Windows XP on a Pentium Pro?

While I was in the process of gutting machines, I considered finally salvaging my old 200 MHz Pentium Pro machine that I've had for the past 6 years. I loved this machine when I first got it, because for about the $2500 I spent at the time 6 years ago, it was a pretty amazing machine considering it outperformed the other Pentium systems on the market and was my first machine with a whopping 128 megabytes of memory. Woo hoo!

After upgrading the machine from its original Windows 95 to Windows NT 4.0 to most recently Windows 2000, I was planning to take the machine apart and salvage the parts thinking this was not a Windows XP compatible machine. The final straw came when the mouse port stopped working and the machine only worked by keyboard.

And anyway, Microsoft has always claimed that Windows XP requires a 233 MHz Pentium II class machine or higher. I stupidly assumed that this was a combination of two things. One, that Windows XP requires a P6 architecture compatible processors (i.e. Pentium II, Pentium III, AMD K6, etc) due to the fact that kernel mode transitions now work using the SYSENTER mechanism introduced in P6 family processors, an instruction missing on the original 486 and Pentium processors. And two, that Windows XP relies on MMX, since all 233 MHz and faster x86 processors have MMX support.

So which is it that makes for the 233 MHz minimum - the P6 requirement, the MMX requirement, or both?

Before taking the screwdriver to this machine, I thought I'd might as well find out. I started the Windows XP setup program and expected to get the annoying "sorry, I can't install on your shitty system" dialog box. Instead, Windows XP installed just fine. No warnings, no errors, nothing.

I then thought, hmmm, let's throw in one of those redundant USB cards that came with one of my Athlon motherboards. I did that, rebooted, plugged in a USB mouse, and presto, the mouse worked.

So now I have this admittedly slow, but working, 200 MHz Pentium Pro system running Windows XP and a working mouse. Not exactly very useful, even compared to a 500 MHz Celeron system, but if you have an old Pentium Pro machine, especially one of those rare dual processor models, don't throw it away on the false assumption that you can't run XP on it. I see no reason why a 180 MHz Pentium MMX won't work either.

I'd like to hear from any readers who have managed to install Windows XP on a less than 200 MHz machine and hear about your experience.