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REPZ

Repeats the CMPS or SCAS instruction while the Zero Flag (ZF) is set to 1 and the RCX register is greater than 0. In each iteration, the instruction compares the operands, decrements RCX, and increments or decrements the index registers (RSI and RDI) based on the Direction Flag (DF).

The table after the description covers what the source and destinations can be.

Support LOCK

The REPZ prefix is only functional when used with CMPS or SCAS. The behavior depends on the current operating mode; in 64-bit mode, the memory operands are typically accessed as qwords unless a size override prefix is used.

To avoid unintended behavior, ensure RCX is initialized to the correct count before execution, as a value of 0 will result in the instruction being skipped entirely. Users SHALL be aware that if the Direction Flag (DF) is not cleared (via CLD) or set (via STD) explicitly, the pointer increments/decrements will be non-deterministic based on the previous state of the processor.