8.1 does not install on existing 8.0 installations
Installed Windows 8 Pro on 4 Windows 7 computers. No issues. Tried to upgrade all 4 PCs to 8.1 and 3 failed to install with error of " ... process doesn't support (some instruction)." Some instructions are: LAHF/SAHF, PrefetchW and ChangeExpress128.  Will shipping 8.1 be different? Is 8.1 just using special instructions for testing. If not, a lot of computers can't be upgraded from 8 to 8.1.
July 30th, 2013 4:21pm

Were these AMD based machines? Please post the msinfo32 from the machines that failed.

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July 30th, 2013 5:18pm

8.1 requires the CMPXCHG16B cpu feature:

http://4sysops.com/forums/topic/windows-server-2012-r2-on-virtual-box-error-0x000000c4/

July 31st, 2013 1:28am

Well I'm not using virtual box, just installing to wiped clean hardware.  One PC is AMD based, next is Pentium D - 3.0 GHz, next is Pentium 4 - 3.06 GHz, and the last one is not available right now. 
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July 31st, 2013 1:29pm

Oh and don't forget, these PCs run Windows 8 just fine.  So why would 8.1 be so different if it's only a point update?

July 31st, 2013 1:36pm

In Windows 8 run AIDA64, go to CPUID and look if the instruction is supported:

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July 31st, 2013 2:51pm

I think we're missing the point.  I don't care what instructions are supported or not, the issue is why is there a difference between 8.0 and 8.1?  I can understand a CPU requirement difference between XP and Vista or between Vista and Win 7.  But why is there a difference between 8 and 8 "blue?"
August 2nd, 2013 4:35pm

You missed the point. Win8.1 requires CMPXCHG16B, I asked this the MSFT developer Gov Maharaj . He told me that he asked the correct team to update the hardware requirement page to show this new/changed requirement.

Win 8.1 uses CMPXCHG16B to have some optimizations over Windows
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August 6th, 2013 1:45am

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