Older systems now won't be able to update to newer versions of Windows due to reliance on an arcane CPU instruction often used for AI neural networks: Report

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Thankfully modern-day Windows 11 machines already support this instruction.

Microsoft Copilot logo on a laptop screen
(Image credit: Shutterstock)

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.

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.

Aaron Klotz
Aaron Klotz
Contributing Writer

Aaron Klotz is a contributing writer for Tom’s Hardware, covering news related to computer hardware such as CPUs, and graphics cards.

48 Comments Comment from the forums
  • evdjj3j
    I'm confused why this article exists, Windows 11 never supported any CPU older that Intel 8xxx or Ryzen 2xxx.
    Reply
  • Amdlova
    That is the famous Ctrl + C and Ctrl + V article
    Reply
  • BTM18
    Because it is click-bait BS story. No one is trying to run 11 on twenty YO pc's. Give me a break.
    Reply
  • AndrewJacksonZA
    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

    OK, so this is a nothingburger. Why is this on Tom's Hardware, other than maybe to help https://twitter.com/NTDEV_ with the latest Tiny11 build? /s
    Reply
  • bit_user
    Hey Microsoft, I found this for you!
    num_1bits = (c * 01001001001ULL & 042104210421ULL) % 017;: D
    Credit: http://www.inwap.com/pdp10/hbaker/hakmem/hacks.html#item167
    Caveats: requires 64-bit and only works for counting the bits in a single byte.
    Reply
  • mitch074
    Non-event. It's merely an indication that Microsoft flipped a bit in the build file - compilers will target a set of instructions by default, this merely tells the VSCPP compiler that it can use 15 years old instructions instead of 20 years old ones (as it was before as of Windows 8.1). I'm pretty sure that Windows 11 was originally compiled with the same instructions as for Windows 10, and then after a few builds where guinea pigs users didn't have problems, they went up a notch, and now they're trying again.
    By Windows 11's EOL in 2027, we might just reach something close to the actual minimum requirements for Win 11 (Haswell for Intel, Zen 1 for AMD).
    Reply
  • stonecarver
    due to reliance on an arcane CPU instruction often used for AI ---neural networks---
    I just wanted to see and understand this subject a little more on how a neural network works and why Microsoft is making it a requirement.

    https://aws.amazon.com/what-is/neural-network/
    Just a few things mentioned from that page.

    What are neural networks used for?Neural networks have several use cases across many industries, such as the following:
    Medical diagnosis by medical image classification
    Targeted marketing by social network filtering and behavioral data analysis
    Financial predictions by processing historical data of financial instruments
    Electrical load and energy demand forecasting
    Process and quality control
    Chemical compound identification
    Computer vision is the ability of computers to extract information and insights from images and videos. With neural networks, computers can distinguish and recognize images similar to humans. Computer vision has several applications, such as the following:
    Visual recognition in self-driving cars so they can recognize road signs and other road users
    Content moderation to automatically remove unsafe or inappropriate content from image and video archives
    Facial recognition to identify faces and recognize attributes like open eyes, glasses, and facial hair
    Image labeling to identify brand logos, clothing, safety gear, and other image details
    So how's that going Tesla ?
    Visual recognition in self-driving cars so they can recognize road signs and other road users
    Reply
  • stonecarver
    Why would this type of feature be on a home computer let alone a requirement. Sure for the business side of the world I get it but make it on a corporate edition of Windows 11.

    Everything good ever made in this world there is historically to flip side on how to extort it. This feels like Microsoft's reaction to those keystroke data collection programs, here hold my beer.
    Reply
  • bit_user
    stonecarver said:
    due to reliance on an arcane CPU instruction often used for AI ---neural networks---
    I just wanted to see and understand this subject a little more
    I think you just fell into a trap.

    The article's author was merely speculating, almost certainly on the basis of what his web search of that instruction turned up. The first piece of evidence I'll present is where he said:
    "This could be the reason why Microsoft is quietly enforcing POPCNT as an additional CPU instruction"
    Second, if you follow the link about using POPCNT in neural networks, it says nothing about Windows or even Microsoft. It's just a purely abstract article about an interesting application of that instruction.

    I don't believe it has anything to do with AI or neural networks. I think @mitch074 hit the nail on the head: Microsoft merely decided to increase their minimum baseline for the CPUs they target, and the first instruction someone happened to encounter, on an older CPU, was POPCNT. That doesn't mean it's the only newer instruction or the primary motivation for MS increasing their baseline ISA target.

    Finally, after reading most articles published on this site for the past year+, I've learned that many of these authors have a proclivity towards padding out their articles with flights of fancy that are loosely based in the actual facts of the matter.
    Reply
  • kano1337
    Was Win11 officially not supported on 7th gen and older Intel PC systems anyway?

    I do not diminish AI, its exciting, but yet with what they have come up for the next gen PCs and OSes seems to be more like the buzzword spinned big time, than something groundbreaking for me otoh. A little performance uplift I expect from these technologies in most of the use cases of the everyday (most often not professional) PC user.
    Reply