Tenorman wrote:

Hello All, […]
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Hello All,

My current main DOS PC is a 486 DX4 100. This is a good speed for many things, but too fast for a lot of older games and way too slow for the later 640x480 games. I would like a platform where I could get much more granular with speed adjustment. Right now, the hard-coded 3x multiplier on the DX4 CPU limits options. I can go 75 or 100 mhz, or disable the L1 cache to bring the machine down to roughly the speed of a slow 386.

My (perhaps unreleastic) goal is to have a single machine where I can manipulate FSB speed and multiplier to support a very wide range of speeds, from slow 386 all the way up to 200 Mhz.

A lot of Socket 7 boards support 50 Mhz bus and 1.5 multiplier for the Pentium 75. Are there any boards that go even lower? I know that there are no Socket 5 or 7 CPUs slower than that, but it would it would be awesome if there was a board that allowed a 33 or 40 Mhz FSB.

There are exceptionally rare S7 boards that have a functioning Turbo... but you're talking golden egg rare. For what you're looking for ... "Slow 386 to 200Mhz" I recommend a VIA C3 (Ezra or Ezra-T) on an MS6905 Slotket in a Slot 1 Motherboard. Slow 386, Fast 386, Slower 486, Faster 486, Slow Pentium, Pentium MMX, and up. Even the K6 CPUs don't scale as smoothly -- skipping from a 486DX33 to a Pentium 133 in speed. As an added bonus because these are slot motherboards you can very easily swap in a fast Pentium III if you ever want more speed for late DOS games.

As for motherboards -- the Gigabyte GA6BXC can do 50, 66, 75, 82, 100, 103, 113 and 133 FSB all changeable from SOFTWARE -- along with the caches and multipliers. You needn't fiddle with jumpers or BIOS to tweak your speed.

I recommend a VIA C3 (Ezra or Ezra-T) on an MS6905 Slotket in a Slot 1 Motherboard.

Very interesting idea. I assumed that anything Slot 1 or newer would be way too fast when I already struggle to smoothly slow down my DX4 sometimes.... I will look into it more. I also like the idea of being able to use newer ATX parts instead of trying to dig for old AT cases and power supplies.

As for motherboards -- the Gigabyte GA6BXC can do 50, 66, 75, 82, 100, 103, 113 and 133 FSB all changeable from SOFTWARE -- along with the caches and multipliers. You needn't fiddle with jumpers or BIOS to tweak your speed.

Are you sure about the 50 Mhz FSB setting and the ability to configure this in software? This looks to be undocumented, or perhaps it was added in a BIOS update at some point.
http://download1.gigabyte.us/Files/Manual/mot … nual_6bxc_e.pdf

Another thought that I had was a Socket 3 board with more options. Some of the later ones are adjustable from 20 - 50 Mhz FSB, which gives a pretty good range of speeds. The downside would be that it would have to be adjusted by jumpers, as you brought up.

Lot of PLL chips I have looked at have also a number of other very slow speed options that are normally not exposed, in addion to normal 25/33/50/66 there's often something like 16 and 8MHz and even slower speeds, you will need to modify the motherboard to expose them though and on the slowest settings often stuff like Keyboard stop working, still worth to try. I got my 486DX2 to run at slower speeds than original IBM PC according to a TOPBENCH using this method.

Are you sure about the 50 Mhz FSB setting and the ability to configure this in software? This looks to be undocumented, or perhaps it was added in a BIOS update at some point.

Found a couple of threads about the C3. I'm assuming now that you were referring to utilities such as SetMul and SoftFSB. I had assumed these were BIOS settings before.

This idea of a Slot 1 DOS machine is new to me. Was not aware of what is all out there. 😀

Lot of PLL chips I have looked at have also a number of other very slow speed options that are normally not exposed, in addion to normal 25/33/50/66 there's often something like 16 and 8MHz and even slower speeds

If you're lucky you can get those with an undocumented jumper setting. On boards that have 3 jumpers for FSB, try all 8 possible combinations and you may get some weird frequency. I suspect that 16MHz is fairly common on 486s, but I've definitely seen a socket 7 board that went lower than 50 as well.

Also, people usually focus on the speed throttling ability of K62/3+ CPUs but I'll note that Mobile Athlon XPs also work. Mine can do anything from 100x3 to 166x12.5, which might be too fast for DOS games but then again if you disable cache, tweak memory timings, have an FSB lower than 100... who knows

Yes, the 50Mhz FSB option is undocumented and it isn't SoftFSB but Rayers SMB that is used to change FSB from within DOS.