OS/2, eCS & ArcaOS - Technical > Utilities

XDF disk extractor

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RickCHodgin:

--- Quote from: Dave Yeo on November 06, 2018, 02:56:29 am ---I'd suggest rereading http://www.os2museum.com/wp/the-xdf-diskette-format/ in particular how there is a disk image hidden in the second FAT that gets loaded if no driver and how the missing 3 sectors are really missing but marked as bad. At least if I'm following along correctly. Michal is pretty good with this stuff.

--- End quote ---

Will do.  I have dyslexia.  Large amounts of reading are difficult for me.  But, in the interest of doing it right, I'll copy-and-paste into my text-to-speech reader. :-)

Valery Sedletski:
2Dave Yeo: Thanks for the link. MichalN blog is a good reading, indeed.

2Rick Hodgin: The root directory end at offset 0x4a00 (sector 37 (if sectors are counted from 0)). But
I see the "hidden root directory" repeating again here (a "WIN1" volume label and a "packing.lst" file again).
Why it's again here? We need to read the article in MichalN blog, and see, I think.

RickCHodgin:

--- Quote from: Dave Yeo on November 06, 2018, 02:56:29 am ---I'd suggest rereading http://www.os2museum.com/wp/the-xdf-diskette-format/ in particular how there is a disk image hidden in the second FAT that gets loaded if no driver and how the missing 3 sectors are really missing but marked as bad. At least if I'm following along correctly. Michal is pretty good with this stuff.

--- End quote ---

It looks like the important parts here:

--- Code: ---When the XDF TSR was loaded, a 3½” HD XDF floppy looked like a
medium with 80 cylinders and 23 sectors per track.  When the XDF
TSR wasn’t loaded, the same medium looked like a teensy disk with
just a few sectors total, usually containing some text file
showing the XDF disk’s contents or a readme file.

The first cylinder of a XDF floppy used standard 512-byte sectors,
only with 19 sectors per track rather than the standard 18.  The
format used 9 sectors per FAT (for the 3½” HD XDF variant, by far
the one most widely used).  It abused the fact that the second
FAT is normally never read by DOS (as long as the first FAT copy
is readable).  The second FAT copy on a XDF disk instead contained
the small 8-sector backwards compatible image.  The XDF sectors
used high sector IDs and were thus not seen by standard DOS/BIOS.
Track 0 (cylinder 0, head 0) was laid out as follows: 1 boot
sector, 11 FAT sectors, 7 sectors of a backward compatible
micro-disk.  That is 19 sectors total.

The second track (cylinder 0, head 1) started with the root
directory (7 sectors) and was followed by the data area.  Now
there’s an obvious problem: The XDF disk claimed to have 23
sectors per track, but physically there were only 19.  What
happened with the missing 4 sectors?  Easy:  XDF marked the
corresponding clusters in the FAT as bad.

It is important to note that XDF images always contained full
XDF-sized tracks, even for cylinder 0.  Some sectors in a XDF
image were thus not written to a disk, but the XDF image was
uniform.
--- End code ---

RickCHodgin:

--- Quote from: Valery Sedletski on November 06, 2018, 11:00:21 am ---2Rick Hodgin: The root directory end at offset 0x4a00 (sector 37 (if sectors are counted from 0)). But
I see the "hidden root directory" repeating again here (a "WIN1" volume label and a "packing.lst" file again).
Why it's again here? We need to read the article in MichalN blog, and see, I think.

--- End quote ---

This portion appears to be in conflict:

--- Code: ---The first cylinder of a XDF floppy used standard 512-byte sectors,
only with 19 sectors per track rather than the standard 18.  The
==>format used 9 sectors per FAT<=== (for the 3½” HD XDF variant, by far
the one most widely used).  It abused the fact that the second
FAT is normally never read by DOS (as long as the first FAT copy
is readable).  The second FAT copy on a XDF disk instead contained
the small 8-sector backwards compatible image.  The XDF sectors
used high sector IDs and were thus not seen by standard DOS/BIOS.
Track 0 (cylinder 0, head 0) was laid out as follows: 1 boot
sector, ==>11 FAT sectors<==, 7 sectors of a backward compatible
micro-disk.  That is 19 sectors total.
--- End code ---

Hmmm....

I'm going to have to examine the disk using a good hex editor, one that can follow links.

Valery Sedletski:
2Rick: Yes, it's 11 sectors per FAT. Not sure, where 9 sectors were used. 1 reserved sectors + two
11-sector FATs. 1+2*11 = 23 sectors == 1 usual track. Though, DOS/BIOS only sees the
hidden image which is 19 sectors long. Hm.

I didn't read the article to the end, yet, though

PS: Yes, 1.44 MB DS/HD indeed uses two 9-sector FATs. XDF format uses 11 sectors per FAT,
so here Michal is right.

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