Bob Pony on X (formally Twitter) reports that the Windows 11 24H2 update now requires the processor to support an instruction that was previously not a showstopper, leaving systems that don't support the feature out in the cold — Windows 11 users who wish to upgrade to Windows 11 24H2 will need a system that supports the underappreciated POPCNT (population count) instruction that debuted in Intel's Nehalem chips back in 2008.
Bob Pony reported the issue, saying Windows 11 24H2 failed to boot with POPCNT deactivated. He also discovered that much of the Windows 11 subsystem requires this instruction, including the kernel and USB XHCI drivers.
Fortunately, compatibility won't be an issue for most users running Windows 11 today. POPCNT is part of the SSE 4.2 instruction set, which came about in the early 2000s. Every Intel CPU since the first Core series, codenamed Nehalem, and every single AMD CPU since the K10 series, codenamed Barcelona (Phenom II), support SSE 4.2 and the POPCNT instruction.
So HUGE discovery found in Windows 11 Version 24H2, since build 25905.A CPU with the instruction "POPCNT" IS NOW REQUIRED!There are various system files requiring the POPCNT CPU instruction, from the Windows 11 kernel to the USB XHCI drivers.Without POPCNT, it doesn't boot! pic.twitter.com/vCWYvzfu6kFebruary 11, 2024
If you don't know what this instruction is or haven't even heard of it, you aren't alone. This unique instruction has gone unnoticed by the wider public for years, debuting back in the 1960s. Despite its large adoption today, it doesn't do a lot. All it does is count the number of ones in a binary representation, and that's it.
This functionality gives it a lot of capability that is useful for many modern workloads. Back in the 1960s, the NSA used it to help decipher encrypted messages by using POPCNT to count distinct characters in intercepted messages that had them.
Today, POPCNT is useful for various workloads such as error correction, and even neural networks. According to Vaibhav Sagar, POPCNT can be used to help run binary convolutional neural networks, which are typically designed to run on lower-end devices, like smartphones and tablets. Neural networks normally use matrix multiplication, but a binary convolutional network runs using binary matrices.
This could be the reason why Microsoft is quietly enforcing POPCNT as an additional CPU instruction that all systems will need to support going forward if they want to keep using Windows 11. Having this requirement in place could mean that all Windows 11 machines starting with the 24H2 will be able to support binary neural networks. And Windows 11 24H2 is Microsoft's upcoming AI-focused patch coming later this year.
As previously mentioned, compatibility won't be an issue for pretty much everyone running Windows 11 already. This new requirement will only affect users who are running modified builds of Windows 11 on super old machines, such as ones featuring Pentiums or Core 2 processors. Those people will not be able to run Windows 11 24H2 unless they figure out a way to bypass the POPCNT requirement as well.
