The Windows 11 24H2 update that Microsoft began testing earlier this month should change the way it approaches installation requirements. But it really only applies to very old PCs and retro hardware enthusiasts.

As reported by user Bob Pony on X, recent builds of Windows 11 refuse to boot on older processors that don’t support an as-yet-obscure instruction called POPCNT. According to programmer Vaibhav Sagar, this instruction is short for population count and is used to “count the number of bits in a machine word,” reports ArsTechnica.

So HUGE discovery under Windows 11 Version 24H2, since build 25905.
A CPU with instruction “POPCNT” IS NOW REQUIRED!
There are various file systems that require the POPCNT CPU instruction, from the Windows 11 kernel to USB XHCI drivers.

Without POPCNT, it doesn’t boot! pic.twitter.com/vCWYvzfu6k

— Bob Pony (@TheBobPony) February 11, 2024

It’s unclear why POPCNT became a core instruction for a number of Windows components, but it appears that the Windows kernel, system USB and network drivers, and other core system files now require this instruction starting with Windows 11 24H2.

In modern x86 processors, POPCNT is implemented as part of the SSE4 instruction set. For Intel chips, it was added as part of SSE4.2 in the original first-generation Core architecture, codenamed Nehalem. In AMD processors, this is part of SSE4a, first used in Phenom, Athlon and Sempron processors based on the K10 architecture. These architectures date back to 2008 and 2007, respectively.

This effectively prevents mid-2000s Intel Core 2 Duo systems and early Athlon 64-based PCs from booting Windows 11 at all. let’s see how it works,” rather than users of real systems.

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