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Instruction general - CMPS/CMPSB/CMPSW/CMPSD—Compare String Operands

Compares the byte, word, or double word specified with the first source operand with the byte, word, or double word specified with the second source operand and sets the status flags in the EFLAGS register according to the results. The first source operand specifies the memory location at the address DS:ESI and the second source operand specifies the memory location at address ES:EDI. (When the operand-size attribute is 16, the SI and DI register are used as the source-index and destination-index registers, respectively.) The DS segment may be overridden with a segment override prefix, but the ES segment cannot be overridden. The CMPSB, CMPSW, and CMPSD mnemonics are synonyms of the byte, word, and doubleword versions of the CMPS instructions. They are simpler to use, but provide no type or segment checking. (For the CMPS instruction, “DS:ESI” and “ES:EDI” must be explicitly specified in the instruction.) After the comparison, the ESI and EDI registers are incremented or decremented automatically according to the setting of the DF flag in the EFLAGS register. (If the DF flag is 0, the ESI and EDI register are incremented; if the DF flag is 1, the ESI and EDI registers are decremented.) The registers are incremented or decremented by 1 for byte operations, by 2 for word operations, or by 4 for doubleword operations. The CMPS, CMPSB, CMPSW, and CMPSD instructions can be preceded by the REP prefix for block comparisons of ECX bytes, words, or doublewords. More often, however, these instructions will be used in a LOOP construct that takes some action based on the setting of the status flags before the next comparison is made.