Consideration on using XCHG Instruction

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The Intel 8086, released in 1978, included an instruction named XCHG. All three of these instructions swapped registers with registers, or registers with memory, but were unable to swap the contents of two memory locations.

On the common x86 architecture, the XCHG instruction with a memory operand has an implicit LOCK prefix, so that the operation is atomic, requiring hundreds of cpu cycles to synchronize with every other device that can access main memory. By comparison, on many processors a single MOV instruction can be issued at the same time as other common instructions in a single clock cycle. Between two registers, XCHG may still be slower than three MOVs, but is smaller, so it may be useful where code size matters. x86s XCHG is primarily useful for its locking form, for writing locking primitives used in threaded or multiprocessing applications.