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

24594 Rev. 3.10 February 2005 AMD64 TechnologyLODSLODSBLODSWLODSDLODSQLoad StringCopies the byte, word, doubleword, or quadword in the memory location pointed to bythe DS:rSI registers to the AL, AX, EAX, or RAX register, depending on the size of theoperand, and then increments or decrements the rSI register according to the state ofthe DF flag in the rFLAGS register.If the DF flag is 0, the instruction increments rSI; otherwise, it decrements rSI. Itincrements or decrements rSI by 1, 2, 4, or 8, depending on the number of bytes beingloaded.The forms of the LODS instruction with an explicit operand address the operand atseg:[rSI]. The value of seg defaults to the DS segment, but may be overridden by asegment prefix. The explicit operand serves only to specify the type (size) of the valuebeing copied and the specific registers used.The no-operands forms of the instruction always use the DS:[rSI] registers to point tothe value to be copied (they do not allow a segment prefix). The mnemonic determinesthe size of the operand and the specific registers used.The LODSx instructions support the REP prefixes. For details about the REP prefixes,see “Repeat Prefixes” on page 10. More often, software uses the LODSx instructioninside a loop controlled by a LOOPcc instruction as a more efficient replacement forinstructions like:mov eax, dword ptr ds:[esi]add esi, 4The LODSQ instruction can only be used in 64-bit mode.Mnemonic Opcode DescriptionLODS mem8 AC Load byte at DS:rSI into AL and then increment or decrement rSI.LODS mem16LODS mem32ADADLoad word at DS:rSI into AX and then increment or decrementrSI.Load doubleword at DS:rSI into EAX and then increment ordecrement rSI.LODSx 187