (Set Interrupt Vector) to set the interrupt   
vectors.                                                                        
                                                                                
36H --                                                                          
GET DISK FREE SPACE                                                             
                                                                                
PURPOSE                                                                         
                                                                                
Returns the disk free space (available clusters, clusters/drive,                
bytes/sector).                                                                  
                                                                                
EXAMPLE                                                                         
                                                                                
MOV     AH,36H                   ; Function Call --                             
; Get disk free space                                                           
MOV     DL,Drive                 ; Drive to query                               
; (0=current, 1=A:,                                                             
; 2=B:, ...)                                                                    
INT     21H                      ; Issue request to DOS                         
CMP     AX,-1                    ; Error?                                       
JE      Error                    ; Yes                                          
MOV     SectAU,AX                ; Save allocation unit                         
; Size                                                                          
MOV     AvailAU,BX               ; Save free allocation                         
;  Units                                                                        
MOV     SectSize,CX              ; Save sector size                             
MOV     TotalAU,DX               ; Save disk size                               
                                                                                
MOV     AX,SectSize              ; Calculate allocation                         
; Unit size                                                                     
MUL     SectAU                                                                  
MOV     CX,AX                    ; CX = bytes/AU                                
MOV     AX,TotalAU               ; Calculate total space                        
MUL     CX                                                                      
MOV     WORD PTR TotalBytes+0,AX ; Save it                                      
MOV     WORD PTR TotalBytes+2,DX                                                
MOV     AX,AvailAU               ; Calculate free space                         
MUL     CX                                                                      
MOV     WORD PTR FreeBytes+0,AX  ; Save it                                      
MOV     WORD PTR FreeBytes+2,DX                                                 
                                                                                
                                                                                
SectAU        DW      ?                ; Sectors in an                          
; Allocation unit                                                               
AvailAU       DW      ?                ; Free allocation units                  
SectSize      DW      ?                ; Bytes in a sector                      
TotalAU       DW      ?                ; Number of allocation                   
; Units on DL disk                                                              
TotalBytes    DD      ?                ; Disk size in bytes                     
FreeBytes     DD      ?                ; Free space in bytes                    
Drive         DD      ?                ; Drive number to get info for           
                                                                                
COMMENTS                                                                        
                                                                                
If the drive number in DL was valid, BX contains the number of available        
allocation units, DX contains the total number of allocation units on the       
drive, CX contains the number of bytes per sector, and AX contains the number   
of sectors for each allocation unit.                                            
                                                                                
38H --                                                                          
GET OR SET COUNTRY DEPENDENT INFORMATION                                        
                                                                                
PURPOSE                                                                         
                                                                                
Sets the Active Country or returns country dependent information.               
                                                                                
EXAMPLE                                                                         
                                                                                
; To set the Current Country                                                    
                                                                                
MOV     AH,38H          ; Function Call -- Get/Set                              
; Country Information                                                           
MOV     AL,CountryID    ; Country ID (-1 if >= 255)                             
MOV     BX,CountryIDX   ; Country ID (if AL=-1)                                 
MOV     DX,-1           ; Indicate set country code                             
INT     21H             ; Issue request to DOS                                  
JC      Error           ; Error code in AX                                      
                                                                                
; To get Country Information                                                    
                                                                                
MOV     AX,SEG Buffer                                                           
MOV     DS,AX                                                                   
MOV     DX,OFFSET Buffer                                                        
MOV     AH,38H                                                                  
MOV     AL,CountryID    ; Country ID (-1 if >= 255)                             
; (0 to get current country)                                                    
MOV     BX,CountryIDX   ; Country ID (if AL=-1)                                 
                                                                                
INT     21H             ; Issue request to DOS                                  
JC      Error           ; Error code in AX                                      
MOV     CountryCode,BX  ; Save current Country Code                             
                                                                                
                                                                                
CountryCode   DW      ?       ; Current country code                            
                                                                                
CountryIDX    DW      ?       ; Extended country code for input                 
                                                                                
Buffer        LABEL   WORD    ; Country information (see format below)          
CountryID     DB      ?       ; Country code for input                          
                                                                                
COUNTRY INFORMATION                                                             
                                                                                
DateFormat    DW      ?         ; Date Format:                                  
;   0 = m d y order                                                             
;   1 = d m y order                                                             
;   2 = y m d order                                                             
$Symbol       DB      "????",0  ; Currency Symbol                               
;   example: "DM",0,?,?                                                         
Sep1000       DB      "?",0     ; Thousands Separator                           
;   example: ",",0                                                              
Sep1          DB      "?",0     ; Fractions Separator                           
;   example: ".",0                                                              
SepDate       DB      "?",0     ; Date Separator                                
;   example: "/",0                                                              
SepTime       DB      "?",0     ; Time Separator                                
;   example: ":",0                                                              
$Format       DB      ?         ; Currency Format:                              
;   0 = currency symbol, value                                                  
;   1 = value, currency symbol                                                  
;   2 = currency symbol, space, value                                           
;   3 = value, space, currency symbol                                           
;   4 = currency symbol is decimal separat                                      
SigDigits     DB      ?         ; Number of Significant Digits in Currency      
TimeFormat    DB      ?         ; Time Format:                                  
;   0 = 12 hour clock                                                           
;   1 = 24 hour clock                                                           
UpperCaseAL@  DD      ?         ; Address of Routine to Upper Case AL           
;   Only for values >=80H                                                       
SepData       DB      "?",0     ; Data List Separator                           
;   example: ",",0                                                              
Reserved      DW      5 DUP(?)  ; Reserved for future                           
                                                                                
COMMENTS                                                                        
                                                                                
The date format has the following values and meaning:                           
                                                                                
CODE      DATE                                                                  
0=USA     m d y                                                                 
1=EUROPE  d m y                                                                 
2=JAPAN   y m d                                                                 
                                                                                
Case Map Call Address:  The register contents for the case map call are:        
                                                                                
+---------------------------------------------------------------------------+   
| ON          REGISTER                                                      |   
| ENTRY       CONTENTS                                                      |   
+---------------------------------------------------------------------------+   
| AL          ASCII code of character to be converted to uppercase          |   
+---------------------------------------------------------------------------+   
                                                                                
+---------------------------------------------------------------------------+   
| ON          REGISTER                                                      |   
| RETURN      CONTENTS                                                      |   
+---------------------------------------------------------------------------+   
| AL          ASCII code of the uppercase input character                   |   
+---------------------------------------------------------------------------+   
                                                                                
The case map call address is in a form suitable for a FAR call indirect.        
                                                                                
RETURNS                                                                         
                                                                                
Error codes are returned in AX.  Issue function call 59H (Get Extended Error)   
for additional information about the error class, suggested action, and         
location.  Function Call 65H, (Get Extended Country Information), returns       
more country information and is preferred.                                      
                                                                                
Setting the Current County Code by using this function call is not              
recommended.  The user can set it by placing a COUNTRY command in the           
CONFIG.SYS file.  The Country Code set by the user should not be changed.       
The NLSFUNC DOS 4.00 extension must be installed to change the Current          
Country.                                                                        
                                                                                
39H --                                                                          
CREATE SUBDIRECTORY (MKDIR)                                                     
                                                                                
PURPOSE                                                                         
                                                                                
Creates the specified directory.                                                
                                                                                
EXAMPLE                                                                         
                                                                                
MOV  AX,SEG    DName  ;Directory Name                                           
MOV  DS,AX                                                                      
MOV  DX,OFFSET DName                                                            
MOV  AH,39H           ;Function-Make a directory                                
INT  21H              ;Issue request to DOS                                     
JC   Error                                                                      
                                                                                
                                                                                
DName          DB      "?? .. ??",0  ; ASCIIZ Name                              
; Example:                                                                      
; "c:\dir",0                                                                    
                                                                                
COMMENTS                                                                        
                                                                                
On entry, DS:DX contains the address of an ASCIIZ string with drive and         
directory path names.  All directory levels other than the last one in the      
name must exist before using this function.  Only one directory level at a      
time can be created with this function.  The maximum length of the ASCIIZ       
string is 64 characters.                                                        
                                                                                
Error codes are returned in AX.  Issue function call 59H (Get Extended Error)   
for additional information about the error class, suggested action, and         
location.                                                                       
                                                                                
Network Access Rights:  Requires Create access rights.                          
                                                                                
3AH --                                                                          
REMOVE SUBDIRECTORY (RMDIR)                                                     
                                                                                
PURPOSE                                                                         
                                                                                
Removes the specified directory.                                                
                                                                                
EXAMPLE                                                                         
                                                                                
MOV  AX,SEG DName              ;Directory name                                  
MOV  DS,AX                                                                      
MOV  DX,OFFSET DName                                                            
MOV  AH,3AH                    ;Function-Remove directory                       
INT  21H                       ;Issue request to DOS                            
JC   Error                                                                      
                                                                                
                                                                                
DName          DB      "?? .. ??",0  ; ASCIIZ Name                              
;   example: "c:\dir",0                                                         
                                                                                
COMMENTS                                                                        
                                                                                
On entry, DS:DX contains the address of an ASCIIZ string with the drive and     
directory path names.  The specified directory is removed from the structure.   
The current directory or a directory with files in it cannot be removed.        
                                                                                
Error codes are returned in AX.  Issue function call 59H (Get Extended Error)   
for additional information about the error class, suggested action, and         
location.                                                                       
                                                                                
Network Access Rights:  Requires Create access rights.                          
                                                                                
3BH --                                                                          
CHANGE THE CURRENT DIRECTORY (CHDIR)                                            
                                                                                
PURPOSE                                                                         
                                                                                
Changes the current directory to the specified directory.                       
                                                                                
EXAMPLE                                                                         
                                                                                
MOV  AX,SEG DName              ;Directory name                                  
MOV  DS,AX                                                                      
MOV  DX,OFFSET DName                                                            
MOV  AH,3BH                    ;Function -- Change directory                    
INT  21H                       ;Issue request to DOS                            
JC   Error                                                                      
                                                                                
                                                                                
DName          DB      "?? .. ??",0  ; ASCIIZ Name                              
;   example: "c:\dir",0                                                         
                                                                                
COMMENTS                                                                        
                                                                                
On entry, DS:DX contains the address of an ASCIIZ string with drive and         
directory path names.  The string is limited to 64 characters and cannot        
contain a network path.  If any member of the directory path does not exist,    
the directory path is not changed.  Otherwise, the current directory is set     
to the ASCIIZ string.                                                           
                                                                                
Error codes are returned in AX.  Issue function call 59H (Get Extended Error)   
for additional information about the error class, suggested action, and         
location.                                                                       
                                                                                
3CH --                                                                          
CREATE A FILE (CREAT)                                                           
                                                                                
PURPOSE                                                                         
                                                                                
Creates a new file or shortens an old file to 0 length in preparation for       
writing.                                                                        
                                                                                
EXAMPLE                                                                         
                                                                                
MOV     AX,SEG FName          ;File name                                        
MOV     DS,AX                                                                   
MOV     DX,OFFSET FName                                                         
MOV     AH,3CH                ;Function -- Create a File                        
MOV     CX,Attribute          ; Attribute of the file                           
; Allowed values                                                                
;  0001H=Read only                                                              
;  0002H=Hidden                                                                 
;  0004H=System                                                                 
;  0008H=Volume label                                                           
INT     21H                   ; Issue request to DOS                            
JC      Error                 ; Error code in AX                                
MOV     Handle,AX             ; Save file handle for                            
                                                                                
                                                                                
FName         DB      "?? .. ??",0  ; ASCIIZ Name                               
;   example: "c:\dir\file.ext",0                                                
Handle        DW      ?             ; File handle                               
Attribute     DW      ?             ; Attributes for directory entry            
                                                                                
