On one of my old computers I am having the problem of upgrading to windows 8.1 because my computer does not support PAE, NX, and SSE2. How was I able to install Windows 8 in the first place? I heard the original requirements for windows 8.0 were PAE, NX, and SSE2.

Anyone know a bypass for this on a machine already running windows 8?

IMO 8.1 should have been released in the form of a service pack not an entirely new Windows.


You can't reliably circumvent them unless you manage to at least obtain the source code for all of Windows and recompile it. (And even then I have some doubts.)

If a program requires SSE2 or something like that, it is generally because it was compiled to use SSE2 instructions – in which case bypassing the requirement checks would mean the program would crash as soon as it reached a place where those instructions were used, because your CPU didn't recognize them. (Having that happen for the OS itself wouldn't be nice, would it?).

Sometimes Windows has code for CPUs both with some feature and without, and detects the supported features at boot time – but I doubt it would bother doing that for something that was listed as required in the first place.

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How to upgrade from Windows 8 64-bit to Windows 8.1 32-bit:

I found the answer but it took me some time.

  1. Acquire this file: en_windows_8_1_x86_dvd_2707392.iso

    If you're not an MSDN subscriber, you can torrent it, but keep in mind sharing copyrighted content is illegal.

  2. Download Virtual CloneDrive and mount the ISO file to a virtual drive.

  3. Copy the entire contents of the drive to a new folder. Add the ei.cfg file to install windows without a product key.

  4. Use ImgBurn to make your new folder into an ISO file.

  5. Create a Windows 8 bootable USB key and boot from the device.

  6. Install Windows 8.1 without a product key. Select the version of Windows you bought (Windows 8 or Windows 8 Pro).

  7. After installation you can activate Windows 8.1 32-bit with your legit OEM product key.

Note: This is what I had to do to install the 32-bit version since my processor does not support PrefetchW and LAHF/SAHF. When I bought Windows 8, it did not come with a 32-bit CD, hence step 1. My product key did not work until after Windows 8.1 was installed, hence step 2-4.

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    Just one more thing to note: You can skip creating an ISO image. Create an active primary partition on the USB device (has to be FAT32 in case you'd like to use UEFI), copy all the files, modify them as needed, install the bootmanager (bootsect /nt60 drive:) and use it straight away. No need for the extra steps. – Mario Nov 13 '13 at 13:50

Windows 8.1 requires 3 new CPU features that Windows 8 doesn't require:

To install a 64-bit OS on a 64-bit PC, your processor needs to support CMPXCHG16b, PrefetchW, and LAHF/SAHF


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  • Right, but I was wondering how to circumvent it. There seems to be one for 8.1 preview: catchtito.blogspot.com/2013/07/… – Jonathan Oct 19 '13 at 6:44
  • @Jonathan You really want to deal with a hack? You will have to wait until an update patch is released – Ramhound Oct 19 '13 at 7:04
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    It's not going to happen. You'd have to patch every single binary file that comes with the OS and then through Windows Update forever more. – ta.speot.is Oct 19 '13 at 8:56
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    @Jonathan when a function is executed which is not present, the box crashes. Stay at Win8 or upgrade the hardware. – magicandre1981 Oct 19 '13 at 9:28
  • Those requirements are for windows 8 64-bit, not 32-bit, and they are CMPXCHG16b, PrefetchW, and LAHF/SAHF, not PAE, NX, and SSE2 like the question. PAE, NX, and SSE2 are required from windows 8 – phuclv Oct 19 '13 at 10:44

I have done many attempts to install Windows 8 RTM in a Pentium 4. None succeeded. The only versions that can be installed are Windows 8 Developer Preview and Windows 8 Consumer Preview. But then forget all upgrades and anything Microsoft (such as IE 11). You can do it for experiment, but not for everyday use. Forget Windows 8 RTM and of course 8.1

I hope that will save you time trying in vain...

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What is your CPU and how old is it? Windows 8 also requires PAE, NX, and SSE2, not only 8.1, look at @magicandre1981's link above. If your PC is running Windows 8 then it already supports these features

If you want to run Windows 8 on your PC, here's what it takes:

  • Processor: 1 gigahertz (GHz) or faster with support for PAE, NX, and SSE2 (more info)

System requirements

CMPXCHG16b, PrefetchW, and LAHF/SAHF are only required for 64-bit Windows 8. You can still install 32-bit Windows 8.1

For unsupported instructions one can write a patch to trap illegal exceptions and then process in software but it would reduce performance dramatically

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    Correct. Though windows might already use these opcodes during boot. In which case timing gets interesting. :) – Hennes Oct 19 '13 at 15:27
  • My specs for the computer are: asia.cnet.com/product/… - I've installed 64bit windows 8, i'm wondering if I could install windows 8.1 32-bit – Jonathan Oct 20 '13 at 19:28
  • according to this your Pentium 4 650 CPU supports CMPXCHG16b but not LAHF/SAHF. I'm afraid that you must stay with windows 8 64-bit or install 32-bit windows 8.1. Another option may be dual booting – phuclv Oct 21 '13 at 1:49

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