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.

Atomic changes

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.

I connected Sudderth with a top-level Microsoft support agent. Together, they determined that the issue was Sudderth’s Intel DP35DP motherboard—an older model that hasn’t received a firmware update since 2009. While the Core 2 chips supported CMPXCHG16b, the motherboard itself did not. No Windows 8.1 for him—or for this other Windows 8 user with an Intel DP35DP.

Unhappy Windows 8 users with an uncertain future

When asked to comment about compatibility issues, Microsoft representatives said technical support questions should go Microsoft’s Answer Desk. However, Microsoft told Neowin that “the number of affected processors are extremely small, since this instruction has been supported for greater than 10 years.”

As the troubles with Sudderth’s shows, however, just because an instruction was supported 10 years ago doesn’t mean it was ubiquitous 10 years ago. The Intel DP35DP was released six years ago, and the Core 2 family of processors was extremely popular.

Searching the Microsoft answers forum for CompareExchange 128 issues with Windows 8.1 returns more than 30 results. The issue has also been raised on numerous other forums.

A tablet running Windows 8.1.

“It’s frustrating that an older but robust machine—one that can play modern PC games and run Windows 8 flawlessly—can be left behind after what is essentially a service pack release,” Sudderth says. Other forum-goers echo the same irritation. “I hope that I’m a rare exception, but I’m also concerned that Windows developers will target Windows 8.1 and leave me behind just as Microsoft did.”

The newly released modern UI Facebook app, for example, only runs in Windows 8.1.

More worrying than that, however, is Microsoft’s plan to discontinue support for Windows 8 “vanilla” after 2015. Beyond then, you’ll need Windows 8.1 installed to receive security patches and other updates.

What can you do if you’re affected?

If you’re stranded on Windows 8 and don’t want to buy a new machine—and why would you if it still rocks Battlefield without skipping a beat?—all might not be lost.

The CMPXCHG16b requirement only applies to the 64-bit installation of Windows 8.1; the 32-bit version should work just fine on your machine. (I say “should” because I haven’t been able to try it.) Now for the bad news: There’s no way to use the free Windows 8.1 update to downgrade from 64-bit to 32-bit, even if you try workarounds that skip the Windows Store and let you burn the update to a flash drive. You’d have to buy the 32-bit OEM version of Windows, which starts at $100, and do a fresh install. (Again: That’s in theory. I have not been able to confirm this.)

Or you could call Microsoft support and complain loudly enough to try to get a free copy. Because to be left marooned by shifting hardware requirements in a "point" update, with no way to downgrade to a compatible version and the threat of discontinued support lingering, is absolutely ludicrous.

I’ll let this irritated forum-goer have the last word: “It’s just amazing how Microsoft chooses to leave some of its users hanging out to dry because of these CPU related requirements.”