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CMPXCHG8B

Compares the value of EDX:EAX with the value of a destination m8. If the values are equal, the value of EDX:EAX is loaded into the destination m8 (no change to the memory). If the values are not equal, the value of m8 is loaded into EDX:EAX and the ZF flag is cleared.

The following table covers what the source and destinations can be:

Support LOCK

This instruction is available in 64-bit mode and compatibility mode. In 64-bit mode, the instruction operates on the EDX:EAX register pair and an 8-byte memory location.

The instruction MUST be used with a memory operand for the destination. Because it atomically compares and swaps a 64-bit value using 32-bit registers, the memory operand SHOULD be aligned on an 8-byte boundary to avoid potential performance penalties or atomicity issues across cache line boundaries.

Failure to initialize EDX:EAX before calling CMPXCHG8B will lead to unpredictable comparison results, as the instruction relies on the current state of these registers to determine the "expected" value.