Its absence also prevents 64-bit Windows prior to Windows 8.1 from having a user-mode address space larger than 8 terabytes. Without CMPXCHG16B one must use workarounds, such as a critical section or alternative lock-free approaches. 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. Similar to CMPXCHG8B, CMPXCHG16B allows for atomic operations on octal words.
Early AMD64 processors lacked the CMPXCHG16B instruction, which is an extension of the CMPXCHG8B instruction present on most post-80486 processors.See also: How prevalent are old 圆4 processors lacking the cmpxchg16b instruction? This instruction may also be referred to as CompareExchange128. It is supported on all modern x86-64 processors, although some early AMD64 CPUs did not support it. The CMPXCHG16B instruction performs an atomic compare-and-exchange on 16-byte values.This requirement is met by all modern processors and is generally only an issue with certain Core 2 and earlier processors. In fact, the same features are required to run 64-bit Windows 8.1.
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