Volume 3: General-Purpose and System Instructions - Stanford ...

AMD64 Technology 24594 Rev. 3.10 February 2005CMPSCMPSBCMPSWCMPSDCMPSQCompare StringsCompares the bytes, words, doublewords, or quadwords pointed to by the rSI and rDIregisters, sets or clears the status flags of the rFLAGS register to reflect the results,and then increments or decrements the rSI and rDI registers according to the state ofthe DF flag in the rFLAGS register. To perform the comparison, the instructionsubtracts the second operand from the first operand and sets the status flags in thesame manner as the SUB instruction, but does not alter the first operand. The twooperands must be the same size.If the DF flag is 0, the instruction increments rSI and rDI; otherwise, it decrements thepointers. It increments or decrements the pointers by 1, 2, 4, or 8, depending on thesize of the operands.The forms of the CMPSx instruction with explicit operands address the first operandat seg:[rSI]. The value of seg defaults to the DS segment, but may be overridden by asegment prefix. These instructions always address the second operand at ES:[rDI]. ESmay not be overridden. The explicit operands serve only to specify the type (size) ofthe values being compared and the segment used by the first operand.The no-operands forms of the instruction use the DS:[rSI] and ES:[rDI] registers topoint to the values to be compared. The mnemonic determines the size of theoperands.Do not confuse this CMPSD instruction with the same-mnemonic CMPSD (comparescalar double-precision floating-point) instruction in the 128-bit media instruction set.Assemblers can distinguish the instructions by the number and type of operands.For block comparisons, the CMPS instruction supports the REPE or REPZ prefixes(they are synonyms) and the REPNE or REPNZ prefixes (they are synonyms). Fordetails about the REP prefixes, see “Repeat Prefixes” on page 10. If a conditionaljump instruction like JL follows a CMPSx instruction, the jump occurs if the value ofthe seg:[rSI] operand is less than the ES:[rDI] operand. This action allowslexicographical comparisons of string or array elements. A CMPSx instruction canalso operate inside a loop controlled by the LOOPcc instruction.110 CMPS