Microsoft appears to finally be putting its foot down on how far back it’s willing to go when it comes to supporting older hardware. As of the upcoming Windows 11 24H2 build, Microsoft will require that your processor supports the POPCNT instruction. If you’re wondering what that is and whether this will affect you, you’re not alone.
This new addition was spotted by Bob Pony on X (formerly Twitter). According to the user, if the CPU doesn’t support the POPCNT instruction or it’s disabled, Windows won’t work at all. Multiple system files now require this instruction, starting with the Windows 11 kernel. Long story short — no POPCNT, no Windows 11 24H2.
But what even is this POPCNT instruction? It stands for “population count.” It’s a simple, but useful instruction that counts the number of set bits (ones) in a machine word. As software engineer Vaibhav Sagar explains: “For example (assuming 8-bit words for simplicity), popcount(00100110) is 3, and popcount(01100000) is 2.”
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/vCWYvzfu6k
— Bob Pony (@TheBobPony) February 11, 2024
Suffice it to say, it’s really, really old — at least by computing standards. It was first introduced in the 1960s as a software solution, reportedly at the behest of the National Security Agency (NSA), as a tool to help decrypt messages. Much later, it became commonplace as a hardware solution in consumer CPUs in the early 2000s, starting with AMD’s Barcelona (Phenom II) processors and Intel’s Nehalem (the first Core series). Those launched in 2007 and 2008, respectively.
These days, POPCNT finds its uses in cryptography, data compression algorithms, bioinformatics, machine learning, and AI, among other things. It seems to be an important, albeit often overlooked instruction. Sagar also points out its usefulness in workloads such as running binary convolutional neural networks.
What’s the point of Microsoft suddenly making a 60-year-old instruction mandatory in its next Windows build? Tom’s Hardware points out that Microsoft might want to ensure that all Windows 11 PCs are capable of supporting binary neural networks.
The good news is that, unless your PC is pretty much ancient, this is nothing you need to worry about. In theory, the Windows 11 TPM 2.0 requirement already locks these computers out, as that is a much newer criterion to meet (although it can be bypassed). Enthusiasts who try to force newer operating systems onto outdated hardware may have a hard time getting past this one, though.
Microsoft might be determined to make AI PCs the norm and that’s that, with features such as auto upscaling said to be coming in the next big patch.
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