COMMENTS                                                                        
                                                                                
If the file did not exist, the file is created in the appropriate directory     
and the file is given the read/write access code.  The file is opened for       
read/write, the read/write pointer is set to the first byte of the file and     
the handle is returned in AX.  Note that function call 43H (Change File Mode)   
can be used later to change the file's attribute.                               
                                                                                
Error codes are returned in AX.  Issue function call 59H (Get Extended Error)   
for additional information about the error class, suggested action, and         
location.                                                                       
                                                                                
This function does not replace an existing volume label.  You must delete the   
existing volume label before issuing this call.                                 
                                                                                
Network Access Rights:  Requires Create access rights.                          
                                                                                
3DH --                                                                          
OPEN A FILE                                                                     
                                                                                
PURPOSE                                                                         
                                                                                
Opens the specified file.                                                       
                                                                                
EXAMPLE                                                                         
                                                                                
MOV  AX,SEG FName          ; File name                                          
MOV  DS,AX                                                                      
MOV  DX,OFFSET FName                                                            
MOV  AH,3DH                ; Function -- Open a File                            
MOV  AL,OpenMode                                                                
INT  21H                   ; Issue request to DOS                               
JC   Error                 ; Error code in AX                                   
MOV  Handle,AX             ; Save file handle for following operations          
                                                                                
                                                                                
FName         DB      "?? .. ??",0  ; ASCIIZ Name                               
;   example: "c:\dir\file.ext",0                                                
Handle        DW      ?             ; File Handle                               
OpenMode      DB      ?             ; Open mode                                 
                                                                                
COMMENTS                                                                        
                                                                                
The read/write pointer is set at the first byte of the file and the record      
size of the file is 1 byte.  The read/write pointer can be changed with         
function call 42H.  The returned file handle must be used for subsequent        
input and output to the file.  The file's date and time can be obtained or      
set through call 57H, and its attribute can be obtained through call 43H.       
                                                                                
Error codes are returned in AX.  Issue function call 59H (Get Extended Error)   
for additional information about the error class, suggested action, and         
location.                                                                       
                                                                                
NOTES:                                                                          
                                                                                
1.  This call opens any normal or hidden file whose name matches the name       
specified.                                                                      
                                                                                
2.  Device names cannot end in a colon.                                         
                                                                                
3.  When a file is closed, any sharing restrictions placed on it by the open    
are canceled.                                                                   
                                                                                
4.  File sharing must be loaded, or the file must be a network file for the     
sharing modes to function.  Refer to the SHARE command.                         
                                                                                
5.  The file read-only attribute can be set when creating the file using        
extended FCBs or specifying the appropriate attribute in CX and using the       
CHMOD interrupt 21H function call or the DOS 4.00 ATTRIB command.               
                                                                                
6.  If the file is inherited by the subordinate process, all sharing and        
access restrictions are also inherited.                                         
                                                                                
7.  If an open file handle is duplicated by either of the DUP function calls,   
all sharing and access restrictions are also duplicated.                        
                                                                                
OPEN MODE                                                                       
                                                                                
The open mode is defined in AL and consists of four bit-oriented fields:        
                                                                                
INHERITANCE FLAG    Specifies if the opened file is inherited by a              
subordinate process.                                                            
SHARING MODE FIELD  Defines which operations other processes can perform on     
the file.                                                                       
RESERVED FIELD                                                                  
ACCESS FIELD        Defines which operations the current process can perform    
on the file.                                                                    
                                                                                
BIT FIELDS                                                                      
                                                                                
The bit fields are mapped as follows:                                           
                                                                                
<I> < S > <R> < A >                                                             
Open Mode bits   7  6 5 4  3  2 1 0                                             
                                                                                
I       Inheritance flag                                                        
                                                                                
If I = 0; File is inherited by subordinate processes.                           
If I = 1; File is private to the current process.                               
                                                                                
S       Sharing Mode                                                            
                                                                                
The file is opened as follows:                                                  
                                                                                
S = 000 -- Compatibility mode                                                   
S = 001 -- DenyRead/Write mode (exclusive)                                      
S = 010 -- DenyWrite mode                                                       
S = 011 -- DenyRead mode                                                        
S = 100 -- DenyNone mode                                                        
                                                                                
Any other combinations are invalid.                                             
                                                                                
When opening a file, you must inform DOS 4.00 which operations any              
other processes, in sharing mode, can perform on the file.                      
                                                                                
The default, compatibility mode, denies all other computers in a                
network access to the file.  If other processes can continue to read            
the file while your process is operating on it, specify DenyWrite.              
DenyWrite prohibits writing by other processes, but allows reading.             
                                                                                
Similarly, you must specify which operations, or access modes, your             
process can perform.  The default access mode, ReadWrite, causes the            
open request to fail if another process on the computer or any other            
computer on a network has the file opened with any sharing mode other           
than DenyNone.  If you intend to read from the file only, your Open             
will succeed unless all other processes have specified DenyNone or              
DenyWrite.  File sharing requires cooperation of both sharing                   
processes.                                                                      
                                                                                
R       Reserved (set this bit field to 0).                                     
                                                                                
A       Access                                                                  
                                                                                
The file access is assigned as follows:                                         
                                                                                
If A = 000; Read access                                                         
If A = 001; Write access                                                        
If A = 010; Read/Write access                                                   
                                                                                
Any other combinations are invalid.                                             
                                                                                
Network Access Rights:  If the Access field (A) of the Open mode field (AL)     
is equal to:                                                                    
                                                                                
000     Requires Read access rights                                             
                                                                                
001     Requires Write access rights                                            
                                                                                
010     Requires Read/Write access rights                                       
                                                                                
COMPATIBILITY MODE                                                              
                                                                                
A file is considered to be in compatibility mode if the file is opened by:      
                                                                                
    Any of the CREATE function calls                                            
    An FCB function call                                                        
    A handle function call with compatibility mode specified.                   
                                                                                
A file can be opened any number of times in compatibility mode by a single      
process, provided that the file is not currently open under one of the other    
four sharing modes.  If the file is marked read-only, and is open in            
DenyWrite sharing mode with Read Access, the file may be opened in              
Compatibility Mode with Read Access.  If the file was successfully opened in    
one of the other sharing modes and an attempt is made to open the file again    
in Compatibility Mode, an interrupt 24H is generated to signal this error.      
The base interrupt 24H error indicates DRIVE NOT READY, and the extended        
error indicates a SHARING VIOLATION.                                            
                                                                                
SHARING MODES                                                                   
                                                                                
The sharing modes for a file opened in compatibility mode are changed by DOS    
4.00 depending on the read-only attribute of the file.  This  allows sharing    
of read-only files.                                                             
                                                                                
+---------------------------------------------------+                           
|                  READ-ONLY                        |                           
| FILE OPENED BY   ACCESS            SHARING MODE   |                           
+---------------------------------------------------+                           
| FCB              Read-Only         DenyWrite      |                           
+---------------------------------------------------+                           
| Handle Read      Read-Only         DenyWrite      |                           
+---------------------------------------------------+                           
| Handle Write     Error             -----          |                           
+---------------------------------------------------+                           
| Handle           Error             -----          |                           
| Read/Write                                        |                           
+---------------------------------------------------+                           
                                                                                
+---------------------------------------------------+                           
|                  NOT READ-ONLY                    |                           
| FILE OPENED BY   ACCESS            SHARING MODE   |                           
+---------------------------------------------------+                           
| FCB              Read/Write        Compatibility  |                           
+---------------------------------------------------+                           
| Handle Read      Read              Compatibility  |                           
+---------------------------------------------------+                           
| Handle Write     Write             Compatibility  |                           
+---------------------------------------------------+                           
| Handle           Read/Write        Compatibility  |                           
| Read/Write                                        |                           
+---------------------------------------------------+                           
                                                                                
DENYREAD/WRITE MODE (EXCLUSIVE)                                                 
                                                                                
If a file is successfully opened in DenyRead/Write mode, access to the file     
is exclusive.  A file currently open in this mode cannot be opened again in     
any sharing mode by any process (including the current process) until the       
file is closed.                                                                 
                                                                                
DENYWRITE MODE                                                                  
                                                                                
A file successfully opened in DenyWrite sharing mode, prevents any other        
write access opens to the file (A = 001 or 010) until the file is closed.  An   
attempt to open a file in DenyWrite mode is unsuccessful if the file is open    
with a write access.                                                            
                                                                                
DENYREAD MODE                                                                   
                                                                                
A file successfully opened in DenyRead sharing mode, prevents any other read    
sharing access opens to the file (A = 000 or 010) until the file is closed.     
An attempt to open a file in DenyRead sharing mode is unsuccessful if the       
file is open in Compatibility mode or with a read access.                       
                                                                                
DENYNONE MODE                                                                   
                                                                                
A file successfully opened in DenyNone mode, places no restrictions on the      
read/write accessibility of the file.  An attempt to open a file in DenyNone    
mode is unsuccessful if the file is open in Compatibility mode.                 
                                                                                
When accessing files that reside on a network disk, no local buffering is       
done when files are opened in any of the following sharing modes:               
                                                                                
    DenyRead                                                                    
    DenyNone.                                                                   
                                                                                
Therefore, in a network environment, DenyRead/Write sharing mode,               
Compatibility sharing mode, and DenyWrite mode opens are buffered locally.      
                                                                                
The following sharing matrix shows the results of opening, and subsequently     
attempting to reopen the same file using all combinations of access and         
sharing modes:                                                                  
                                                                                
Y     :2nd,3rd,...open is allowed                                               
N     :2nd,3rd,...open is denied                                                
DRW   :DenyRead/Write Mode (Exclusive)                                          
DW    :DenyWrite Mode                                                           
DR    :DenyRead Mode                                                            
ALL   :Read/Write Mode                                                          
I     :Read Only Access                                                         
O     :Write Only Access                                                        
IO    :Read/Write Access                                                        
                                                                                
3EH --                                                                          
CLOSE A FILE HANDLE                                                             
                                                                                
PURPOSE                                                                         
                                                                                
Closes the specified file handle.                                               
                                                                                
EXAMPLE                                                                         
                                                                                
MOV     AH,3EH          ; Function Call --                                      
; Close a Handle                                                                
MOV     BX,Handle                                                               
INT     21H             ; Issue request to DOS                                  
JC      Error           ; Error code in AX                                      
                                                                                
                                                                                
Handle        DW      ?       ; File Handle (from Open / Create)                
                                                                                
COMMENTS                                                                        
                                                                                
On entry, BX contains the file handle that was returned by Open or Create.      
On return, the file is closed, the directory is updated, and all internal       
buffers for that file are flushed.                                              
                                                                                
Error codes are returned in AX.  Issue function call 59H (Get Extended Error)   
for additional information about the error class, suggested action, and         
location.                                                                       
                                                                                
3FH --                                                                          
READ FROM A FILE OR DEVICE                                                      
                                                                                
PURPOSE                                                                         
                                                                                
Transfers the specified number of bytes from a file into a buffer location.     
                                                                                
EXAMPLE                                                                         
                                                                                
MOV  AX,SEG Buffer             ; Address data buffer                            
MOV  DS,AX                                                                      
MOV  DX,OFFSET Buffer                                                           
MOV  AH,3FH                    ; Function -- Read from a file                   
MOV  BX,Handle                                                                  
MOV  CX,BufSize                ; Buffer size                                    
INT  21H                       ; Issue request to DOS                           
JC   Error                     ; Error code in AX                               
CMP  AX,0                      ; At End Of File?                                
JE   EOF                       ; Yes!                                           
MOV  SizeRead,AX               ; Save Amount Read                               
                                                                                
N             EQU   512       ; Typical buffer size                             
Handle        DW    ?         ; File Handle (from Open /Create)                 
BufSize       DW    N         ; Buffer Size, N is                               
Buffer        DB    N DUP(?)  ; Data Buffer                                     
SizeRead      DW    ?         ; Amount of Data in Buffer                        
                                                                                
