Marius on Motherboards

by Marius Gleeson

Firstly let's examine which form of expansion slots you should look at. The expansion slots are where you plug your video card, hard drive controller, sound card and other cards into your motherboard. Over the last few years there has been a war over which expansion slot design will become the standard, so understanding the various options is important.

The ISA bus (Industry Standard Architecture) has been used since XT's were introduced, and whilst still the base standard, is definitely a dying architecture. Due to the need to send as much data around the motherboard as possible, ISA has been held back by it's old and out dated design. You will still find ISA slots on all mother boards today due to the large amount of cards that still use it, and many cards like internal modems and sound cards don't need anything more than what ISA delivers. It's when it comes to performance with video and hard drive controller cards that ISA really shows its limitations. Limited to sending 8 or 16 bits of information at a time and a speed of only a little more than 8Mhz (Very slow in today's motherboards where 33Mhz is the norm), the ISA bus just can't keep up with today's high speed demands made by graphics cards and hard drives.

Since ISA was developed, there have been several attempts to upgrade the bus architecture including MCA (Micro Channel Architecture) by IBM, and EISA (Enhanced Industry Standard Architecture). MCA is a 32bit design which was never really accepted by the industry despite being in many ways superior to ISA and EISA. It's main competition, EISA also incorporated a 32 bit address bus was still held back due to its 8.33Mhz bus speed. EISA saw a reasonable amount of interest when it was introduced but was quickly out done by better designs.

At the moment the two main Bus architectures that are available on a personal computer are VLB (VESA Local Bus) and PCI (Peripheral Component Interconnect local bus). Both of these designs have a 32 bit bus and use the clock speed of the CPU. No, PCI doesn't have a 64 bit bus as is commonly thought. However when combined with a 64bit CPU like the pentium, it's throughput does double.

As I said, both architectures use the clock speed of the CPU (25Mhz in a 486DX25, and 33Mhz in a 486DX33) making them considerable faster than previous designs, as well as having several other features that make these two architectures capable of performing to today's requirements. VLB was the first to be available and is still favoured in low cost systems, however PCI is slowly overtaking VLB, with almost all of the higher end systems (read faster and more expensive) using PCI. When incorporated with a Pentium, the PCI bus gives a considerable increase in speed and with it's friendliness to Plug and Play, you will see PCI in more and more systems.

As I said before the speed of your bus lines is determined by how fast your CPU is running. This isn't to say however that a Pentium 90 has 90Mhz bus lines. In fact it's bus lines run at 30Mhz, the same speed as a Pentium 60. 33Mhz is the speed that most bus lines run at and this includes the 486 DX33, 486 DX2-66 (actually a 486DX33, but running at 66Mhz inside the CPU which is known as clock doubling), the 486DX4-100 (actually a clock tripled 486DX33), and the Pentium 66 and Pentium 100. So is this to say that a 486 33 has 10% faster bus lines than Pentium 90 ? Yes, but as far as overall system performance goes, the Pentium 90 still wins.

If you are the kind (like me) who just can't handle having a 486DX33 doing anything faster than their Pentium 90, some BIOS's will allow you to increase the speed on your P90 bus lines to a more respectable 40Mhz. BIOS settings are something that most people shy away from, with the false view that making a find could permanently ruin your computer or damage it. All up there is probably no need for your average user to find any of their BIOS settings, but if you know what you are doing it is possible to increase the performance of certain aspects of your computer. As I said above, with a Pentium 90 it is possible to increase the speed of the bus lines from 30 to 33Mhz or more with the right BIOS.

This may not seem like a considerable increase in speed but for a free 10% boost in computer performance it is worth doing (DISCLAIMER: I will take no responsibility if your Graphics card reduces itself to base plastic and silicone :-). There are many things in the BIOS that can be find to attempt an increase in system performance. For example, changing the wait states of your memory. These settings are usually set to give the best performance when you buy your computer, but on occasion it is possible to speed this up a little. If you are interested in playing with your BIOS, get hold of some diagnostic programs, like MFT which comes with Qemm or PCbench. Test your computer before making any finds then tweak the settings, reboot and test it again. Make sure however that you remember which settings you started on and what difference certain settings make.

MFT will give you some basic benchmarks for your memory speed, and also general system information. PCbench which is up to version 9 comes with a lot of diagnostic tests, unfortunately though these tests only give you a raw value that is meaningless unless you have some other results to compare them too. When buying a motherboard though, also make sure you get a flash BIOS, this will allow you to update the BIOS when a newer version comes around, you may even get a disk when you buy your machine that has a latter version than what is already installed, meaning that you should update it straight away.

