Windows 8.1 fixes many of Windows 8's most glaring flaws, but not everyone is able to bask in the bountiful new features. Owners of some older PCs have found themselves stranded on Windows 8, trapped by a subtle tweak in Windows 8.1's hardware requirements. And beyond the irritation factor, the issue could have troubling support implications for affected users, as Microsoft has told Windows 8 users they'll need to upgrade to Windows 8.1 by 2015 to continue receiving critical system updates.
"I'm rather frustrated, because I see no need for Microsoft to have produced a point release OS update -- or rather, a service pack -- with significantly different system requirements that would leave my PC 'marooned' on Windows 8," says Chip Sudderth, a PCWorld reader affected by the issue.
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That issue is Windows 8.1's sudden need for CMPXCHG16b support in the 64-bit version. CMPXCHG16b allows for atomic memory exchanges. While modern 64-bit processors support CMPXCHG16b, some older hardware does not -- but the requirement wasn't a must-have for the 64-bit version of Windows 8. Trying to upgrade to Windows 8.1 on newly non-compatible hardware results in a message that your "CPU does not support CompareExchange 128."
While we were investigating the issue, Microsoft confirmed to Neowin that some older AMD processors, such as the Athlon 64 X2 and Opteron 185, lack CMPXCHG16b compatibility and won't work with Windows 8.1.
Now, Microsoft can't support previous-gen processors in perpetuity, but a service pack seems like an odd place to pull the rug out from underneath owners of older machines. And the issue isn't limited to AMD processors; Sudderth's rig runs on an Intel Core 2 Quad, a chip that was a beast when it was released in 2008 and one still capable of playing today's games. Core 2 processors are still the fourth most popular processor found in PCs, according to CPU-World's user data.
Sudderth's compatibility issues don't stem from the chip itself, however.
After running into the CompareExchange 128 error with his original Q300 Core 2 CPU, Sudderth tried swapping in another Core 2 Quad chip, the Q9550S, which explicitly supports CMPXCHG16b. No dice.