COMMENTS                                                                        
                                                                                
On entry, BX contains the file handle.  CX contains the number of bytes to      
read.  DS:DX contains the buffer address.  On return, AX contains the number    
of bytes read.                                                                  
                                                                                
This function call attempts to transfer (CX) bytes from a file into a buffer    
location.  It is not guaranteed that all bytes will be read.  For example,      
when DOS 4.00 reads from the keyboard, at most one line of text is              
transferred.  If this read is performed from the standard input device, the     
input can be redirected.  If the value in AX is 0, then the program has tried   
to read from the end of file.                                                   
                                                                                
Error codes are returned in AX.  Issue function call 59H (Get Extended Error)   
for additional information about the error class, suggested action, and         
location.                                                                       
                                                                                
Network Access Rights:  Requires Read access rights.                            
                                                                                
40H --                                                                          
WRITE TO A FILE OR DEVICE                                                       
                                                                                
PURPOSE                                                                         
                                                                                
Transfers the specified number of bytes from a buffer into a specified file.    
                                                                                
EXAMPLE                                                                         
                                                                                
MOV  AX,SEG Buffer       ;Data Buffer                                           
MOV  DS,AX                                                                      
MOV  DX,OFFSET Buffer                                                           
MOV  CX,BufSize                                                                 
MOV  AH,40H              ;Function-Write to a File                              
MOV  BX,Handle                                                                  
MOV  DX,OFFSET Buffer                                                           
INT  21H                 ; Issue request to DOS                                 
JC   Error               ; Error code in AX                                     
CMP  AX,CX               ; Disk Full?                                           
JB   FullDisk            ; Yes!                                                 
                                                                                
N             EQU    512      ; Typical buffer size                             
Handle        DW     ?        ; File Handle (from Open / Create)                
BufSize       DW     N        ; Buffer Size                                     
Buffer        DB     N DUP(?) ; Data Buffer                                     
                                                                                
COMMENTS                                                                        
                                                                                
On entry, BX contains the file handle.  CX contains the number of bytes to      
write.  DS:DX contains the address of the data to write.                        
                                                                                
This function call attempts to transfer (CX) bytes from a buffer into a file.   
AX returns the number of bytes actually written.  If the carry flag is not      
set and this value is not the same as the number requested (in CX), it should   
be considered an error.  Although no error code is returned, your program can   
compare these values.  Normally, the reason for the error is a full disk.  If   
this write is performed to the standard output device, the output can be        
redirected.                                                                     
                                                                                
To truncate a file at the current position of the file pointer, set the         
number of bytes (CX) to 0 before issuing the interrupt 21H.  The file pointer   
can be moved to the desired position by reading, writing, and performing        
function call 42H, (Move File Read/Write Pointer.)                              
                                                                                
If the file is read-only, the write to the file or device is not performed.     
                                                                                
Error codes are returned in AX.  Issue function call 59H (Get Extended Error)   
for additional information about the error class, suggested action, and         
location.                                                                       
                                                                                
Network Access Rights:  Requires Write access rights.                           
                                                                                
41H --                                                                          
DELETE A FILE FROM A SPECIFIED DIRECTORY (UNLINK)                               
                                                                                
PURPOSE                                                                         
                                                                                
Removes a directory entry associated with a filename.                           
                                                                                
EXAMPLE                                                                         
                                                                                
MOV  AX,SEG FName        ; File Name                                            
MOV  DS,AX                                                                      
MOV  DX,OFFSET FName                                                            
MOV  AH,41H              ; Function-Delete a File                               
INT  21H                 ; Issue request to DOS                                 
JC   Error               ; Error code in AX                                     
                                                                                
                                                                                
FName       DB    "?? .. ??",0  ; ASCIIZ Name                                   
;   example: "c:\dir\File.ext",0                                                
                                                                                
COMMENTS                                                                        
                                                                                
Global filename characters are not allowed in any part of the ASCIIZ string.    
Read-only files cannot be deleted by this call.  To delete a read-only file,    
you can first use call 43H to change the file's read-only attribute to 0,       
then delete the file.                                                           
                                                                                
Error codes are returned in AX.  Issue function call 59H (Get Extended Error)   
for additional information about the error class, suggested action, and         
location.                                                                       
                                                                                
Network Access Rights:  Requires Create access rights.                          
                                                                                
42H --                                                                          
MOVE FILE READ WRITE POINTER (LSEEK)                                            
                                                                                
PURPOSE                                                                         
                                                                                
Moves the read/write pointer according to the method specified.                 
                                                                                
EXAMPLE                                                                         
                                                                                
MOV     AH,42H                  ; Function Call --                              
; Move Read/Write Pointer                                                       
MOV     AL,Method               ; Method of Positioning:                        
;   0 = From Beginning of File                                                  
;       (BOF)                                                                   
;   1 = From Current Position                                                   
;   2 = From End of File (EOF)                                                  
MOV     BX,Handle               ; Select File                                   
MOV     DX,WORD PTR Position+0  ; New Position = Position + METHOD              
MOV     CX,WORD PTR Position+2                                                  
INT     21H                     ; Issue request to DOS                          
JC      Error                   ; Error code in AX                              
MOV     WORD PTR Position+0,AX  ; Set new File Position                         
MOV     WORD PTR Position+2,DX                                                  
                                                                                
                                                                                
Handle        DW      ?               ; File Handle (from Open /                
; Create)                                                                       
Position      DD      ?               ; File Offset (may be                     
; negative)                                                                     
Method        DB      ?                                                         
                                                                                
COMMENTS                                                                        
                                                                                
On entry, AL contains a method value.  BX contains the file handle.  CX:DX      
contains the desired offset in bytes with CX containing the most significant    
part.  On return, DX:AX contains the new location of the pointer with DX        
containing the most significant part if the carry flag is not set.              
                                                                                
Error codes are returned in AX.  Issue function call 59H (Get Extended Error)   
for additional information about the error class, suggested action, and         
location.                                                                       
                                                                                
This function call moves the read/write pointer according to the following      
methods:                                                                        
                                                                                
+---------+-----------------------------------------------------------------+   
| AL      | DESCRIPTION                                                     |   
+---------+-----------------------------------------------------------------+   
| 0       | The pointer is moved CX:DX bytes (offset) from the beginning of |   
|         | the file.                                                       |   
+---------+-----------------------------------------------------------------+   
| 1       | The pointer is moved to the current location plus offset.       |   
+---------+-----------------------------------------------------------------+   
| 2       | The pointer is moved to the end-of-file plus offset.  This      |   
|         | method can be used to determine file's size.                    |   
+---------+-----------------------------------------------------------------+   
                                                                                
NOTE:  If an LSEEK operation is performed on a file that resides on a network   
disk that is open in either DenyRead or DenyNone sharing mode, the              
read/write pointer information is adjusted on the computer where the            
file actually exists.  If the file is opened in any other sharing               
mode, the read/write pointer information is kept on the remote                  
computer.                                                                       
                                                                                
43H --                                                                          
CHANGE FILE MODE (CHMOD)                                                        
                                                                                
PURPOSE                                                                         
                                                                                
Changes the file mode of the specified file.                                    
                                                                                
EXAMPLE                                                                         
                                                                                
; To Get Attributes                                                             
                                                                                
MOV     AX,SEG FName          ;File Name                                        
MOV     DS,AX                                                                   
MOV     DX,OFFSET FName       ;DS:DX points to ASCIIZ path name                 
MOV     AL,0                  ;Indicate get                                     
MOV     AH,43H                ;Function-Change File Mode                        
INT     21H                   ;Issue request to DOS                             
JC      Error                 ;Error code in AX                                 
MOV     Attribute,CX          ;Save Attribute                                   
                                                                                
; To Set Attributes                                                             
                                                                                
MOV    AX,SEG FName           ;File Name                                        
MOV    DS,AX                                                                    
MOV    DX,OFFSET FName        ;DS:DX points to ASCIIZ path name                 
MOV    AL,1                   ;Indicate set                                     
MOV    AH,43H                 ;Function-Change File Mode                        
MOV    CX,Attribute           ;Set Attribute                                    
INT    21H                    ;Issue request to DOS                             
JC     Error                  ;Error code in AX                                 
                                                                                
                                                                                
Fname         DB      64 Dup (0)    ;ASCIIZ Name                                
;  example: "c:\dir\File.ext",0                                                 
Attribute     DW      ?             ;File Attribute                             
;  example: 0001H to set Read-Only                                              
                                                                                
COMMENTS                                                                        
                                                                                
On entry, AL contains a function code, and DS:DX contains the address of an     
ASCIIZ string with the drive, path, and filename.                               
                                                                                
If AL contains 01H, the file's attribute is set to the attribute in CX.  See    
"The Disk Directory" on page 2-3 for the attribute byte description.  If AL     
is 00H the file's current attribute is returned in CX.                          
                                                                                
Error codes are returned in AX.  Issue function call 59H (Get Extended Error)   
for additional information about the error class, suggested action, and         
location.                                                                       
                                                                                
NOTE:  Only the Archive (20H), Read-Only (01H), System (04H) and Hidden (02H)   
bits can be changed.  All other bits of CX must be 0, otherwise, an             
error may be indicated.                                                         
                                                                                
Network Access Rights:  To change the archive bit (AL=20H), no access rights    
are required.  To change any other bit, Create access rights are required.      
                                                                                
44H --                                                                          
I/O CONTROL FOR DEVICES                                                         
                                                                                
PURPOSE                                                                         
                                                                                
Sets or gets device information associated with open device handles, or sends   
control strings to the device handle or receives control strings from the       
device handle.                                                                  
                                                                                
See Appendix C for full details on this function call.                          
                                                                                
45H --                                                                          
DUPLICATE A FILE HANDLE (DUP)                                                   
                                                                                
PURPOSE                                                                         
                                                                                
Returns a new file handle for an open file that refers to the same file at      
the same position.                                                              
                                                                                
EXAMPLE                                                                         
                                                                                
MOV     AH,45H          ; Function Call --                                      
; Duplicate a Handle                                                            
MOV     BX,Handle       ; Select File                                           
INT     21H             ; Issue request to DOS                                  
JC      Error           ; Error code in AX                                      
MOV     NewHandle,AX    ; Save New Handle                                       
                                                                                
                                                                                
Handle        DW      ?       ; File Handle (from Open / Create)                
NewHandle     DW      ?       ; File Handle that duplicates Handle              
                                                                                
COMMENTS                                                                        
                                                                                
On entry, BX contains the file handle.  On return, AX contains the returned     
file handle.                                                                    
                                                                                
Error codes are returned in AX.  Issue function call 59H (Get Extended Error)   
for additional information about the error class, suggested action, and         
location.                                                                       
                                                                                
NOTE:  If you move the read/write pointer of either handle by a read, write,    
or LSEEK function call, the pointer for the other handle is also                
changed.                                                                        
                                                                                
46H --                                                                          
FORCE A DUPLICATE OF A HANDLE (FORCDUP)                                         
                                                                                
PURPOSE                                                                         
                                                                                
Forces the handle in CX to refer to the same file at the same position as the   
handle in BX.                                                                   
                                                                                
EXAMPLE                                                                         
                                                                                
MOV     AH,46H          ; Function Call --                                      
; Force Duplicate a Handle                                                      
MOV     BX,Handle       ; Select file                                           
MOV     CX,NewHandle    ; Select new definition of File                         
INT     21H             ; Issue request to DOS                                  
JC      Error           ; Error code in AX                                      
                                                                                
                                                                                
Handle        DW      ?       ; File Handle (from Open/Create)                  
NewHandle     DW      ?       ; File Handle that duplicates Handle              
                                                                                
COMMENTS                                                                        
                                                                                
On entry, BX contains the file handle.  CX contains a second file handle.  On   
return, the CX file handle refers to the same file at the same position as      
the BX file handle.  If the CX file handle was an open file, it is closed       
first.  If you move the read/write pointer of either handle, the pointer for    
the other handle is also changed.                                               
                                                                                