The standard BIOS on most Motherboards is the AMI BIOS. With this BIOS you are able to find quite a substantial amount of settings, really customising your systems performance. Another good BIOS' is the Award 4.5 BIOS which offers a similar array of settings as AMI. The other main BIOS which appears is the Phoenix BIOS. I have had two motherboards with this BIOS and neither has performed as well as the AMI or Award BIOS's. So much so, that I will not buy a motherboard shipped with it.

A groovy picture of a motherboard would be here if you had graphix on! Here's a picture of the motherboard from Derek's system. Pentium wise, it's now an older style of motherboard. It supports a 66Mhz Pentium using an Award BIOS. It has 3 PCI slots, 4 ISA and one shared ISA/PCI and can handle up to 192M of main memory and 512k of 2nd level cache memory.

More modern motherboards feature Intel Triton chipsets, support for EDO ram, pipelined cache and have fast serial ports, parallel ports, games ports, floppy drive controllers and Enhanced IDE controllers built onto the board as well.



I won't go any further into BIOS's at this stage as I wish to keep this article of a more general nature. However I am working on a article dedicated to BIOS's and Derek has asked for it for next issue or the one after, so keep an ear out. Another part of motherboards that is often overlooked, is what chipset it uses. A 'Chipset' is a set of microchips on your motherboard which are used to control how the CPU talks between it's bus lines, memory and other things. Different motherboards will use chipsets made from different companies. The speed of these chip sets can quite considerably affect the overall speed of your system. For instance a Pentium 90 with Intel's Neptune chip set, would on occasion be running up to 30% faster than a Pentium 90 with the Opti chipset! So which chipset will give the best performance? On 486's or below the chips sets only vary a little in speed difference, a name brand motherboard however is always recommended over a cheap one. With Pentiums however chipsets do seem to make a difference, comparisons between cheap motherboards and expensive ones with good chipsets can be quite dramatic. To be on the safe side I would recommend either Intel's Neptune, or Mercury chipsets. Both of these have good reputations as being well designed and fast chipsets.

Another chipset that is being used on Pentium motherboards is the SIS chipset which is gaining a reputation comparable to the Intel designs. When buying a Pentium, keep in mind that a motherboard with both PCI and VLB expansion slots will run considerably slower than a board with just one or the other. This is because the only chipset that can currently handle both forms of expansion slots on the one motherboard is the slower Opti chipset. You will often see advertised with computer systems an amount of level 2 cache. What does this memory do? Basically today's CPU's need a faster memory than can be made with the standard DRAM memory that comes with a system. DRAM is what is meant when you talk about how much memory is on your system, for example 8 or 16 meg of memory. The speed of this memory is generally about 70 nanoseconds (ns), the smaller this value the faster the memory will run, but the faster the memory you buy the more expensive it becomes. Because DRAM is too slow for modern CPU's, a small amount of a faster type of memory is used between it and your CPU. This is your level 2 cache and is made up of SRAM memory.

If you were to buy fast SRAM memory instead of DRAM you could almost double the cost of your entire system. So therefore there is only a small SRAM memory buffer between your standard memory and the CPU. What this memory does is hold the next set of instructions that the CPU will be asking for, and because it is very fast there is very little delay between when the CPU requests an instruction and the memory gives it. This cache ranges in size from 64k to 1 meg. The exact amount depends on what your particular system, for a 486 you probably should have at least 128k and preferably 256k of secondary cache with a speed of 25ns or faster. With a Pentium you can have anywhere from 256k to 1 meg (If your motherboard can handle it), but 512k of 15ns memory is what you should be looking for. In some advertisements you will also see an amount of Level 1 cache. A 486 will have 8k and a Pentium will have 16k. This is nothing to boast about as all of these CPU's come with this built into them and can not be findd. This Level 1 cache is even faster than level 2 cache and is directly inside the CPU.

By the way, Intel's P6 chip, the successor to the Pentium, has 256k of level 2 cache built into the chip itself. Not only does this save space on the motherboard, but it speeds up the accessing of this memory. When it comes to the amount of actual memory on a system, a new standard has been introduced. Any new motherboard that you buy now will come with 72 pin memory SIMM (Single Inline Memory Module) slots. These are newer and better than the standard 30 pin memory SIMMs that has been used in the past.

