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. >
Some older AMD CPUs do not support Windows 8.1
Posted on Thursday, October 24 2013 @ 13:59 CEST by Thomas De Maesschalck