Error codes are returned in AX.  Issue function call 59H (Get Extended Error)   
for additional information about the error class, suggested action, and         
location.                                                                       
                                                                                
47H --                                                                          
GET CURRENT DIRECTORY                                                           
                                                                                
PURPOSE                                                                         
                                                                                
Places the full path name (starting from the root directory) of the current     
directory for the specified drive in the area pointed to by DS:SI.              
                                                                                
EXAMPLE                                                                         
                                                                                
MOV     AX,SEG DName     ; Directory Name Buffer                                
MOV     DS,AX                                                                   
MOV     SI,OFFSET DName  ; OS:SI points to buffer                               
MOV     DL,Drive         ; Select Drive                                         
MOV     AH,47H           ; Function-Get Current Dir                             
INT     21H              ; Issue request to DOS                                 
JC      Error            ; Error code in AX                                     
                                                                                
                                                                                
Drive    DB  ?            ; Drive (0=current, 1=A:, 2=b:, ...)                  
DName    DB  64 DUP(?)    ; ASCIIZ Directory Name Returned                      
;   example: "dir1\dir2",0                                                      
                                                                                
COMMENTS                                                                        
                                                                                
The drive letter is not part of the returned string.  The string does not       
begin with a backslash and is terminated by a byte containing 00H.              
                                                                                
Error codes are returned in AX.  Issue function call 59H (Get Extended Error)   
for additional information about the error class, suggested action, and         
location.                                                                       
                                                                                
48H --                                                                          
ALLOCATE MEMORY                                                                 
                                                                                
Allocates the requested number of paragraphs of memory.                         
                                                                                
EXAMPLE                                                                         
                                                                                
MOV     AH,48H          ; Function Call --                                      
; Allocate Memory                                                               
MOV     BX,Paragraphs   ; Paragraphs Desired                                    
INT     21H             ; Issue request to DOS                                  
JNC     Done                                                                    
MOV     AH,48H          ; Function Call --                                      
; Allocate memory                                                               
; BX set to largest available memory                                            
INT     21H             ; Issue request to DOS                                  
Done:                                                                           
MOV     BlockSeg,AX     ; Save BlockSeg of memory                               
MOV     Paragraphs,BX                                                           
                                                                                
                                                                                
Paragraphs    DW      ?       ; Size requested in paragraphs                    
; (Bytes allocated is 16 * Paragraphs)                                          
BlockSeg      DW      ?       ; BlockSeg address of allocated memory            
                                                                                
COMMENTS                                                                        
                                                                                
On entry, BX contains the number of paragraphs requested.  On return, AX:0      
points to the allocated memory block.  If the allocation fails, BX returns      
the size of the largest block of memory available in paragraphs.                
                                                                                
Error codes are returned in AX.  Issue function call 59H (Get Extended Error)   
for additional information about the error class, suggested action, and         
location.                                                                       
                                                                                
49H --                                                                          
FREE ALLOCATED MEMORY                                                           
                                                                                
PURPOSE                                                                         
                                                                                
Frees the specified allocated memory.                                           
                                                                                
EXAMPLE                                                                         
                                                                                
MOV     AH,49H          ; Function Call -- Free Memory                          
MOV     ES,BlockSeg     ; Set address to free                                   
INT     21H             ; Issue request to DOS                                  
JC      Error                                                                   
                                                                                
                                                                                
BlockSeg       DW      ?      ; BlockSeg address of allocated memory            
                                                                                
COMMENTS                                                                        
                                                                                
On entry, ES contains the segment of the block to be returned to the system     
pool.  On return, the block of memory is returned to the system pool.           
                                                                                
Error codes are returned in AX.  Issue function call 59H (Get Extended Error)   
for additional information about the error class, suggested action, and         
location.                                                                       
                                                                                
4AH --                                                                          
MODIFY ALLOCATED MEMORY BLOCKS (SETBLOCK)                                       
                                                                                
PURPOSE                                                                         
                                                                                
Modifies allocated memory blocks to contain the new specified block size.       
                                                                                
EXAMPLE                                                                         
                                                                                
MOV     AH,4AH          ; Function Call --                                      
; Modify Allocated Memory; allocate memory                                      
MOV     ES,BlockSeg     ; Set address to free                                   
MOV     BX,BlockSize    ; New size (may be larger or smaller)                   
INT     21H             ; Issue request to DOS                                  
JNC     Done                                                                    
MOV     AH,4AH          ; Function Call --                                      
; Allocate memory                                                               
; BX set to largest available Size                                              
INT     21H             ; Issue request to DOS                                  
Done:                                                                           
MOV     Size,BX                                                                 
                                                                                
                                                                                
BlockSeg      DW      ?       ; Segment address of allocated memory             
BlockSize     DW      ?       ; Size requested in paragraphs                    
; (Bytes allocated is 16 * Size)                                                
                                                                                
COMMENTS                                                                        
                                                                                
Error codes are returned in AX.  Issue function call 59H (Get Extended Error)   
for additional information about the error class, suggested action, and         
location.                                                                       
                                                                                
NOTE:  This call is often used to set the size of a program before using        
function call 31H (Terminate Process and Remain Resident).  Use the             
program segment prefix.  This value can be obtained using function              
call 62H (Get Program Segment Prefix Address).  Another use is to               
release memory to prepare for using function call 4BH (Load or Execute          
a Program).                                                                     
                                                                                
4BH --                                                                          
LOAD OR EXECUTE A PROGRAM (EXEC)                                                
                                                                                
PURPOSE                                                                         
                                                                                
Allows a program to load another program into memory and may choose to begin    
execution of it.                                                                
                                                                                
EXAMPLE                                                                         
                                                                                
; To Execute a Program                                                          
                                                                                
MOV  AH,4BH                  ; Function Call -- Execute a Program               
MOV  AL,0                    ; Indicate execute program                         
MOV  CX,SEG Parms            ; Program parameters                               
MOV  ES,CX                                                                      
MOV  BX,OFFSET Parms         : ES:BX points to parameter block                  
MOV  CX,SEG PName            ; Program name                                     
MOV  DS,CX                                                                      
MOV  DX,OFFSET PName         ; DS:DX points to program name                     
MOV  WORD PTR StackSave+0,SP ; Save stack pointer                               
MOV  WORD PTR StackSave+2,SS                                                    
INT  21H                     ; Issue request to DOS                             
JC   Error                   ; Error code in AX                                 
; Note:  All Registers (except CS:IP) Destroyed                                 
                                                                                
; Program Runs here                                                             
                                                                                
CLI                          ; Protect from stack usage                         
MOV  SS,WORD PTR StackSave+2 ; Restore stack pointer                            
MOV  SP,WORD PTR StackSave+0                                                    
STI                          ; Enable interrupts                                
MOV  AH,4DH                  ; Function Call -- Get Return Code                 
INT  21H                     ; Issue request to DOS                             
MOV  RetCode,AX              ; Save return code                                 
                                                                                
; To Load an Overlay                                                            
MOV  AH,4BH                  ; Function Call -- Execute a Program               
MOV  AL,3                    ; Indicate load overlay                            
MOV  CX,SEG OParms           ; Overlay parameters                               
MOV  ES,CX                                                                      
MOV  BX,OFFSET OParms        ; ES:BX points to parameter block                  
MOV  CX,SEG PName            ; Overlay name                                     
MOV  DS,CX                                                                      
MOV  DX,OFFSET PName         ; DS:DX points to overlay filename                 
INT  21H                     ; Issue request to DOS                             
JC   Error                   ; Error code in AX                                 
                                                                                
PName      DB    64 Dup (0)   ; ASCIIZ Name                                     
;   example: "c:\dir\File.ext",0                                                
Parms      LABEL WORD         ; Program parameters                              
Env@       DW    ?            ; Environment segment address                     
;   Value of 0000H indicates copy EXEC'ers                                      
;   Environment                                                                 
Cmd@       DD    ?            ; Command line address                            
FCB1@      DD    ?            ; FCB Image to set to New PSP+5CH                 
FCB2@      DD    ?            ; FCB Image to set to New PSP+6CH                 
                                                                                
StackSave  DD    ?            ; Stack pointer save area                         
RetCode    DW    ?            ; Program return code                             
; (see function code 4DH for more informatio                                    
OParms     LABEL WORD         ; Overlay parameters                              
Load@      DW    ?            ; Overlay load segment address                    
RelocFactor DW    ?            ; Relocation factor to apply (for .EXE files     
                                                                                
COMMENTS                                                                        
                                                                                
Error codes are returned in AX.  Issue function call 59H (Get Extended Error)   
for additional information about the error class, suggested action, and         
location.  Refer to "Responding to Errors" on page B-5 and "Extended Error      
Codes" on page B-5 for more information on the codes returned from function     
call 59H.                                                                       
                                                                                
The following function values are allowed in AL:                                
                                                                                
+---------------------------------------------------------------------------+   
| FUNCTION                                                                  |   
| VALUE     DESCRIPTION                                                     |   
+---------------------------------------------------------------------------+   
| 00H       Load and execute the program.  A program segment prefix is      |   
|           established for the program; terminate and Ctrl-Break addresses |   
|           are set to the instruction after the EXEC system call.          |   
|                                                                           |   
|           NOTE:  When control is returned, all registers are changed,     |   
|                  including the stack.  You must restore SS, SP, and any   |   
|                  other required registers before proceeding.              |   
+---------------------------------------------------------------------------+   
| 03H       Load, do not create the program segment prefix, and do not      |   
|           begin execution.  This is useful in loading program overlays.   |   
+---------------------------------------------------------------------------+   
                                                                                
All open files of a process are duplicated in the newly created process after   
an EXEC, unless the file was opened with the inheritance bit set to 1.  This    
means that the parent process has control over the meanings of standard         
input, output, auxiliary, and printer devices.  The parent could, for           
example, write a series of records to a file, open the file as standard         
input, open a listing file as standard output, and then execute a sort          
program that takes its input from standard input and writes to standard         
output.                                                                         
                                                                                
Also inherited (or copied from the parent) is an "environment."  This is a      
block of text strings (less than 32KB total) that conveys various               
                                                                                
configuration parameters.  The following is the format of the environment       
(always on a paragraph boundary):                                               
                                                                                
+---------------------------------------------------------------------------+   
| Byte ASCIIZ string 1                                                      |   
+---------------------------------------------------------------------------+   
| Byte ASCIIZ string 2                                                      |   
+---------------------------------------------------------------------------+   
|       ...                                                                 |   
+---------------------------------------------------------------------------+   
| Byte ASCIIZ string n                                                      |   
+---------------------------------------------------------------------------+   
| Byte of 0                                                                 |   
+---------------------------------------------------------------------------+   
                                                                                
Typically the environment strings have the form:                                
                                                                                
parameter=value                                                                 
                                                                                
Following the byte of 0 in the environment is a WORD that indicates the         
number of other strings following.  Following this is a copy of the DS:DX       
filename passed to the child process.  For example, the string VERIFY=ON        
could be passed.  A 0 value of the environment address causes the newly         
created process to inherit the original environment unchanged.  The segment     
address of the environment is placed at offset 2CH of the program segment       
prefix for the program being invoked.                                           
                                                                                
Errors codes are returned in AX.  Refer to "Responding to Errors" on page B-5   
and "Extended Error Codes" on page B-5 for more information on the codes        
returned.                                                                       
                                                                                
NOTE:  When your program received control, all available memory was allocated   
to it.  You must free some memory (see call 4AH) before EXEC can load           
the program you are invoking.  Normally, you would shrink down to the           
minimum amount of memory you need, and free the rest.                           
                                                                                
4CH --                                                                          
TERMINATE A PROCESS (EXIT)                                                      
                                                                                
PURPOSE                                                                         
                                                                                
Terminates the current process and transfers control to the invoking process.   
                                                                                
