VirtualBox CompareExchange128 Error

Today, I attempted to upgrade my Windows 8 machine to the yet-to-be-released Windows 8.1, and I got the following error:

You can't install Windows 8.1 because your processor doesn't support CompareExchange128.


I am running 64-bit Mac as my primary operating system, and have set up 64-bit Windows 8 on a VM using VirtualBox.


One of the additional system requirements for 64-bit Windows 8.1 installation is that the CPU must also support CMPXCHG16b, PrefetchW and LAHF/SAHF. And this setting is disabled in VirtualBox by default.

Please check out this link for the other system requirements for Windows 8.1.


Open up the command prompt of your choice – Terminal / iTerm, and execute the following command:

VBoxManage setextradata [VM_Name] VBoxInternal/CPUM/CMPXCHG16B 1

And, that’s it! You’re ready to install Windows 8.1!


2 responses to “VirtualBox CompareExchange128 Error

  1. Jim Mclaughlin October 31, 2013 at 09:43

    I have a very similar situation but am running Win 8 in Vbox within Vista 64 and get the same message when upgrading to Win 8.1. Would same advice apply and where would commands be inserted ? within Vista or within virtual win 8 ?

    • Sanchit Bahal October 31, 2013 at 10:20

      Yes, the same commands would apply. The problem is with VirtualBox, so the commands have to be executed on the host machine, Vista in your case.

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