pg_0505

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3-465

INSTRUCTION SET REFERENCE

OUTS/OUTSB/OUTSW/OUTSDOutput String to Port

Description

These instructions copy data from the source operand (second operand) to the I/O port specified

with the destination operand (first operand). The source operand is a memory location, the

address of which is read from either the DS:EDI or the DS:DI registers (depending on the

address-size attribute of the instruction, 32 or 16, respectively). The DS segment may be over-

ridden with a segment override prefix. The destination operand is an I/O port address (from 0 to

65,535) that is read from the DX register. The size of the I/O port being accessed (that is, the

size of the source and destination operands) is determined by the opcode for an 8-bit I/O port or

by the operand-size attribute of the instruction for a 16- or 32-bit I/O port.

At the assembly-code level, two forms of this instruction are allowed: the explicit-operands

form and the no-operands form. The explicit-operands form (specified with the OUTS

mnemonic) allows the source and destination operands to be specified explicitly. Here, the

source operand should be a symbol that indicates the size of the I/O port and the source address,

and the destination operand must be DX. This explicit-operands form is provided to allow docu-

mentation; however, note that the documentation provided by this form can be misleading. That

is, the source operand symbol must specify the correct type (size) of the operand (byte, word,

or doubleword), but it does not have to specify the correct location. The location is always spec-

ified by the DS:(E)SI registers, which must be loaded correctly before the OUTS instruction is

executed.

The no-operands form provides short forms of the byte, word, and doubleword versions of the

OUTS instructions. Here also DS:(E)SI is assumed to be the source operand and DX is assumed

to be the destination operand. The size of the I/O port is specified with the choice of mnemonic:

OUTSB (byte), OUTSW (word), or OUTSD (doubleword).

After the byte, word, or doubleword is transferred from the memory location to the I/O port, the

(E)SI register is incremented or decremented automatically according to the setting of the DF

flag in the EFLAGS register. (If the DF flag is 0, the (E)SI register is incremented; if the DF flag

is 1, the (E)SI register is decremented.) The (E)SI register is incremented or decremented by ne

for byte operations, by two for word operations, or by four for doubleword operations.

Opcode

Instruction

Description

OUTS DX, m8

Output byte from memory location specified in DS:(E)SI to

I/O port specified in DX

OUTS DX, m16

Output word from memory location specified in DS:(E)SI

to I/O port specified in DX

OUTS DX, m32

Output doubleword from memory location specified in

DS:(E)SI to I/O port specified in DX

OUTSB