EXAMPLE                                                                         
                                                                                
MOV     AH,4CH          ; Function Call -- Terminate a Process                  
MOV     AL,ErrorCode    ; Set ERRORLEVEL                                        
INT     21H             ; Issue request to DOS                                  
INT     20H             ; Be safe if running on PC/DOS 1.1                      
                                                                                
                                                                                
ErrorCode     DB      ?       ; Error Code (sets ERRORLEVEL if EXEC'ed          
; by COMMAND.COM)                                                               
                                                                                
COMMENTS                                                                        
                                                                                
In addition, a return code can be sent.  The return code can be interrogated    
by the batch subcommands IF and ERRORLEVEL and by the wait function call 4DH.   
All files opened by this process are closed.                                    
                                                                                
4DH --                                                                          
GET RETURN CODE OF A SUBPROCESS (WAIT)                                          
                                                                                
PURPOSE                                                                         
                                                                                
Gets the return code specified by another process either through function       
call 4CH or function call 31H.  It returns the Exit code only once.             
                                                                                
EXAMPLE                                                                         
                                                                                
MOV     AH,4DH          ; Function Call -- Get Return Code                      
INT     21H             ; Issue request to DOS                                  
MOV     RetCode,AX      ; Save return code                                      
                                                                                
RetCode       LABEL   WORD    ; Program return code                             
ExitCode      DB      ?       ; ERRORLEVEL value                                
ExitType      DB      ?       ; Method used to exit:                            
;   00H -- for normal termination                                               
;   01H -- for termination by Ctrl-Break                                        
;   02H -- for termination as a result                                          
;          of a critical device error                                           
;   03H -- for termination by call 31H                                          
                                                                                
COMMENTS                                                                        
                                                                                
The low byte of the exit code contains the information sent by the exiting      
routine.                                                                        
                                                                                
4EH --                                                                          
FIND FIRST MATCHING FILE (FIND FIRST)                                           
                                                                                
PURPOSE                                                                         
                                                                                
Finds the first filename that matches the specified file specification.         
                                                                                
EXAMPLE                                                                         
                                                                                
MOV     AH,1AH          ; Function Call -- Set DTA Address                      
MOV     CX,SEG DTA      ; Address buffer for found file                         
MOV     DS,CX                                                                   
MOV     DX,OFFSET DTA                                                           
INT     21H             ; Issue request to DOS                                  
MOV     AH,4EH          ; Function Call -- ASCIIZ Find First                    
MOV     CX,SEG FName    ; Directory or filename                                 
MOV     DS,CX                                                                   
MOV     DX,OFFSET FName ; DS:DX points to ASCII filename                        
MOV     CX,Attribute    ; Set Match Attribute                                   
INT     21H             ; Issue request to DOS                                  
JC      Error           ; Error code in AX                                      
                                                                                
                                                                                
DTA           LABEL   BYTE              ; Find return information               
DB      21 DUP(0)         ; Reserved for DOS 4.00 to continu                    
FileAttr      DB      ?                 ; Matched files attribute low byte      
FileTime      DW      ?                 ; File time                             
FileDate      DW      ?                 ; File date                             
FileSize      DD      ?                 ; File size                             
FileNameExt   DB      "????????.???",0  ; Filename and extension                
                                                                                
FName         DB      64 DUP (0)        ; ASCIIZ Name                           
;   example: "c:\dir\*.*",0                                                     
Attribute     DW      ?                 ; Select files attribute                
; Combination of following:                                                     
;  0002H=Hidden                                                                 
;  0004H=System                                                                 
;  0008H=Volume label                                                           
;  0010H=Directory                                                              
                                                                                
NOTES:                                                                          
                                                                                
If an extended FCB is used, the following search pattern is used:               
                                                                                
1.  If the attribute is 0, only normal file entries are found. Entries for      
volume label, sub-directories, hidden and system files are not returned.        
                                                                                
2.  If the attribute field is set for hidden or system files, or directory      
entries, it is to be considered as an inclusive search. All normal file         
entries plus all entries matching the specified attributes are returned.        
To look at all directory entries except the volume label, the attribute         
byte may be set to hidden + system + directory (all 3 bits on).                 
                                                                                
3.  If the attribute field is set for the volume label, it is considered an     
exclusive search, and only the volume label entry is returned.                  
                                                                                
COMMENTS                                                                        
                                                                                
The filename in DS:DX can contain global filename characters.  The ASCIIZ       
string cannot contain a network path.  See function call 11H (Search for        
First Entry) for a description of how the attribute bits are used for           
searches.                                                                       
                                                                                
Error codes are returned in AX.  Issue function call 59H (Get Extended Error)   
for additional information about the error class, suggested action, and         
location.  Refer to "Responding to Errors" on page B-5 and "Extended Error      
Codes" on page B-5 for more information on the codes returned from function     
call 59H.                                                                       
                                                                                
NOTE:  The name and extension of file found is returned as an ASCIIZ string.    
All blanks are removed from the name and extension, and, if an                  
extension is present, it is preceded by a period.                               
                                                                                
4FH --                                                                          
FIND NEXT MATCHING FILE (FIND NEXT)                                             
                                                                                
PURPOSE                                                                         
                                                                                
Finds the next directory entry matching the name that was specified on the      
previous Find First or Find Next function call.                                 
                                                                                
EXAMPLE                                                                         
                                                                                
MOV     AH,1AH          ; Function Call -- Set DTA Address                      
MOV     CX,SEG DTA      ; Address buffer for found file                         
MOV     DS,CX                                                                   
MOV     DX,OFFSET DTA                                                           
INT     21H             ; Issue request to DOS                                  
MOV     AH,4FH          ; Function Call -- Find next                            
INT     21H             ; Issue request to DOS                                  
JC      Error           ; Error code in AX                                      
                                                                                
                                                                                
DTA           LABEL   BYTE          "  ; Find Return Information                
DB      21 DUP(0)     "  ; Reserved for DOS to Continue Find                    
"  ; Set by Find First or Previous Fin                                          
FileAttr      DB      ?             "  ; Matched Files Attribute Low Byte       
FileTime      DW      ?             "  ; File Time                              
FileDate      DW      ?             "  ; File Date                              
FileSize      DD      ?             "  ; File Size                              
FileNameExt   DB      "????????.???",0 ; File Name and Extension                
                                                                                
COMMENTS                                                                        
                                                                                
If a matching file is found, the DTA is set as described in call 4EH (Find      
First Matching File (FIND FIRST)).  If no more matching files are found, an     
error code is returned.                                                         
                                                                                
Error codes are returned in AX.  Issue function call 59H (Get Extended Error)   
for additional information about the error class, suggested action, and         
location.                                                                       
                                                                                
54H --                                                                          
GET VERIFY SETTING                                                              
                                                                                
PURPOSE                                                                         
                                                                                
Returns the value of the verify flag.                                           
                                                                                
EXAMPLE                                                                         
                                                                                
MOV     AH,54H          ; Function Call -- Get VERIFY Setting                   
INT     21H             ; Issue request to DOS                                  
MOV     VERIFY,AL       ; Save VERIFY State                                     
                                                                                
VERIFY         DB      ?       ; VERIFY State:                                  
; 0 = OFF                                                                       
; 1 = ON                                                                        
                                                                                
COMMENTS                                                                        
                                                                                
On return, AL returns 00H if verify is OFF, 01H if verify is ON.  Note that     
the verify switch can be set through call 2EH (Set/Reset Verify Switch).        
                                                                                
56H --                                                                          
RENAME A FILE                                                                   
                                                                                
PURPOSE                                                                         
                                                                                
Renames the specified file.                                                     
                                                                                
EXAMPLE                                                                         
                                                                                
MOV     AH,56H              ; Function Call -- ASCIIZ Rename File               
MOV     CX,SEG FName        ; File Name                                         
MOV     DS,CX                                                                   
MOV     DX,OFFSET FName     ; DS:DX points to original name                     
MOV     CX,SEG NewName      ; New File Name                                     
MOV     ES,CX                                                                   
MOV     DI,OFFSET NewName   ; ES:DI points to rename                            
INT     21H                 ; Issue request to DOS                              
JC      Error               ; Error code in AX                                  
                                                                                
                                                                                
FName         DB      64 DUP (0)    ; ASCIIZ Name                               
;   example: "c:\dir\abc.lst",0                                                 
NewName       DB      64 DUP (0)    ; ASCIIZ Name                               
;   example: "\dir\xyz.lst",0                                                   
                                                                                
COMMENTS                                                                        
                                                                                
If a drive is used in the NewName string, it must be the same as the drive      
specified or implied in the Name string.  The directory paths need not be the   
same, allowing a file to be moved to another directory and renamed in the       
process.  Directory names can be changed but not moved.  Global filename        
characters are not allowed in the filename.                                     
                                                                                
Error codes are returned in AX.  Issue function call 59H (Get Extended Error)   
for additional information about the error class, suggested action, and         
location.  Refer to "Responding to Errors" on page B-5 and "Extended Error      
Codes" on page B-5 for more information on the codes returned from function     
call 59H.                                                                       
                                                                                
Network Access Rights:  Requires Create access rights.                          
                                                                                
57H --                                                                          
GET/SET FILE'S DATE AND TIME                                                    
                                                                                
PURPOSE                                                                         
                                                                                
Gets or sets a file's date and time.                                            
                                                                                
EXAMPLE                                                                         
                                                                                
; To Get a File's Date and Time                                                 
                                                                                
MOV     AH,57H          ; Function Call -- Get/Set Date and Time                
MOV     AL,0            ; Indicate get                                          
MOV     BX,Handle       ; Select file                                           
INT     21H             ; Issue request to DOS                                  
JC      Error           ; Error code in AX                                      
MOV     FileTime,CX     ; Save Time                                             
MOV     FileDate,DX     ; Save Date                                             
                                                                                
; To Set a File's Date and Time                                                 
                                                                                
MOV     AH,57H          ; Function Call -- Get/Set Date and Time                
MOV     AL,1            ; Indicate Set                                          
MOV     BX,Handle       ; Select file+                                          
MOV     CX,FileTime     ; Set Time                                              
MOV     DX,FileDate     ; Set Date                                              
INT     21H             ; Issue request to DOS                                  
JC      Error           ; Error code in AX                                      
                                                                                
                                                                                
Handle        DW      ?       ; File Handle (from Open / Create)                
FileTime      DW      ?       ; File Time                                       
FileDate      DW      ?       ; File Date                                       
                                                                                
COMMENTS                                                                        
                                                                                
The date and time formats are the same as those for the directory entry         
described in Chapter 5 of this book.                                            
                                                                                
Error codes are returned in AX.  Issue function call 59H (Get Extended Error)   
for additional information about the error class, suggested action, and         
location.  Refer to "Responding to Errors" on page B-5 and "Extended Error      
Codes" on page B-5 for more information on the codes returned from function     
call 59H.                                                                       
                                                                                
59H --                                                                          
GET EXTENDED ERROR                                                              
                                                                                
PURPOSE                                                                         
                                                                                
Returns additional error information, such as the error class, location, and    
recommended action.                                                             
                                                                                
EXAMPLE                                                                         
                                                                                
PUSH    DX              ; Save Registers                                        
PUSH    SI                                                                      
PUSH    DI                                                                      
PUSH    ES                                                                      
PUSH    DS                                                                      
MOV     AH,59H          ; Function Call -- Get Extended Error                   
MOV     BX,0            ; Version 0 information                                 
INT     21H             ; Issue request to DOS                                  
POP     DS              ; Restore registers                                     
POP     ES                                                                      
POP     DI                                                                      
POP     SI                                                                      
POP     DX                                                                      
MOV     ExtError,AX     ; Save error code                                       
MOV     ErrorClass,BH   ; Save error class                                      
MOV     ErrorAction,BL  ; Save error action                                     
MOV     ErrorLocation,CH; Save error location                                   
                                                                                
                                                                                
ExtError      DW      ?       ; DOS extended error                              
ErrorClass    DB      ?       ; Class of error                                  
ErrorAction   DB      ?       ; Suggested action                                
ErrorLocation DB      ?       ; System area effected                            
                                                                                
