For a while now, some tech geeks have managed to get Windows 11 to start on older machines, but it looks like they could soon be stuffed, judging by a post saying that Microsoft's code will now need an instruction not found on old CPUs.

The POPCNT instruction first showed up in Intel Core processors at the end of 2008 as part of the SSE4.2 instruction set extension and AMD's Barcelona architecture in 2007.

POPCNT counts the number of bits in a machine word that are on (or not zero.) You might see it in code-breaking, and it has been hiding in CPU architectures for years before Intel and AMD's version by ages.

Its use here means that if your CPU is 15 years old – or older – there is every chance the next big release of Windows 11, expected to be 24H2, won't start. But to put this in context, the instruction came in the first lot of Core i5 and i7 processors, which used the Nehalem microarchitecture.

The dodgy Windows 11 hardware compatibility list leaves out anything earlier than eighth-generation Intel chips, so the finding is unlikely to matter much to most people. But customers using tricks to avoid the Windows 11 hardware check would do well to make sure their hardware is not too old before trying an install.

The report of the requirement, found by X user @TheBobPony in recent Windows 11 24H2 builds, comes a month after some Windows 10 users couldn't open some Windows apps after an update.

Some users guessed that a Microsoft engineer had turned on some newer CPU instructions by mistake, cutting off support for several older processors – not that Vole is likely to care.