OUTS/OUTSB/OUTSW/OUTSD -- Output String to Port

Opcode   Instruction     Clocks          Description

6E       OUTS DX,r/m8    14,pm=8*/28**   Output byte [(E)SI] to port in DX
6F       OUTS DX,r/m16   14,pm=8*/28**   Output word [(E)SI] to port in DX
6F       OUTS DX,r/m32   14,pm=8*/28**   Output dword [(E)SI] to port in DX
6E       OUTSB           14,pm=8*/28**   Output byte DS:[(E)SI] to port in
                                         DX
6F       OUTSW           14,pm=8*/28**   Output word DS:[(E)SI] to port in
                                         DX
6F       OUTSD           14,pm=8*/28**   Output dword DS:[(E)SI] to port in
                                         DX

Notes

*If CPL <= IOPL **If CPL > IOPL or if in virtual 8086 mode

Operation

IF AddressSize = 16
THEN use SI for source-index;
ELSE (* AddressSize = 32 *)
   use ESI for source-index;
FI;

IF (PE = 1) AND ((VM = 1) OR (CPL > IOPL))
THEN (* Virtual 8086 mode, or protected mode with CPL > IOPL *)
   IF NOT I-O-Permission (DEST, width(DEST))
   THEN #GP(0);
   FI;
FI;
IF byte type of instruction
THEN
   [DX] := [source-index]; (* Write byte at DX I/O address *)
   IF DF = 0 THEN IncDec := 1 ELSE IncDec := -1; FI;
FI;
IF OperandSize = 16
THEN
   [DX] := [source-index]; (* Write word at DX I/O address *)
   IF DF = 0 THEN IncDec := 2 ELSE IncDec := -2; FI;
FI;
IF OperandSize = 32
THEN
   [DX] := [source-index]; (* Write dword at DX I/O address *)
   IF DF = 0 THEN IncDec := 4 ELSE IncDec := -4; FI;
   FI;
FI;
source-index := source-index + IncDec;

Description

OUTS transfers data from the memory byte, word, or doubleword at the source-index register to the output port addressed by the DX register. 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 ESI is used for the source-index register.

OUTS does not allow specification of the port number as an immediate value. The port must be addressed through the DX register value. Load the correct value into DX before executing the OUTS instruction.

The address of the source data is determined by the contents of source-index register. Load the correct index value into SI or ESI before executing the OUTS instruction.

After the transfer, source-index register is advanced automatically. If the direction flag is 0 (CLD was executed), the source-index register is incremented; if the direction flag is 1 (STD was executed), it is decremented. The amount of the increment or decrement is 1 if a byte is output, 2 if a word is output, or 4 if a doubleword is output.

OUTSB, OUTSW, and OUTSD are synonyms for the byte, word, and doubleword OUTS instructions. OUTS can be preceded by the REP prefix for block output of CX bytes or words. Refer to the REP instruction for details on this operation.

Flags Affected

None

Protected Mode Exceptions

#GP(0) if CPL is greater than IOPL and any of the corresponding I/O permission bits in TSS equals 1; #GP(0) for an illegal memory operand effective address in the CS, DS, or ES 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

#GP(0) fault if any of the corresponding I/O permission bits in TSS equals 1; #PF(fault-code) for a page fault

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