COMMENTS                                                                        
                                                                                
This function call returns the error class, location, and recommended action,   
in addition to the return code.  Use this function call from:                   
                                                                                
    Interrupt 24H error handlers                                                
    Interrupt 21H function calls that return an error in the carry bit          
    FCB function calls that return FFH.                                         
                                                                                
On return, the registers contents of DX, SI, DI, ES, CL, and DS are             
destroyed.                                                                      
                                                                                
ERROR RETURN IN CARRY BIT                                                       
                                                                                
For function calls that indicate an error by setting the carry flag, the        
correct method for performing function call 59H is:                             
                                                                                
    Load registers.                                                             
    Issue interrupt 21H.                                                        
    Continue operation, if carry not set.                                       
    Disregard the error code and issue function call 59H to obtain additional   
information.                                                                    
    Use the value in BL to determine the suggested action to take.              
                                                                                
ERROR STATUS IN AL                                                              
                                                                                
For function calls that indicate an error by setting AL to FFH, the correct     
method for performing function call 59H is:                                     
                                                                                
    Load registers.                                                             
                                                                                
    Issue interrupt 21H.                                                        
    Continue operation, if error is not reported in AL.                         
    Disregard the error code and issue function call 59H to obtain additional   
information.                                                                    
    Use the value in BL to determine the suggested action to take.              
                                                                                
5AH --                                                                          
CREATE UNIQUE FILE                                                              
                                                                                
PURPOSE                                                                         
                                                                                
Generates a unique filename, and creates that file in the specified             
directory.                                                                      
                                                                                
EXAMPLE                                                                         
                                                                                
MOV     AH,5AH                 ; Function Call -- Create a                      
; Unique File                                                                   
MOV     CX,SEG DirName         ; Directory name                                 
MOV     DS,CX                                                                   
MOV     DX,OFFSET DirName                                                       
MOV     CX,Attribute           ; File attribute                                 
INT     21H                    ; Issue request to DOS                           
JC      Error                  ; Error code in AX                               
MOV     Handle,AX              ; Save file handle for following                 
; Operations                                                                    
                                                                                
                                                                                
DirName       DB      "?? .. ??\",0  ; ASCIIZ name                              
DB      "????????.???"                                                          
;   example in : "c:\dir\",0                                                    
;   example out: "c:\dir\file",0                                                
Handle        DW      ?              ; File handle                              
Attribute     DW      ?              ; Select file's attribute                  
                                                                                
COMMENTS                                                                        
                                                                                
On entry, AH contains 5AH.  If no error has occurred, the file is opened in     
compatibility mode with Read/Write access.  The read/write pointer is set at    
the first byte of the file and AX contains the file handle and the filename     
is appended to the path specified in DS:DX.                                     
                                                                                
This function call generates a unique name and attempts to create a new file    
in the specified directory.  If the file already exists in the directory,       
then another unique name is generated and the process is repeated.  Programs    
that need temporary files should use this function call to generate unique      
filenames.                                                                      
                                                                                
Error codes are returned in AX.  Issue function call 59H (Get Extended Error)   
for additional information about the error class, suggested action, and         
location.  Refer to "Responding to Errors" on page B-5 and "Extended Error      
                                                                                
Codes" on page B-5 for more information on the codes returned from function     
call 59H.                                                                       
                                                                                
NOTE:  The file created using this function call is not automatically deleted   
at program termination.                                                         
                                                                                
Network Access Rights:  Requires Create access rights.                          
                                                                                
5BH --                                                                          
CREATE NEW FILE                                                                 
                                                                                
PURPOSE                                                                         
                                                                                
Creates a new file.                                                             
                                                                                
EXAMPLE                                                                         
                                                                                
MOV     AH,5BH          ; Function Call -- Create a New File                    
MOV     CX,SEG FName    ; File Name                                             
MOV     DS,CX                                                                   
MOV     DX,OFFSET FName                                                         
MOV     CX,Attribute                                                            
INT     21H             ; Issue request to DOS                                  
JC      Error           ; Error code in AX                                      
MOV     Handle,AX       ; Save File Handle for following operations             
                                                                                
                                                                                
FName         DB      64 DUP (0)    ; ASCIIZ Name                               
;   example: "c:\dir\file",0                                                    
Handle        DW      ?             ; File Handle                               
Attribute     DW      ?             ; Select File's Attribute                   
                                                                                
COMMENTS                                                                        
                                                                                
This function call is the same as function call 3CH (Create), except it will    
fail if the filename already exists.  The file is created in compatibility      
mode for reading and writing and the read/write pointer is set at the first     
byte of the file.                                                               
                                                                                
Error codes are returned in AX.  Issue function call 59H (Get Extended Error)   
for additional information about the error class, suggested action, and         
location.  Refer to "Responding to Errors" on page B-5 and "Extended Error      
Codes" on page B-5 for more information on the codes returned from function     
call 59H.                                                                       
                                                                                
Network Access Rights:  Requires Create access rights.                          
                                                                                
5CH --                                                                          
LOCK/UNLOCK FILE ACCESS                                                         
                                                                                
PURPOSE                                                                         
                                                                                
Locks or unlocks a single range of bytes in an opened file.  This function      
call provides database services that are useful in maintaining database         
integrity in a network environment.                                             
                                                                                
EXAMPLE                                                                         
                                                                                
; To Lock a Single Range                                                        
                                                                                
MOV     AH,5CH                  ; Function Call --                              
; Access Lock/Unlock File                                                       
MOV     AL,0                    ; Indicate lock                                 
MOV     BX,Handle               ; Select file                                   
MOV     DX,WORD PTR Position+0  ; Set position                                  
MOV     CX,WORD PTR Position+2                                                  
MOV     DI,WORD PTR Llength+0   ; Set length                                    
MOV     SI,WORD PTR Llength+2                                                   
INT     21H                     ; Issue request to DOS                          
JC      Error                   ; Error code in AX                              
                                                                                
; To Unlock a Single Range                                                      
                                                                                
MOV     AH,5CH                  ; Function Call --                              
; Lock/Unlock File Access                                                       
MOV     AL,1                    ; Indicate unlock                               
MOV     BX,Handle               ; Select file                                   
MOV     DX,WORD PTR Position+0  ; Set position                                  
MOV     CX,WORD PTR Position+2                                                  
MOV     DI,WORD PTR Llength+0   ; Set length                                    
MOV     SI,WORD PTR Llength+2                                                   
INT     21H                     ; Issue request to DOS                          
JC      Error                   ; Error code in AX                              
                                                                                
                                                                                
Handle        DW      ?               ; File Handle                             
Position      DD      ?               ; Start of Range                          
Llength       DD      ?               ; Length of Range                         
                                                                                
COMMENTS                                                                        
                                                                                
The Lock/Unlock function calls should only be used when a file is opened        
using the DenyRead or DenyNone sharing modes.  These modes do no local          
buffering of data when accessing files on a network disk.                       
                                                                                
AL = 00H  LOCK                                                                  
                                                                                
Provides a simple mechanism for excluding other processes' read/write access    
to regions of the file.  If another process attempts to read or write in such   
a region, its system call is retried the number of times specified with the     
system retry count set by IOCTL.  If, after those retries, no success occurs,   
a general failure error is generated, signaling the condition.  The number of   
                                                                                
retries, as well as the length of time between retries, can be changed using    
function call 440BH (IOCTL Change Sharing Retry Count).                         
                                                                                
The recommended action is to issue function call 59H (Get Extended Error) to    
get the error code, in addition to the error class, location, and recommended   
action.  The locked regions can be anywhere in the logical file.  Locking       
beyond end-of-file is not an error.  It is expected that the time in which      
regions are locked will be short.  Duplicating the handle duplicates access     
to the locked regions.  Access to the locked regions is not duplicated across   
the EXEC system call.  Exiting with a file open and having issued locks on      
that file has undefined results.                                                
                                                                                
Programs that may be cancelled using INT 23H (File Size) or INT 24H (Set        
Relative Record Field) should trap these and release the locks before           
exiting.  The proper method for using locks is not to rely on being denied      
read or write access, but to attempt to lock the region desired and examining   
the error code.                                                                 
                                                                                
AL = 01H  UNLOCK                                                                
                                                                                
Unlock releases the lock issued in the lock system call.  The region            
specified must be exactly the same as the region specified in the previous      
lock.  Closing a file with locks still in force has undefined results.          
Exiting with a file open and having issued locks on that file has undefined     
results.                                                                        
                                                                                
Programs that may be aborted using INT 23H (File Size) or INT 24H (Set          
Relative Record Field) should trap these and release the lock before exiting.   
The proper method for using locks is not to rely on being denied read or        
write access, but attempting to lock the region desired and examining the       
error code.                                                                     
                                                                                
Error codes are returned in AX.  Issue function call 59H (Get Extended Error)   
for additional information about the error class, suggested action, and         
location.  Refer to "Responding to Errors" on page B-5 and "Extended Error      
Codes" on page B-5 for more information on the codes returned from function     
call 59H.                                                                       
                                                                                
5E00H --                                                                        
GET MACHINE NAME                                                                
                                                                                
PURPOSE                                                                         
                                                                                
Returns the character identifier of the local computer.                         
                                                                                
EXAMPLE                                                                         
                                                                                
MOV    AX,SEG CNAME      ; Name buffer                                          
MOV    DS,AX                                                                    
MOV    DX OFFSET CNAME                                                          
MOV    AX,5E00H          ; Function Call --                                     
; Get Machine Name                                                              
INT    21H               ; Issue request to DOS                                 
JC     Error             ; Error code in AX                                     
MOV    NameFlag,CH       ; Save name number indicator                           
MOV    NameID,CL         ; Save NETBIOS name number                             
                                                                                
                                                                                
CName           DB      "???????????????",0    ; ASCIIZ computer name           
NameFlag        DB      ?                      ; 0 = Name is not set            
; 1 = Name is set                                                               
NameID          DB      ?                      ; NETBIOS name number            
                                                                                
COMMENTS                                                                        
                                                                                
Get Machine Name returns the text of the current computer name to the caller.   
The computer name is a 15-character byte string padded with spaces and          
followed by a 00H byte.  If the computer name was never set, register CH is     
returned with 00H and the value in the CL register is invalid.  The IBM PC      
Local Area Network program must be loaded for the function call to execute      
properly.                                                                       
                                                                                
5E02H --                                                                        
SET PRINTER SETUP                                                               
                                                                                
PURPOSE                                                                         
                                                                                
Specifies an initial string for printer files.                                  
                                                                                
EXAMPLE                                                                         
                                                                                
MOV     AX,5E02H        ; Function Call --                                      
; Set Printer Setup                                                             
MOV     BX,Index        ; Redirection List Index                                
MOV     CX,size         ; String size                                           
MOV     SI,SEG String   ; String Buffer                                         
MOV     DS,SI                                                                   
MOV     SI,OFFSET String                                                        
INT     21H             ; Issue request to DOS                                  
JC      Error           ; Error code in AX                                      
                                                                                
                                                                                
Index         DW     ?        ; Redirection List Index                          
Size          DW     N        ; String size (Maximum 64)                        
String        DB     N DUP(?) ; Printer Setup String                            
                                                                                
COMMENTS                                                                        
                                                                                
The string specified is put in front of all files destined for a particular     
network printer.  Set Printer Setup allows multiple users of a single printer   
to specify their own mode of operation for the printer.  BX is set to the       
same index that is used in function call 5F02H (Get Redirection List Entry).    
An error code is returned if print redirection is paused or if the IBM PC       
Local Area Network program is not loaded.                                       
                                                                                
Error codes are returned in AX.  Issue function call 59H (Get Extended Error)   
for additional information about the error class, suggested action, and         
location.  Refer to "Responding to Errors" on page B-5 and "Extended Error      
Codes" on page B-5 for more information on the codes returned from function     
call 59H.                                                                       
                                                                                
