Microsoft confirms some older AMD processors do not support Windows 8.1

If you own a PC with an older AMD processor inside and tried to update from Windows 8 to Windows 8.1 this past week, you might have discovered that Windows no longer supports that chip. Today, Microsoft has confirmed to Neowin that it has eliminated support for a number of older AMD chips in Windows 8.1 that lack certain software instructions.

We were alerted to this a few days ago by Neowin reader "eladbs" who told us:

Short story: AMD's early CPUs, i.e. Athlon x2 64, Opteron 185 lack CMPXCHG16b instructions. Until Windows 8 (apparently) MS implemented a software solution into the OS to support this instruction (so I read). Windows 8.1 refuses to install on machines based on those CPUs. Windows 8 works great.

We contacted Microsoft for comment and received a response from a spokesperson. The statement confirms the removal of Windows 8.1 support for these AMD chips, adding:

This was a change done to improve the security effectiveness of Windows by removing code that cannot use instructions such as CMPXCHG16b. Based on our data, the number of affected processors are extremely small, since this instruction has been supported for greater than 10 years.

Obviously, people who are affected by this issue might wish to consider upgrading their PC with a more recent processor or motherboard so they can utilize Windows 8.1. 

Update: It's been pointed out in the comments that the 32-bit version of Windows 8.1 could still work, although it's been shown that most PCs running Windows 8.1 are using the 64-bit version.

Image via AMD

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I have an old Intel Pentium Dual Core that suffers from the same problem. I had to switch back to 32bits when I upgraded it to W8.1.

Even if it doesn't show up, that is not the real issue. I am obviously talking about the issue of an "update" that tends to be treated by MS as such raising system requirements.

Yeah, I don't like that practice either with a .1 update doing this. Same thing with Windows Server 2012 that gets its R2 release because underneath it's still mostly the same system and changing requirements causes confusion. They both should have been named differently.

Weird, I had an X2 less than 10 years ago. Shouldn't x86-64 be considered x86-64? I'm surprised that they are affected since AMD came up with the extensions.

So as long as I stay with 32-bit Windows, I suppose my AMD Athlon 64 X2 4200+ (Manchestor core) should be fine then? I'm kind of relieved I haven't switched to 64-bit for Windows.

So in other words, corners were cut when AMD made those 64bit processors 10 years ago, and the results of that are showing now in their inability to upgrade to win 8.1 x64?

I didn't find any info on AMD's website, so I'll take a bet that any early socket 939 Athlon 64, Athlon 64 x2, Athlon FX, Opteron 1xx and socket 940 Opteron 2xx and 8xx are affected.

Important to note, this only applies to the 64bit version of Windows 8.1, the 32bit versions of Windows 8.1 will still work on these CPUs

"CMPXCHG16B" is part of the AMD64 instruction set and thus would never have been used by 32bit windows.

I'm running an older dual core (not core 2) e5200 - oc'd to 3.5ghz, 6gb ddr2 ram (667mhz) & 5 hard drives - without any problem. 8.1 runs way better than 8.

Wall-swe said,
That's really strange.. Is it Windows 8.1 64bit?

Yes it is. Did a Clean install though, not an update. Motherboard is some cheap asrock.

benthebear said,
I remember Apple receiving a good amount of heat around here when Mountain Lion was announced.

And those people, myself included, were called "haters" who needed to get with the program and buy new hardware. Then those same people complained about the WP7 devices not being able to run WP8.

BigBoy said,
Note though that you can still install 32 bit Windows 8.1

I was going to say. I have already updated 12 Athlon x2 64 machines to 8.1 at work with no issues at all. Never tried 64bit version on them though. At least know I know it would fail. Anyway, the article should make it clear that the 32bit works perfectly.

Does this apply to mobile phenom II? I just put one one those in my old laptop for a considerable speed bump and I have an extra Windows 8/8.1 laying around I was days away from installing....

If my previous motherboard did not fry, I would still use an Athlon X2 4200+ with 4GB of RAM. If you don't do anything that requires CPU cycles, you don't need a super i7

Believe it or not, I JUST replaced the Opteron 165 in my main system this year. That would have been included in the not supported list. That was a hell of a CPU for a long time though. Monster overclocker!

I'm running Windows Server 2012 R2 on an AMD Athlon 64 X2 5000+, if that means anything. I'm not sure if this is one of the chips that's not supposed to work (this post mentions the X2s), but it does.

I actually thought about it yesterday so I tried running the installer of Windows Server 2012 R2 and I didn't face the same restrictions.
I didn't install it yet... I might plan to do so next week when I'll have the time and check for sure.

If anyone is willing to check until then and tell us i'll be grateful!

@ataris_kid - Your Athlon is AM2 version I guess so this instruction set is implemented in your CPU. You can run Sysinternals Coreinfo the check that.

That's what I feared. I have a Socket 939 x2 4400+ Toledo with Home Server 2011 that I wanted to upgrade to 2012 R2. I guess that's not going to happen anymore...

Exactly. Anyway, unless you're on a laptop an AMD CPU that supports the instruction set would be pretty cheap. I doubt there are many users affected by this, it would be a very small minority running Win8.1 on 64 X2's

Still a very capable machine to host a small web server, a Minecraft server, to use as a NAS, etc.

Can't blame MS though. Sooner or later, you have to abandon legacy hardware support.

myxomatosis said,
Still a very capable machine to host a small web server, a Minecraft server, to use as a NAS, etc.

Can't blame MS though. Sooner or later, you have to abandon legacy hardware support.

Then you don't need Windows 8.1. Just run an older version of Windows or use Linux.

Figure 8 Dash said,
The Athlon 64 X2 was released in 2005. If your PC is still running on an eight year old chip then it's had a good run.

Until I got my Surface Pro earlier this year, I was still using a machine with that CPU as my daily driver, hooked up to 3 monitors. I'm constantly connected to other machines via RDP so the local machine doesn't have much of a workload (except for running the displays), so really, I could've kept using it until it died. As Windows 8.1 boots faster and is more lightweight than any of its predecessors, it might've been worth installing.

Bottom line--yeah, it's had a great run, but it could've kept going if not for this new limitation.

Does anyone know if the Athlon 64 TF-20 processor is compatible with Windows 8.1? I upgraded my older 2009 emachines e627 laptop to Windows 8 last year and I just wanted to know because one of my family member is using it.

Lone Wanderer Chicken said,
Disregard, I found thats its compatible on cpu-world

yeah this article is quite bland, as far as I could investigate this instruction is only missing on the first gen Athlon 64 X2 (Socket 939, stepping E4 & E6, codenames Manchester and Toledo respectively). Considering that most people stuck on an old X2 64 are on the AM2 platform this should not be affecting that many people.