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

24594 Rev. 3.10 February 2005 AMD64 TechnologySCASSCASBSCASWSCASDSCASQScan StringCompares the AL, AX, EAX, or RAX register with the byte, word, doubleword, orquadword pointed to by ES:rDI, sets the status flags in the rFLAGS register accordingto the results, and then increments or decrements the rDI register according to thestate of the DF flag in the rFLAGS register.If the DF flag is 0, the instruction increments the rDI register; otherwise, itdecrements it. The instruction increments or decrements the rDI register by 1, 2, 4, or8, depending on the size of the operands.The forms of the SCASx instruction with an explicit operand address the operand atES:rDI. The explicit operand serves only to specify the size of the values beingcompared.The no-operands forms of the instruction use the ES:rDI registers to point to the valueto be compared. The mnemonic determines the size of the operands and the specificregister containing the other comparison value.For block comparisons, the SCASx instructions support 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. A SCASxinstruction can also operate inside a loop controlled by the LOOPcc instruction.Mnemonic Opcode DescriptionSCAS mem8SCAS mem16SCAS mem32SCAS mem64AEAFAFAFCompare the contents of the AL register with the byte at ES:rDI,and then increment or decrement rDI.Compare the contents of the AX register with the word at ES:rDI,and then increment or decrement rDI.Compare the contents of the EAX register with the doubleword atES:rDI, and then increment or decrement rDI.Compare the contents of the RAX register with the quadword atES:rDI, and then increment or decrement rDI.SCASx 265