IMPORTANT: The redirection index value may change if function call 5F03H        
(Redirect Device) or function call 5F04H (Cancel Redirection) is issued         
between the time the redirection list is scanned and the function call 5E02H    
(Set Printer Setup) is issued.  Therefore, we recommend that you issue Set      
Printer Setup immediately after you issue "Get Redirection List ."              
                                                                                
5E03H --                                                                        
GET PRINTER SETUP                                                               
                                                                                
PURPOSE                                                                         
                                                                                
Returns the printer setup string for printer files.                             
                                                                                
EXAMPLE                                                                         
                                                                                
MOV     AX,5E03H        ; Function Call --                                      
; Get Printer Setup                                                             
MOV     BX,Index        ; Redirection List Index                                
MOV     CX,SEG String   ; String Buffer                                         
MOV     ES,CX                                                                   
MOV     DI,OFFSET String                                                        
INT     21H             ; Issue request to DOS                                  
JC      Error           ; Error code in AX                                      
MOV     Ssize, CX       ; Save String size                                      
                                                                                
                                                                                
Index         DW    ?         ; Redirection List Index                          
Ssize         DW    ?         ; String size                                     
String        DB    64 DUP(?) ; Printer Setup String                            
                                                                                
COMMENTS                                                                        
                                                                                
This function call returns the printer setup string which was specified using   
the function call 5E02H (Set Printer Setup).  The setup string is attached to   
all files destined for a particular printer.  The value in BX is set to the     
same index issued in function call 5F02H (Get Redirection List).  Error code    
1 (invalid function number) is returned if the IBM PC Local Area Network is     
not loaded.                                                                     
                                                                                
Error codes are returned in AX.  Issue function call 59H (Get Extended Error)   
for additional information about the error class, suggested action, and         
location.  Refer to "Responding to Errors" on page B-5 and "Extended Error      
Codes" on page B-5 for more information on the codes returned from function     
call 59H.                                                                       
                                                                                
IMPORTANT: The redirection index value may change if function call 5F03H        
(Redirect Device) or function call 5F04H (Cancel Redirection) is issued         
between the time the redirection list is scanned and the function call 5E03H    
(Get Printer Setup) is issued.  Therefore, we recommend that you issue "Get     
Printer Setup" immediately after you issue "Get Redirection List."              
                                                                                
5F02H --                                                                        
GET REDIRECTION LIST ENTRY                                                      
                                                                                
PURPOSE                                                                         
                                                                                
Returns nonlocal network assignments.                                           
                                                                                
EXAMPLE                                                                         
                                                                                
MOV      BX,0               ; Start at beginning of list                        
Get_Loop:                        ; Get next entry                               
MOV      Index,BX           ; Redirection list index                            
MOV      AX,5F02H           ; Function Call --                                  
; Get redirection list entry                                                    
MOV      SI,SEG device      ; "PRN" is possible                                 
MOV      DS,SI                                                                  
MOV      SI,OFFSET device   ; DS:SI points to local name                        
MOV      DI,SEG info                                                            
MOV      ES,DI                                                                  
MOV      DI,OFFSET inf      ; ES:DI points to buffer address of netwo           
PUSH     BX                                                                     
PUSH     DX                 ; Save registers                                    
PUSH     BP                                                                     
INT      21H                ; Issue request to DOS                              
POP      BP                 ; Restore registers                                 
POP      DX                                                                     
JC       CheckEnd           ; Error code in AX                                  
MOV      Status,BH          ; Save status                                       
MOV      Type,BL            ; Save type                                         
MOV      UserParm,CX        ; Save user parmameter                              
POP      BX                                                                     
INC      BX                 ; Set to next entry                                 
JMP      Get_Loop                                                               
                                                                                
CheckEnd:                                                                       
POP     BX                 ; Balance state                                      
CMP     AX,18              ; End of list?                                       
JNE     Error              ; No!                                                
                                                                                
                                                                                
Index       DW      ?               ; Redirection list index (0 based)          
Device      DB      128 DUP(?)      ; ASCIIZ device name                        
;  example: "LPT1",0                                                            
;               "A:",0                                                          
Status      DB      ?               ; Device status                             
; Bit 0=0 : Device is OK                                                        
; Bit 0=1 : Device in Error                                                     
; Bit 7-1 reserved                                                              
UserParm    DW      ?               ; User parameter                            
Type        DB      ?               ; Device type                               
; 3 = NET USE device                                                            
; 4 = NET USE drive                                                             
Info        DB      128 DUP(?)      ; NET USE network path                      
;   example: "\\MYNODE\CDRIVE",0                                                
                                                                                
COMMENTS                                                                        
                                                                                
The Get Redirection List Entry function call returns the list of network        
redirections that were created through function call 5F03H (Redirect Device).   
Each call returns one redirection, so BX should be increased by one each time   
to step through the list.  The contents of the list may change between calls.   
The end-of-list is detected by error code 18 (no more files).  Error code 1     
(invalid function number) is returned if the IBM PC Local Area Network          
program is not loaded.                                                          
                                                                                
If either disk or print redirection is paused, the function is not effected.    
                                                                                
Error codes are returned in AX.  Issue function call 59H (Get Extended Error)   
for additional information about the error class, suggested action, and         
location.  Refer to "Responding to Errors" on page B-5 and "Extended Error      
Codes" on page B-5 for more information on the codes returned from function     
call 59H.                                                                       
                                                                                
5F03H --                                                                        
REDIRECT DEVICE                                                                 
                                                                                
PURPOSE                                                                         
                                                                                
Causes a Redirector/Server connection to be made.                               
                                                                                
EXAMPLE                                                                         
                                                                                
MOV     AX,5F03H              ; Function Call --                                
; Redirect Device                                                               
MOV     SI,SEG Device         ; Device Buffer                                   
MOV     DS,SI                                                                   
MOV     SI,OFFSET Device                                                        
MOV     DI,SEG Net Path info  ; Information Buffer                              
MOV     ES,DI                                                                   
MOV     DI,OFFSET Net Path                                                      
MOV     BL,Type               ; Set Type                                        
MOV     CX,UserParm           ; Set User Parameter                              
INT     21H                   ; Issue request to DOS                            
JC      Error                 ; Error code in AX                                
                                                                                
                                                                                
Device        DB      "....",0      ; ASCIIZ Device Name                        
;  example: "LPT1",0                                                            
;               "A:",0                                                          
UserParm      DW      ?             ; User Parameter                            
Type          DB      ?             ; Device Type                               
; 3 = NET USE Device                                                            
; 4 = NET USE Drive                                                             
Net Path      DB     128 DUP(0)                                                 
                                                                                
COMMENTS                                                                        
                                                                                
This call defines the current directories for the network and defines           
redirection of network printers.                                                
                                                                                
    If BL = 03, the source specifies a printer, the destination specifies a     
network path, and the CX register has a word that DOS 4.00 maintains for        
the programmer.  For compatibility with the IBM PC Local Area Network           
program, CX should be set to 0.  Values other than 0 are reserved for the       
IBM PC Local Area Network program.  This word may be retrieved through          
function call 5F02H (Get Redirection List).  All output destined for the        
specified printer is buffered and sent to the remote printer spool for          
that device.  The printers are redirected at the INT 17H level.                 
                                                                                
The source string must be PRN , LPT1, LPT2, or LPT3 each ended with a           
00H.  The destination string must point to a network name string of the         
following form:                                                                 
                                                                                
[\\computername\¤shortname|printdevice‡]                                        
                                                                                
The destination string must be ended with a 00H.                                
                                                                                
The ASCIIZ password (0 to 8 characters) for access to the remote device         
should immediately follow the network string.  The password must end with       
a 00H.  A null (0 length) password is considered to be no password.             
                                                                                
    If BL = 4, the source specifies a drive letter and colon ending with 00H,   
the destination specifies a network path ending with 00H, and the CX            
register has a word that DOS maintains for the programmer.  For                 
compatibility with the IBM PC Local Area Network program, CX should be          
set to 00H.  Values other than 00H are reserved for the IBM PC Local Area       
Network program.  The value may be retrieved through function call 5F02H        
(Get Redirection List).  If the source was a drive letter, the                  
association is made between the drive letter and the network path.  All         
subsequent references to the drive letter are translated to references to       
the network path.  If the source is an empty string, the system attempts        
to grant access to the destination with the specified password without          
redirecting any device.                                                         
                                                                                
The ASCIIZ password for access to the remote path should immediately            
follow the network string.  A null (0 length) password ended with 00H is        
considered to be no password.                                                   
                                                                                
Error codes are returned in AX.  Issue function call 59H for additional         
information about the error class, suggested action, and location.  Refer       
to "Responding to Errors" on page B-5 and "Extended Error Codes" on             
page B-5 for more information on the codes returned from function call          
59H (Get Extended Error) .                                                      
                                                                                
NOTES:                                                                          
                                                                                
1.  Devices redirected through this function call are not displayed by the      
NET USE command.                                                                
                                                                                
2.  An error is returned if you try to redirect a drive while disk              
redirection is paused, or if you try to redirect a printer while print          
redirection is paused.                                                          
                                                                                
5F04H --                                                                        
CANCEL REDIRECTION                                                              
                                                                                
PURPOSE                                                                         
                                                                                
Cancels a previous redirection.                                                 
                                                                                
EXAMPLE                                                                         
                                                                                
MOV     AX,5F04H        ; Function Call --                                      
; Cancel redirection                                                            
MOV     SI,SEG Device   ; Device buffer                                         
MOV     DS,SI                                                                   
MOV     SI,OFFSET Device                                                        
INT     21H             ; Issue request to DOS                                  
JC      Error           ; Error code in AX                                      
                                                                                
                                                                                
Device        DB      "....",0        ; ASCIIZ Device Name                      
;  example: "LPT1",0                                                            
;               "A:",0                                                          
;               "\\Computer\Path",0                                             
                                                                                
COMMENTS                                                                        
                                                                                
The redirection created by the Redirect Device function call (5F03H) is         
removed through the Cancel Redirection call.  If the buffer points to a drive   
letter and the drive is associated with a network name, the association is      
ended and the drive is restored to its physical meaning.  If the buffer         
points to PRN, LPT1, LPT2, or LPT3, and the device has an association with a    
network device, the association is terminated and the device is restored to     
its physical meaning.  If the buffer points to a network path ending with 00H   
and a password ending with 00H, the association between the local machine and   
the network directory is terminated.                                            
                                                                                
An error is returned if you try to cancel a redirected file device while disk   
redirection is paused, or if you try to cancel a redirected printer while       
print redirection is paused.  Error code 1 (invalid function number) is         
returned if the IBM PC Local Area Network program is not loaded.                
                                                                                
Error codes are returned in AX.  Issue function call 59H (Get Extended Error)   
for additional information about the error class, suggested action, and         
location.  Refer to "Responding to Errors" on page B-5 and "Extended Error      
Codes" on page B-5 for more information on the codes returned from function     
call 59H.                                                                       
                                                                                
62H --                                                                          
GET PROGRAM SEGMENT PREFIX ADDRESS                                              
                                                                                
PURPOSE                                                                         
                                                                                
Returns the program prefix address.                                             
                                                                                
EXAMPLE                                                                         
                                                                                
MOV     AH,62H          ; Function Call --                                      
; Get Program Segment Prefix Address                                            
INT     21H             ; Issue request to DOS                                  
JC      Error           ; Error code in AX                                      
MOV     PSPSeg,BX       ; Save PSP address                                      
                                                                                
                                                                                
PSPSeg        DW      ?       ; Segment address of my PSP                       
                                                                                
COMMENTS                                                                        
                                                                                
The internal PSP address for the currently executing process is returned in     
BX.                                                                             
                                                                                
65H --                                                                          
GET EXTENDED COUNTRY INFORMATION                                                
                                                                                
PURPOSE                                                                         
                                                                                
Returns extended country information.                                           
                                                                                
