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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

up: Chapter 17 -- 80386 Instruction Set
prev: LOCK Assert LOCK# Signal Prefix
next: LOOP/LOOPcond Loop Control with CX Counter