Underclock a Pentium/Overdrive/486

Mr Ian Primus ian_primus at yahoo.com
Sun Oct 14 06:23:48 CDT 2007

--- Grant Stockly <grant at stockly.com> wrote:

> I'm wondering about underclocking a pentium.  I've
> read about running 
> Windows XP on a Pentium Overdrive underclocked to
> 8MHz with 20MB RAM.
> I was curious if anyone in here knew enough about
> the design of an 
> early pentium or 486 to say if its possible to go
> lower, like 2MHz.

I don't know, but what I do know is that XP won't run
on a 486. Period. I've tried it :)

> For a joke I thought it would be fun to have an
> intel machine running 
> XP or Vista at an Altair paced clock speed.  XP took
> 30 minutes to 
> boot at 8MHz, so I imagine a bare bones Vista would
> take a few hours 
> at 2MHz, that is if Vista would even run on a
> Pentium...

That would be cool. But be forewarned - Vista doesn't
even really run right on a Pentium 4... I tried it. It
brought a 2.4 ghz P4 to it's knees.

> To me this is kind of like the project where someone
> booted Mac OS X 
> on a 25MHz Centris 650 using a PPC emulator.  Except
> most likely faster.  : )

Yes, that was a pretty cool hack too. Didn't that take
like half a day to boot? I wonder if it would run

> Maybe I'm just looking for something to carry me
> through the Alaskan 
> winter.  : )

Maybe. But you might need something with more heat
output. Think ECL logic minicomputer.

> Any thoughts?  My first one is to set the jumpers as
> low as they 
> would go and then start changing the onboard 
> crystals/oscillators.  Chances are the BIOS would
> still report 8MHz 
> to windows and the screen shot wouldn't look as
> cool?

Well, the guys the did XP at 25mhx removed the fan
from the overdrive, so it defaulted to a multiplier of
1. Then you could start swapping crystals to bring the
bus speed down. I don't know what the minimum bus
speed is, but if you were to install a socket for the
crystal, you could keep fiddleing. I have no idea if
the bios would report the proper speed though.


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