EXAMPLE                                                                         
                                                                                
; To get information                                                            
                                                                                
MOV     AH,65H          ; Function Call --                                      
; Get extended country information                                              
MOV     AL,InfoID       ; Data/function requested                               
; (1, 2, 4, 6 or 7).                                                            
MOV     BX,CodePage     ; Set desired code page                                 
; (-1=current, Set by function call 6602H).                                     
MOV     CX,SizeBuffer   ; Maximum data to return                                
; (must be >= 5)                                                                
MOV     DX,CountryID    ; Set desired Country ID                                
; (-1=current, set by function call 38H).                                       
MOV     DI,SEG Buffer   ; Information return buffer                             
MOV     DS,DI                                                                   
MOV     DI,OFFSET Buffer                                                        
INT     21H             ; Issue request to DOS                                  
JC      Error           ; Error code in AX                                      
                                                                                
                                                                                
Buffer        LABEL   BYTE                                                      
; Format depends on AL value                                                    
; AL=1 : Extended Country Information                                           
                                                                                
SizeBuffer    DW      ?       ; Size of data block to return                    
InfoID        DB      ?       ; Type of info to get                             
CIinfoSize    DW      ?       ; amount of data that follows                     
; (limited by CX input)                                                         
CountryID     DW      ?       ; Selected CountryID                              
CodePage      DW      ?       ; Selected Code Page                              
; See function call 38H for the format of the remainder of this buffer          
                                                                                
                                                                                
; AL=2 : Upper Case Table                                                       
DB      2               ; Indicates Upper Case Table                            
UpperCase@    DD      ?               ; Address of Upper Case Table             
                                                                                
; AL=4 : File Upper Case Table                                                  
DB      4               ; Indicates File Upper Case Table                       
UpperCase@    DD      ?               ; Address of File Upper Case Table        
                                                                                
; AL=6 : Collating Table                                                        
DB      6               ; Indicates Collate Table                               
Collate@      DD      ?               ; Address of Collate Table                
                                                                                
; AL=7 : DBCS Vector Table                                                      
DB      7               ; Indicates DBCS Vector Table                           
DBCS@         DD      ?               ; Address of DBCS Vector Table            
                                                                                
COMMENTS                                                                        
                                                                                
On entry, DX contains the ID of the country for which the extended              
information is needed.  AL contains the ID value for the country.               
                                                                                
    If the country code and code page do not match, or if either one or both    
are invalid, an error code of 2 (file not found) is returned in AX.             
    The size requested in CX must be 5 or greater.  If it is less than 5, an    
error code of 1 is returned in AX.                                              
    If the amount of information returned is greater than the size requested    
in CX, it is ended and no error is returned in AX.                              
                                                                                
NOTE:  For further information on the country information, see function call    
38H (Get or Set Country Dependent Information).                                 
                                                                                
The NLSFUNC DOS extension must be installed to get information for              
countries other than the Current Country.                                       
                                                                                
The uppercase table and the filename uppercase tables are 130 bytes long,       
consisting of a length field (2 bytes), followed by 128 uppercase values for    
the upper 128 ASCII characters.  They have the following layout:                
                                                                                
Tsize         DW      128             ; Table Size                              
Table         DB      128 DUP(?)      ; Upper case versions of 80H to FFH       
                                                                                
The following formula can be used to determine the address of an uppercase      
equivalent for a lowercase character (ASCII in) in the uppercase table or the   
filename uppercase table.                                                       
                                                                                
EXAMPLE                                                                         
                                                                                
ASCII in -(256-table len)+table start= address of ASCII out                     
                                                                                
WHERE                                                                           
                                                                                
ASCII in    = character to be generated                                         
                                                                                
table len   = length of list of uppercase                                       
values (2 bytes)                                                                
                                                                                
table start = starting address of uppercase                                     
table (4 bytes)                                                                 
                                                                                
ASCII out   = uppercase value for ASCII in                                      
                                                                                
If the value of ASCII in is equal to or greater than (256-table len), there     
is an uppercase equivalent for ASCII in in the table.  If it is lower than      
(256-table len), no uppercase equivalent exists in the table.                   
                                                                                
The collate table is 258 bytes long, consisting of a length field (2 bytes)     
followed by 256 ASCII values, in the appropriate order.  It has the following   
layout:                                                                         
                                                                                
Tsize         DW      256             ; Table Size                              
Table         DB      256 DUP(?)      ; Sort Weights for 00H to FFH             
                                                                                
The DBCS vector is variable in length, consisting of a length field (two        
bytes) followed by one or more pairs of bytes.  It has the following layout:    
                                                                                
Tsize   DW    Nx2          ;List size                                           
1       DB    Start,end    ;DBCS vector 1                                       
2       DB    Start,end    ;DBCS vector 2                                       
:                                                                               
N       DB    Start,end    ;DBCS vector n                                       
DB    0,0          ;End marker                                                  
                                                                                
66H --                                                                          
GET/SET GLOBAL CODE PAGE                                                        
                                                                                
PURPOSE                                                                         
                                                                                
This function gets or sets the code page for the current country.               
                                                                                
MOV     AX,6601H        ; Function Call --                                      
; Get Global Code Page                                                          
INT     21H             ; Issue request to DOS                                  
JC      Error           ; Error code in AX                                      
MOV     GlobalCP,BX     ; Save Global Code Page                                 
MOV     SystemCP,DX     ; Save DOS System Code Page                             
                                                                                
or                                                                              
                                                                                
MOV     AX,6602H        ; Function Call --                                      
; Set Global Code Page                                                          
MOV     BX,GlobalCP     ; New Global Code Page                                  
INT     21H             ; Issue request to DOS                                  
JC      Error           ; Error code in AX                                      
                                                                                
                                                                                
GlobalCP      DW      ?       ; Current Code Page of DOS Country Informati      
SystemCP      DW      ?       ; Code Page of DOS messages                       
; Often the default Code Page for the                                           
; current country                                                               
                                                                                
COMMENTS                                                                        
                                                                                
DOS 4.00 moves the new code page data from the COUNTRY.SYS file to a resident   
country buffer area.  DOS 4.00 uses the new code page to perform a Select to    
all attached devices that are set up for code page switching, (that is, have    
a code page switching device driver specified in CONFIG.SYS).  If any device    
fails to be selected, an error code of 65 is returned in AX.  The code page     
must be recognizable by the current country, and DOS 4.00 must be able to       
open and read from the country information file.  Otherwise, the carry flag     
will be set on return and AX will contain 02 (file not found).                  
                                                                                
NOTE:  NLSFUNC must be installed to use this function call, and all the         
devices must be prepared in order for the Select function to be                 
successful.                                                                     
                                                                                
67H --                                                                          
SET HANDLE COUNT                                                                
                                                                                
PURPOSE                                                                         
                                                                                
Permits more than 20 open files per process.                                    
                                                                                
EXAMPLE                                                                         
                                                                                
EntryPoint:                                                                     
MOV     AH,62H          ; Function Call --                                      
; Get PSP Address                                                               
INT     21H             ; Issue request to DOS                                  
JC      Error           ; Error code in AX                                      
MOV     ES,BX           ; Set Segment                                           
MOV     AH,4AH          ; Function Call --                                      
; Set Memory Block Size                                                         
MOV     BX,paragraphs   ; Set Size                                              
INT     21H             ; Issue request to DOS                                  
JC      Error           ; Error code in AX                                      
MOV     AH,67H          ; Function Call - Set Handle Count                      
MOV     BX,NewHandles   ; Set new handle count                                  
INT     21H             ; Issue request to DOS                                  
JC      Error           ; Error code in AX                                      
                                                                                
                                                                                
NewHandles      DW     ?                            ; Number of Handles need    
; by this DOS 4.00 proce                                                        
ListSize        EQU    (Endofprogram - EntryPoint)  ; Number of bytes           
Paragraphs      EQU    (ListSize / 10H)             ; Number of paragraphs      
End_of_program  LABEL  BYTE                         ; Last used position for    
;  this program                                                                 
                                                                                
COMMENTS                                                                        
                                                                                
The maximum number of file handles allowed for this interrupt is 64KB.  If      
the the specified number of allowable handles is less than the current number   
allowed, the specified number will become current only after all the handles    
above the specified number have been closed.  If the specified number is less   
than 20, the number is assumed to be 20.  Data base applications can use this   
function to reduce the need to swap handles.                                    
                                                                                
You must release memory for DOS 4.00 to contain the extended handle list.       
You can do this by using the SET BLOCK (4AH) function call.                     
                                                                                
68H --                                                                          
COMMIT FILE                                                                     
                                                                                
PURPOSE                                                                         
                                                                                
Causes all buffered data for a file to be written to the device. This           
function can be used instead of a close-open sequence.                          
                                                                                
EXAMPLE                                                                         
                                                                                
MOV     AH,68H          ; Function Call -- Commit File                          
MOV     BX,Handle       ; Select file                                           
INT     21H             ; Issue request to DOS                                  
JC      Error           ; Error code in AX                                      
                                                                                
                                                                                
Handle        DW      ?       ; Handle from previous Open or Create             
                                                                                
COMMENTS                                                                        
                                                                                
Commit File provides a faster and more secure method of committing data in      
multi-user environments such as the IBM PC Local Area Network.                  
                                                                                
6CH --                                                                          
EXTENDED OPEN/CREATE                                                            
                                                                                
PURPOSE                                                                         
                                                                                
Optionally opens and creates a file.                                            
                                                                                
EXAMPLE                                                                         
                                                                                
MOV     AH,6CH              ; Extended open                                     
MOV     AL,0                ; Reserved                                          
MOV     BX,MODE             ; Open mode                                         
; Format : 0WF00000ISSS0AAA                                                     
; AAA=Access code 0=Read                                                        
;                 1=Write                                                       
;                 2=Read/Write                                                  
; SSS=Sharing mode 0=Compatibility                                              
;                  1=DenyRead/Write                                             
;                  2=DenyWrite                                                  
;                  3=DenyRead                                                   
;                  4=DenyNone                                                   
                                                                                
; I 0=Pass handle to chile, 1=No inherit                                        
; F 0=INT 24H, 1=Return error                                                   
;  on this open and any I/O to this handle                                      
; W 0=No commit, 1=Auto-Commit on write                                         
MOV     CX,ATTR             ; Create attribute (ignored if open)                
MOV     DX,FLAG             ; Function control, Format=0000000NNNNEEEE          
; NNNN=Does not exist action                                                    
;   0=Fail, 1=Create                                                            
; EEEE=Exists action                                                            
;   0=Fail, 1=open, 2=Replace/Open                                              
                                                                                
MOV     SI,SEG file name   ; Name to open or create                             
MOV     DS,SI                                                                   
MOV     SI,OFFSET file name                                                     
INT     21H                                                                     
JC      ERROR                                                                   
; AX=Handle                                                                     
; CX=Action taken code                                                          
; 1=File opened                                                                 
; 2=File created/opened                                                         
; 3=File replaced/opened                                                        
                                                                                
;----                                                                           
Mode     DW        ?         ; Open mode bit                                    
; Definitions                                                                   
Attr     DW        ?         ; File attributes                                  
Flag     DW        ?         ; Function definition                              
File name    64 DUP (0)                                                         
                                                                                
COMMENTS                                                                        
                                                                                
Function 6CH combines the functions currently available with OPEN, CREATE and   
a CREATE NEW.                                                                   
                                                                                
If F is 1, the critical error handler (Interrupt 24) is disabled for the        
handle returned by Extended Open.  Any I/O issued to this handle will never     
generate the critical error but only the extended error.                        
                                                                                
If F is 0, no actions are taken.                                                
                                                                                
If W is 1, any disk write using the handle returned by Extended Open will       
accompany with a commit call (see Interrupt 21H (AL=68H)).                      
                                                                                
If W is 0, no actions are taken.