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CMPS

Compares two operands by subtracting the source from the destination. The result is not stored, but the CPU flags (CF, ZF, SF, OF, PF) are updated based on the unsigned and signed difference. Both the source and destination pointers are incremented or decremented according to the Direction Flag (DF).

The table below covers what the source and destinations can be.

DO NOT support LOCK

The instruction SHALL be used with the RSI register for the source and RDI register for the destination. It is available in 64-bit mode, compatibility mode, and 32-bit protected mode. The operand size is determined by the prefix (byte, word, dword, qword) or the default size of the current mode.

The behavior of CMPS is strictly dependent on the Direction Flag (DF). If DF is 0, the pointers RSI and RDI are incremented after the comparison; if DF is 1, they are decremented. Failure to clear (CLD) or set (STD) the Direction Flag before execution MAY lead to memory access violations or incorrect pointer offsets. The instruction MUST be used with the REP or REPE/REPN prefixes if comparing blocks of memory to avoid manually updating pointers in a loop.