LODS/LODSB/LODSW/LODSD -- Load String Operand

LODS/LODSB/LODSW/LODSD -- Load String Operand Opcode Instruction Clocks Description AC LODS m8 5 Load byte [(E)SI] into AL AD LODS m16 5 Load word [(E)SI] into AX AD LODS m32 5 Load dword [(E)SI] into EAX AC LODSB 5 Load byte DS:[(E)SI] into AL AD LODSW 5 Load word DS:[(E)SI] into AX AD LODSD 5 Load dword DS:[(E)SI] into EAX Operation IF AddressSize = 16 THEN use SI for source-index ELSE (* AddressSize = 32 *) use ESI for source-index; FI; IF byte type of instruction THEN AL = [source-index]; (* byte load *) IF DF = 0 THEN IncDec = 1 ELSE IncDec = -1; FI; ELSE IF OperandSize = 16 THEN AX = [source-index]; (* word load *) IF DF = 0 THEN IncDec = 2 ELSE IncDec = -2; FI; ELSE (* OperandSize = 32 *) EAX = [source-index]; (* dword load *) IF DF = 0 THEN IncDec = 4 ELSE IncDec = -4; FI; FI; FI; source-index = source-index + IncDec Description LODS loads the AL, AX, or EAX register with the memory byte, word, or doubleword at the location pointed to by the source-index register. After the transfer is made, the source-index register is automatically advanced. If the direction flag is 0 (CLD was executed), the source index increments; if the direction flag is 1 (STD was executed), it decrements. The increment or decrement is 1 if a byte is loaded, 2 if a word is loaded, or 4 if a doubleword is loaded. If the address-size attribute for this instruction is 16 bits, SI is used for the source-index register; otherwise the address-size attribute is 32 bits, and the ESI register is used. The address of the source data is determined solely by the contents of ESI/SI. Load the correct index value into SI before executing the LODS instruction. LODSB, LODSW, LODSD are synonyms for the byte, word, and doubleword LODS instructions. LODS can be preceded by the REP prefix; however, LODS is used more typically within a LOOP construct, because further processing of the data moved into EAX, AX, or AL is usually necessary. Flags Affected None Protected Mode Exceptions #GP(0) for an illegal memory operand effective address in the CS, DS, ES, FS, or GS segments; #SS(0) for an illegal address in the SS segment; #PF(fault-code) for a page fault Real Address Mode Exceptions Interrupt 13 if any part of the operand would lie outside of the effective address space from 0 to 0FFFFH Virtual 8086 Mode Exceptions Same exceptions as in Real Address Mode; #PF(fault-code) for a page fault