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 PostPost subject: Why Windows Vista/7 doesn't work on a Pentium        Posted: Mon May 18, 2020 4:15 am 
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Windows Vista/7 for a long time has been rather commonly understood to not be compatible with P5-based Intel Pentium and Intel Pentium MMX CPUs. This is despite the fact that according to Geoff Chappell Windows Vista only requires the CMPXCHG8B and RDTSC features, both of which are supported on the original Pentium and the Pentium MMX. Clearly, further investigation will be necessary.

Some testing had already been done: Vista on a real Pentium/Pentium MMX will give a BSOD with the code 0x78.The BSOD only is known to affect Intel P5 family CPUs, not AMD K5/K6, Cyrix MII, or Intel P6 family CPUs, One hint comes in the form of a video which shows Windows 7 SP1 booting on a Pentium 90MHz system:

In the description, they reference KB980358, a KB article relating to the issue. As it turns out, it had been a bug that was patched with a hotfix and one that is included in Service Pack 1 for Windows 7. Another hint comes in the fact that the hotfix only updates mcupdate_GenuineIntel.dll, which presumably controls the built-in microcode updates for Intel CPUs.

Later, doing testing in Bochs' pentium_mmx preset revealed that Vista specifically threw a 0x78 BSOD when I disabled “Ignore RDMSR/WRMSR to unknown MSR register”:
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After seeing this I immediately went to check Bochs’ own logging, which revealed the following line:
Code:
WRMSR: Unknown register 0x8b

A quick google search of this register reveals it relates to microcode updates, confirming that it is indeed mcupdate_GenuineIntel.dll. The DLL appears to write to the register, which is not implemented on Intel P5 CPUs as they do not support microcode updates. Reading from/writing to an unimplemented MSR on an Intel CPU will usually cause a general protection fault. Windows doesn’t expect this and bugchecks as a result.


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 PostPost subject: Re: Why Windows Vista/7 doesn't work on a Pentium        Posted: Mon May 18, 2020 9:47 pm 
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Very interesting information! I remember attempting to install Windows 8 onto a Pentium 4 system and having the setup wizard error out, probably because it required a Core2 architecture or newer. I wonder what specific CPU instructions in the Pentium 4 was lacking... in any case, I'm surprised Microsoft released a patch for Windows 7 for those older Pentium CPUs.


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 PostPost subject: Re: Why Windows Vista/7 doesn't work on a Pentium        Posted: Mon May 18, 2020 11:13 pm 
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Windoze98 wrote:
Very interesting information! I remember attempting to install Windows 8 onto a Pentium 4 system and having the setup wizard error out, probably because it required a Core2 architecture or newer. I wonder what specific CPU instructions in the Pentium 4 was lacking... in any case, I'm surprised Microsoft released a patch for Windows 7 for those older Pentium CPUs.

PAE and NX (which normally cascades into VT-X) generally. Windows 8 started to enforce them, and I think both of them weren't a thing in Pentium 4.


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 PostPost subject: Re: Why Windows Vista/7 doesn't work on a Pentium        Posted: Wed May 20, 2020 1:20 am 
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I believe some later Pentium 4 models had NX, but I don't know about PAE.

Oddly, 32bit Pentium M 760 and above had both NX and PAE. I was able to easily install 8.1 stock/unmodified on a system equipped with a 780.

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 PostPost subject: Re: Why Windows Vista/7 doesn't work on a Pentium        Posted: Fri May 22, 2020 6:38 am 
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I know Michael MJD tried to install Vista on a shitty 90's PC one time. He ended up upgrading the specs in order to install Vista.


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 PostPost subject: Re: Why Windows Vista/7 doesn't work on a Pentium        Posted: Sun May 24, 2020 5:41 pm 
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You're getting it all wrong. NT 6 can indeed run on a Pentium 4 system (I actually used to run 7 on a P-IV Prescott), but it can't run on an original Pentium, maybe because it lacks PAE (introduced in the Pentium Pro). I've also successfully ran Windows 7 on a AMD Duron, which only has MMX, 3DNow!+, and SSE, so Windows 7 could run on a PIII, maybe because of SSE, or maybe because of PAE, need to test later.

And Windows 8/8.1 can run on a P4, but as mentioned earlier, it requires NX, which was on the Prescott architecture and onwards (and on the Athlon 64 onwards).

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 PostPost subject: Re: Why Windows Vista/7 doesn't work on a Pentium        Posted: Sun May 24, 2020 9:02 pm 
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Have you even read the first post? Don't you see a successful attempt to run Win7 SP1 on original Pentium?


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