100MHz Bus Speed & Socket-7 Motherboardsby Anand Lal Shimpi on September 27, 1997 6:46 PM EST
- Posted in
What has been keeping us from the 100MHz bus speed? Two things, our memory and our PCI peripherals. The Random Access Memory (RAM) of your system operates AT your system's Bus Speed, and while most SDRAM and even EDO DRAM modules have no problems running at speeds up to 83.3MHz, when reaching 100MHz things get a bit tricky. With the quality of the SDRAM floating around the market increasing dramatically as the release of the first 100MHz Bus Speed Socket-7 motherboards approaches the problem of getting RAM to work at that high of a bus speed is slowly deflating itself to more of a smaller issue than a major worry.
The other half of the story is the PCI Bus, and the components that rest on it. Unlike the memory bus, the PCI bus operates at 1/2 the Bus Speed, meaning for a 66MHz bus speed your PCI peripherals will operate at 33MHz which is perfectly fine. When running at a 75MHz bus speed however, things get a bit more tricky, since half of 75MHz is 37.5MHz, a 13% increase in the PCI bus speed. A great deal of PCI cards work flawlessly at this speed, however a number still won't function properly, taking the bus speed up to 83MHz also increases the PCI bus speed to 41.6MHz which even fewer PCI cards will function at. As you might be able to guess, at 100MHz, the PCI bus operates at 50MHz, a full 51% increase over the specification for most PCI cards. While the PCI 2.1 specification theoretically supports speeds up to 66MHz, very few cards actually stand by that theoretical limit, in fact, most manufacturers just assume 33MHz is the limit.
So how do we get past this barrier? The answer is quite simple, instead of dividing the Bus Speed by 2 in order to obtain the PCI bus speed, use a 1/3 divisor (100MHz x 1/3 = 33.3MHz) which results in a PCI bus of, yep you guessed it, 33MHz. Problem solved...not really though, it is still the responsibility of the manufacturer to provide us with this setting, however you can rest assured that most motherboards that officially support the 100MHz bus speed will come with this feature.
What about Intel?
Earlier we were posed with the question, "Has Intel had changed their minds about supporting the 100MHz bus speed with their TX Chipset..?" The answer...no. However if we remember back to the days of the VX chipset, Intel didn't support the 75MHz bus speed yet it did appear on motherboards based on their chipsets. The same situation exists here, while searching for a hidden 83.3MHz bus speed setting on an Intel TX based motherboard I stumbled across a 100MHz bus speed setting, here's where things get interesting.
The Game Continues
So which motherboard would be the first Socket-7 motherboard available for sale with unofficial support for the 100MHz bus speed? The Tyan Titan Turbo ATX-2. Tyan has always been a manufacturer who has relied on their sheer quality and reliability to get sales, none of their motherboards (until now) even supported the 75MHz bus speed, much less the 83MHz bus speed. In any case, while searching for the 83.3MHz setting on the Titan Turbo ATX-2 I managed to stumble across this little setting:
JP23 1-2 = OFF
JP23 3-4 = ON
JP31 = ON
Upon setting the clock multiplier to 2.5x and using the above bus speed setting I noticed that the BIOS detected the processor I had installed (AMD K6) as being a 250MHz processor, a few benchmarks and tests later I confirmed that the system did indeed boot at 100MHz x 2.5 for a clock speed of 250MHz. I should mention this now, that the Tyan Titan Turbo ATX-2 is very shaky at the 100MHz bus speed setting, and by no means do I expect you to be able to run a system for even limited amounts of time at this setting. I tried a total of 5 different SDRAM combinations: 2 x 32MB Corsair SDRAM, 2 x 32MB SmarTech SDRAM; 2 x 32MB Advanced Megatrends SDRAM; 1 x 64MB Corsair ECC SDRAM; 1 x 64MB Advanced Megatrends SDRAM. Out of all of those test modules, the SmarTech, Advanced Megatrends, and Corsair (64MB Chip only) would actually allow the system to boot, and only the Advanced Megatrends SDRAM (64MB Chip only) would allow me to get some results out of the test system.
In order to increase stability the BIOS Memory timings had to be configured at the slowest possible settings otherwise Windows would come back with a handful of errors. The Hard Disk used in the test System (WD Caviar) was set to a PIO Mode 2 instead of the standard PIO Mode 4 setting to avoid disk corruption. The Matrox Millennium being used with the test system had no problem running using a 50MHz PCI bus frequency however, and it provided a nice little performance boost as well. The AMD K6 233 used in the tests was one of the first K6/233's to make it out into the retail market, and is therefore a very horrid overclocker, at 250MHz the system was too erratic to benchmark, and at 300MHz the system wouldn't boot.
What about the Pentium MMX and Cyrix 6x86MX?
Why did I chose to use the AMD K6 in the tests instead of the Pentium MMX or Cyrix 6x86MX? Well, first of all, the Cyrix 6x86MX wouldn't even boot at the 100MHz bus speed setting. At 100 x 2.0 the system returned a dead screen (no video) and obviously at all higher speeds the system couldn't manage to do more than make a few noises. The Intel Pentium MMX went a bit further than that, the processor would boot at 100 x 2.0 as well as 100 x 2.5 (100 x 3.0 wouldn't boot at all, which is funny since 83.3 x 3.5 works perfectly fine) however Windows 95 failed to load with a Pentium MMX installed using the 100MHz bus speed.
The conclusion? As they stand today, the Pentium MMX and the 6x86MX are not designed for use with the 100MHz bus, and you shouldn't expect to be able to use the processors at bus speeds that great unless either of the manufacturers plan some significant changes to their chips, for now, the only x86 processors which will run reliably using the 100MHz bus speed are the AMD K6 and the Pentium II. Now that we've settled that, let's move onto the test...