POP4.2 Runs Windows 11 26H2 on AMD Phenom and K10 CPUs Without SSE4.2

The tool is not a general way to put Windows 11 on any old PC. It fixes one narrow gap between two CPU features, swaps in a few older system files, and comes with known problems.

The 26H2 release​

Microsoft announced the Windows 11 2026 Update (version 26H2) on September 29, 2026. It reaches eligible devices running Windows 11 24H2 or 25H2 as an enablement package. That is a small update that turns on features already delivered through earlier monthly updates, because all three versions share one servicing branch and codebase. Microsoft is using a controlled feature rollout and may place safeguard holds on devices with known compatibility problems. Business customers are told to run targeted deployments that check apps, devices and important workflows before rolling out widely. Installing 26H2 restarts the support clock: 24 months for Home and Pro, 36 months for Enterprise and Education.

The shared codebase is the reason a bypass built for 24H2 and 25H2 could carry over to 26H2. The kernel check it patches arrived with 24H2 and is still there.

Summary: 26H2 is an enablement-package update built on the same base as 24H2 and 25H2, so whatever blocks old CPUs on those versions blocks them on 26H2 too.

How the CPU block came about​

The requirement was added in stages during the 24H2 Insider cycle. Bob Pony, a Windows enthusiast, found that POPCNT became mandatory starting with build 25905, and that "various system files requiring the POPCNT CPU instruction, from the Windows 11 kernel to the USB XHCI drivers. Without POPCNT, it doesn't boot!" Tom's Hardware later reported that the SSE 4.2 instruction set is now required to run Windows 11 24H2, starting with build 26080. If the instruction is lacking, the OS won't boot.

This is a different kind of block from the original Windows 11 requirements. WindowsForum has covered before why Rufus, registry edits, and installer-file swaps cannot cross it: a processor missing the requisite instructions cannot execute code that Windows now expects from the earliest stages of startup. There is no installer screen to modify if the boot environment or kernel itself cannot run on the hardware.

Tom's Hardware also noted at the time that AMD chips which only go up to SSE4a won't work. These are older AMD processors such as Barcelona-era K10 parts, which support POPCNT but not Intel's full SSE4.2 set. POP4.2 is aimed at exactly that group.

How POP4.2 works​

The developer, Smu1zel, argues that Windows checks for SSE4.2 but rarely uses it. In the project's words, while Windows 11 checks for both POPCNT and SSE4.2, it does not actually need SSE4.2 for 99% of its files, with only a few exceptions that can be worked around.

The developer says the check runs through an undocumented kernel routine, RtlDetectProcessorFeatures, which reads its requirements from the kernel's .rdata section. For kernel 26100.7171, the README points to an offset found in the IDA disassembler where a requirement value sits as the byte 0D. Changing it to 0C makes Windows effectively skip the check and continue booting. Microsoft doesn't publish Windows source code, so this account comes from the developer's reverse engineering. Microsoft has not confirmed it, and the exact offsets may differ between kernel builds.

The kernel isn't the only problem. The README says Microsoft compiled WindowsCodecs.dll and msxml6.dll with SSE4.1 instructions (PMOVSXBW and PMOVZXBW), and these CPUs can't run them:

• Without a replacement WindowsCodecs.dll, Windows Setup crashes and restarts. Deploying the WIM image by hand doesn't help, because Windows later drops into Automatic Recovery when it tries to finish setup.
• Without a replacement msxml6.dll, Windows looks like it installed but can't register any AppX packages. The result is a broken system that can't even start Explorer.

POP4.2 deals with this by substituting the Windows 11 23H2 versions of those DLLs. It tracks problem binaries in a badblobs.xml file, where each entry can be set to patch, replace or ignore a file.

Summary: POP4.2 changes one kernel requirement value, patches the boot loader and provisioning tool, and replaces two SSE4.1-compiled DLLs with their 23H2 versions.

