Windows 8.1 brought with it a host of improvements to its core version, but a specific class of hardware is blocked from upgrading to the service pack from vanilla Windows 8. Adding to the problem is Microsoft's declaration that it will no longer provide security updates or other patches for the original Windows 8 release after 2015.
The flaw affects motherboard and processor combinations in 64-bit that block the CMPXCHG16b memory swap instruction, which is required in Windows 8.1, but not in Windows 8. While most Core 2 Duo processors explicitly support the instruction, some motherboards do not, thus rendering the equipment incapable of executing the command. Attempting to upgrade the OS to 8.1 on affected machines forces the process to terminate, stating that the computer's CPU "does not support CompareExchange 128."
Microsoft admits the issue, and notes that older AMD processors, including the Athlon 64 X2 and Opteron 185 lack the compatibility and are completely incompatible with version 8.1 of the operating system. Microsoft claims that "the number of affected processors are extremely small, since this instruction has been supported for greater than 10 years."
However, when motherboards are considered, the problem grows. One motherboard, the Intel DP35DP which was produced as late as 2010 and last updated in 2009, doesn't support the instruction regardless of processor installed. Other Core 2 Duo and Core 2 Quad-compatible motherboards by manufacturers such as Gigabyte and MSI are also incapable of executing the CMPXCHG16b instruction.
Users have the option of obtaining a 32-bit version of Windows 8, then using the equally lower-bit update, bypassing the problem, but taking a performance hit in the process. The OEM 32-bit Windows 8 retails for $100.