What is PrefetchW ?

[Geek-9pm]

I went to install Windows 8.1 trial and it asked me if I wanted to check for dow2nloads. I said yes, it took awhile, then it said:

You can't install Windows 8.1 because your processor doesn't support PrefetchW.

What? Never here of it. 

[BC_Programmer]

It is a CPU instruction.

[Salmon Trout]

It is a CPU instruction.

Not available on certain old CPUs, e.g. some Pentium 4s, Celerons etc. Your CPU does not support Windows 8.1.

http://answers.microsoft.com/en-us/windows/forum/windows_8-windows_update/cannot-upgrade-to-windows-81-because-of-prefetchw/d5106cb9-eb0a-48df-9922-409e693a993b

[Salmon Trout]

For 64 bit installations of Windows 8.1 the CPU must support CMPXCHG16b, PrefetchW and LAHF/SAHF instructions. PrefetchW is an AMD 3DNow! instruction and Intel desktop processors became compatible from family 15/model 6/stepping 1 (Cedar Mill, the 65 nM final revision of the Pentium 4 released on January 5, 2006) thus I should think all 90 nM Pentium 4 era CPUs and earlier are incompatible (Prescott and earlier).

[Salmon Trout]

Geek, on your profile your listed machines use the following CPUs:

Dell E1705 cpu: Intel Core Duo T2500
Dell GX520 cpu: Intel Pentium 4

Which machine are you talking about? If it is the Optiplex GX520 desktop, that uses a Prescott era Pentium 4 and would not be Windows 8.1 compatible.

[Geek-9pm]

Dell GX520 CPU Intel Pentium 4
I was trying a 64 bit Windows 8.1

Strange, I had to replace that CPU because the chip set   did not fully support the new CPU in that group. After that I tried to install Windows 8.1 and is stalled opr gave the error message.

The machine runs Windows 7 64 bit with not problem.

Anyway, I am not going to rip it open and swap  the CPU again. Instead, I have Windows 8.1 now on a Dell 755 machine and it works.

[patio]

Your an adventerous guy Geek...

[Geek-9pm]

Your an adventerous guy Geek...

For some reason Windows 7 64 does not seem to care about a missing code. But it seems to matter to Windows 8.1 and I can not run it on that machine.

Thanks to Salmon Trout for the research.  So did Intel have t o buy a bunch of PrefetchW jelly bean bits from AMD?
This may one of  the things science will never understand. 

[Geek-9pm]

OK, some looking around shows this issue is well known. Some Intell Pentium CPU chips just don't have it. Hard to accept! 
Look at this:
OK for Windows 8, but not 8.1

The CPU must also support the following
LAHF-SAHF * Supports LAHF/SAHF instructions in 64-bit mode
CX16 * Supports CMPXCHG16B instruction
PREFETCHW * Supports PREFETCHW instruction
The Intel Pentium D 820 does NOT support the last one :-( so it seems that I'm stuck on 8.0

Not likely MS will continue to support Windows 8.0 just for a few.
This may mean that a few people with the wrong CPU have to get both a new motherboard and a CPU if the chip set does not support the newer CPU. Thanks a lot, Intel!

[Salmon Trout]

This may mean that a few people with the wrong CPU have to get both a new motherboard and a CPU if the chip set does not support the newer CPU. Thanks a lot, Intel!

I think you just have to suck it up; why should Microsoft hold back OS development to cater for people with hardware that is fit for the dumpster?

[BC_Programmer]

I think you just have to suck it up; why should Microsoft hold back OS development to cater for people with hardware that is fit for the dumpster?

Bah Humbug! Why, this new "386 enhanced mode" is just a ploy by Intel I tell you!

[Salmon Trout]

I see I miswrote the unit for nanometres: the SI symbol is nm - small n small m

[Salmon Trout]

OK, some looking around shows this issue is well known.

Why didn't you do that before starting this thread?

[Geek-9pm]

Why didn't you do that before starting this thread?

Because I had never heard of it and did not know it was a problem. I just could not believe there was anything missing on my Pentium D dual core 64 bit Intel CPU.  It is not like  I was running an old Pentium that was missing part of the math math tables years ago. I thought that by now  Intel would have it all over with.

As was said in the thread, it is an instruction introduced by AMD.  It is part of the 3D set.
http://en.wikipedia.org/wiki/3DNow!

3DNow performance-enhancement instructions
    FEMMS – Faster entry/exit of the MMX or floating-point state
    PREFETCH/PREFETCHW – Prefetch at least a 32-byte line into L1 data cache (this is the non-deprecated instruction

I am glad I asked. The info from  Salmon Trout and others helped me understand. I am a slow learner an d I value the help of forum members. Besides, It is fun to come here. 

[Salmon Trout]

It is fun to come here. 

A fair comment!