Which CPUs it supports​

The README describes the target as CPUs with POPCNT, but without SSE4.2 (such as the AMD Phenom and K10 lines of CPUs). Rule of thumb:

The Core 2 exclusion will disappoint a lot of people. The README says those chips lack POPCNT, which Windows uses heavily, and patching a check can't add an instruction the CPU doesn't have. Neowin explains the exclusion as Core 2 lacking SSE4a. AMD shipped POPCNT alongside SSE4a, but that's general background. The developer's own reason, missing POPCNT, is what matters.

Neowin showed a screenshot of 25H2 running on an Athlon II X2 M320 laptop chip. It also reports that the developer tested with Bob Pony on his AMD Phenom X4 9600, a 2008 quad-core K10 chip. Gamegpu separately reported a 26H2 run on a K10 machine, confirmed by validation CPU-Z, functioning natively without using software emulation.

What has actually been tested on 26H2​

A version of the README indexed about 11 days ago said the patcher has been tested on Windows 11 25H2 v2. Other builds have not been tested, but should work. The current README now says it has been tested on 25H2 v2 and 26H2, which matches Neowin's report of an update. That's encouraging, but it is still the developer testing their own tool on a handful of machines. No one has independently checked it across a range of hardware.

Known problems​

• Nvidia nForce chipsets. The README says certain nForce chipsets hang at boot and need more investigation. This hit close to home: Bob Pony first reported he couldn't get it installed on bare metal hardware as it doesn't like nForce chipsets, but it worked in a VM on it! nForce boards were common in the Athlon and Phenom era, so many of the target machines may be affected.
• Driver signature enforcement. Gamegpu reports that the current version requires mandatory disabling of driver digital signature verification at boot. The README text available doesn't mention this. If it's accurate, it's a real security cost.
• The project calls itself experimental and asks users to report bugs on GitHub.

Using it​

Per the README, you need Windows 8 or newer, Python 3.7+ with the pefile package, and the Windows ADK Deployment Tools for oscdimg.exe, which the script downloads if it's missing. You also have to supply the 23H2 DLLs yourself: 64-bit WindowsCodecs.dll plus both the 64-bit and 32-bit msxml6.dll, all placed in the blobs/ folder, with the 32-bit WOW64 copy renamed to msxml6_wow64.dll.

The recommended method:

1. Start with an original Windows 11 ISO.
2. Run python pop42.py build with your input ISO (-i), output ISO (-o), the --clean flag and an --index number for the edition you want.
3. Approve the administrator prompt. The script needs elevation to mount WIM images and edit offline registry hives.
4. The output is a patched ISO that boots on both UEFI and BIOS.

There are also standalone commands for patching ntoskrnl.exe, winload.efi/winload.exe and provtool.exe one at a time, which is handy for Windows PE or testing. If you leave out -o, the tool overwrites the input file in place, so work on copies. The old C patcher is marked deprecated.

On an unsupported machine, a successful install means Windows reaches a working desktop and Explorer starts. If Setup keeps restarting, the codecs DLL is probably missing. If you get a "Why did my PC restart?" loop with no shell after setup, check msxml6.dll. If the system hangs at boot on an nForce board, that's the known chipset issue, not a mistake on your part.

Should you use it?​

For hobbyists with an old Phenom box and a free weekend, this is good reverse-engineering work, and the developer's point that the check is stricter than the code needs is a fair one. You'll need to read the README closely.

Everyone else should keep in mind that getting Windows to boot doesn't make the PC supported. Microsoft's 26H2 announcement talks only about eligible devices, safeguard holds and controlled rollout. Nothing in it covers patched kernels or 23H2-era DLLs in a 26H2 install. Every cumulative update could replace the patched files, change the kernel offsets, or add new SSE4.1 code somewhere else. Meanwhile, the hardware involved is 15 to 18 years old. For a home lab, try it. For a PC that runs payroll, don't.

Summary: POP4.2 brings Windows 11 24H2 through 26H2 to AMD CPUs that have POPCNT but lack SSE4.2. Intel Core 2 isn't supported, nForce boards may hang, and it should be treated as an unsupported experiment, not a way to upgrade a PC you depend on.