Putting memory in your system is not just a matter of adding in a SIMM of the appropriate size. Due to the design of PC's, memory is 'Banked'. For example, on the older 30 pin SIMM motherboards, 8 memory slots are a typically available. These are divided up into two 'Banks' of 4 slots each. To add memory to the system you have to completely fill a bank. Thus if you have a 4 meg system, you will see 4 x 1 meg modules installed, whilst a 20 meg system would have 4 x 4 meg in bank 0 and 4 x 1 meg in bank 1.

The advantage of the newer 72 pin slot is that more memory can be put onto each individual SIMM card, and the banks are arranged differently. On a 486 you don't have to bank the memory at all, i.e. 20 meg of memory can be installed using a 4 meg SIMM and a 16 meg SIMM. However on Pentiums, you still have to bank the memory, but the bank are 2 slots instead of 4. I.e. 16 meg can be installed using 2 x 8 meg SIMMs.

The 72 pin standard also allows much more memory to be placed onto a motherboard, but it will cause a problem for people who are upgrading their motherboards from 30 pin to 72 pin and want to take their memory with them, because the new slot will not accept the older 30 pin SIMMs. There was a rumour around though about an adaptation that available that you could plug into a 72 pin socket and then connect 30 pin SIMMs to the adapter. Whether this actually available or not is yet to be seem.

So how much memory do you need for your system? Well 8 meg is becoming the standard, and should be recommended as the minimum amount of memory to buy. Most people will not need to go to 16 meg for a little while yet, but there is a noticeable increase in performance when running Operating systems like OS/2. DOS on the other hand does not take much advantage of memory over 8 meg and any memory beyond this amount is probably only useful for caching your hard drive.

The final thing to be considered about motherboards is the speed of the CPU. The 486DX2-66 is at the moment the standard system to buy. This will find several times over the next year. With the 486DX4-100 dropping to ridiculous prices it will soon overtake the 66 as the entry level machine. The lower end Pentiums however are already dropping and will be fairly commonplace over the next year. The Pentium 90 is the top of the range CPU to buy at the moment and is reasonably priced, unlike the Pentium 100 which is outrageously expensive and not really worth considering. This will find however when Intel release their Pentium 150's and the new P6 chips. There will also be a few new upstarts appearing on the market quite soon.

These Include the new Pentium Compatible chips designed By NexGen, AMD, and Cyrix who are designing chips that are compatible with the x86 series, yet are not clones. They offer new features that do not appear in the Pentium and all of course, claim to be faster.

NexGen's CPU known as Nx586 comes with 32k of Level 1 cache, twice that of the Pentium, and has it's Level 2 cache controller built in the CPU instead of in the chipset on the motherboard. All this is supposed to allow it to run at a slightly slower clock speed but still deliver the same performance as a Pentium. The naming convention for the speed of the chip is actually taken from how it compares to the Pentium, so a P90 Nx586 actually has a clock speed of 84Mhz, not 90. The other speeds to be released for this chip are the P75 ( actually 70Mhz), the P80 (75Mhz) and the P100 (94Mhz). This CPU however may not be widely used in it's early days as it does not come with a Floating Point Unit which increases mathematical performance. The other factor is that this CPU will not fit into a standard Pentium socket, and therefore will require it's own motherboard design and chipset.

AMD's chip, the K5 series will be released as a 100Mhz processor and a 150Mhz to be released in 1996. The speed of these chips will apparently be 30% faster than a Pentium of an equivalent clock. It manages this by increasing the level 1 cache to 24k (Not as much as the NexGen CPU) and by allowing up to 4 instructions to be completed at a time, the Pentium can perform 2 instructions simultaneously. The K5 will also be pin compatible with the Pentium, and will therefore not need a specific motherboard designed for it.

The Cyrix CPU on offer this year will be the M1. This will offer many of the features of the other chips released this year and will be pin compatible with the Pentium. The results released so far show that the M1 will perform 30% to 50% faster than it's Pentium equivalent. It will also benefit from branch-prediction, something the Nx586's will also perform. Branch-prediction is based around the idea that when a decision has to be made, the CPU will in advance predict which path will be taken through the code, and start to go down that track. If it is found that the wrong path was chosen, it will scrap the extra work it has done and perform the correct choice. If however, it chose correctly it has already performed the instructions.

It will be interesting to see how these machines perform under independent test, as the released results would have been performed under ideal conditions. There is already a fine line between what is a standard CPU and a power CPU at the moment, and with these new additions that gap will shrink even further over the next year.