3-463
INSTRUCTION SET REFERENCE
OUTOutput to Port
Description
This instruction copies the value from the second operand (source operand) to the I/O port spec-
ified with the destination operand (first operand). The source operand can be register AL, AX,
or EAX, depending on the size of the port being accessed (8, 16, or 32 bits, respectively); the
destination operand can be a byte-immediate or the DX register. Using a byte immediate allows
I/O port addresses 0 to 255 to be accessed; using the DX register as a source operand allows I/O
ports from 0 to 65,535 to be accessed.
The size of the I/O port being accessed 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 machine code level, I/O instructions are shorter when accessing 8-bit I/O ports. Here, the
upper eight bits of the port address will be 0.
This instruction is only useful for accessing I/O ports located in the processors I/O address
space. Refer to Chapter 10, Input/Output of the Intel Architecture Software Developers Manual,
Volume 1, for more information on accessing I/O ports in the I/O address space.
Intel Architecture Compatibility
After executing an OUT instruction, the Pentium
®
processor insures that the EWBE# pin has
been sampled active before it begins to execute the next instruction. (Note that the instruction
can be prefetched if EWBE# is not active, but it will not be executed until the EWBE# pin is
sampled active.) Only the P6 family of processors has the EWBE# pin; the other Intel Architec-
ture processors do not.
Opcode
Instruction
Description
E6 ib
OUT imm8, AL
Output byte in AL to I/O port address imm8
E7 ib
OUT imm8, AX
Output word in AX to I/O port address imm8
E7 ib
OUT imm8, EAX
Output doubleword in EAX to I/O port address imm8
EE
OUT DX, AL
Output byte in AL to I/O port address in DX
EF
OUT DX, AX
Output word in AX to I/O port address in DX
EF
OUT DX, EAX
Output doubleword in EAX to I/O port address in DX
