Windows 10 64-bit requirements: Does my CPU support CMPXCHG16b, PrefetchW and LAHF/SAHF?

Your processor supports these features. In fact, the same features are required to run 64-bit Windows 8.1. This requirement is met by all modern processors and is generally only an issue with certain Core 2 and earlier processors.

What are these instructions?

• The CMPXCHG16B instruction performs an atomic compare-and-exchange on 16-byte values. It is supported on all modern x86-64 processors, although some early AMD64 CPUs did not support it. This instruction may also be referred to as CompareExchange128. See also: How prevalent are old x64 processors lacking the cmpxchg16b instruction?

• Early AMD64 processors lacked the CMPXCHG16B instruction, which is an extension of the CMPXCHG8B instruction present on most post-80486 processors. Similar to CMPXCHG8B, CMPXCHG16B allows for atomic operations on octal words. This is useful for parallel algorithms that use compare and swap on data larger than the size of a pointer, common in lock-free and wait-free algorithms. Without CMPXCHG16B one must use workarounds, such as a critical section or alternative lock-free approaches. Its absence also prevents 64-bit Windows prior to Windows 8.1 from having a user-mode address space larger than 8 terabytes. The 64-bit version of Windows 8.1 requires the instruction.

• The PREFETCHW instruction is a hint to the processor to prefetch data from memory into the cache in anticipation for writing (Intel Instruction Set Reference, PDF page 888). This instruction was introduced in AMD's 3DNow! instruction set, which is deprecated except for the PREFETCH and PREFETCHW instructions. All AMD processors since the Athlon 64 support this instruction. However, this instruction may not be supported in some older 64-bit Intel processors predating Nehalem.

• The LAHF and SAHF load and store the contents of the AH register into the flags register, respectively (Intel Instruction Set Reference, PDF pages 530 and 1025). Some older Intel processors without hardware virtualization (VT-x) functionality do not support this instruction when running in 64-bit long mode; these are mostly limited to certain low-end processors predating Nehalem. Some very old AMD64 processors also lack this feature.

• Early AMD64 and Intel 64 CPUs lacked LAHF and SAHF instructions in 64-bit mode. AMD introduced these instructions (also in 64-bit mode) with their Athlon 64, Opteron and Turion 64 revision D processors in March 2005 while Intel introduced the instructions with the Pentium 4 G1 stepping in December 2005. The 64-bit version of Windows 8.1 requires this feature.

What does this mean for me?

• All Intel Core i7, i5, or i3 processors, as well as all Pentium or Celeron processors based on the Clarkdale, Arrandale, Sandy Bridge, or newer microarchitectures, support these features, as well as Intel Atom and Celeron Silvermont processors. For AMD, all but the oldest 64-bit processors have these features.

• You generally only need to be concerned about these instructions if you have a processor that predates the above. The Get Windows 10 app will tell you if you can upgrade to Windows 10. If the processor doesn't meet requirements, you'll get "The CPU isn't supported."

If you can use the command line in Windows then:

• Get SysInternals coreinfo.exe from technet
• Run coreinfo > coreinfo.txt
• Open the file in a text editor, and do a find (case insensitive) for each of the instructions

PS: In Linux you would use grep flags /proc/cpuinfo | head -1