OUT--Output to Port

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

Description

Copies the value from the second operand (source operand) to the I/O port specified 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 processor's I/O address space. See Chapter 9, Input/Output, in the IA-32 Intel(R) Architecture Software Developer's Manual, Volume 1, for more information on accessing I/O ports in the I/O address space.

Intel(R) 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 Pentium processor family has the EWBE# pin; the other IA-32 processors do not.

Operation

IF ((PE 1) AND ((CPL > IOPL) OR (VM 1)))
THEN (* Protected mode with CPL > IOPL or virtual-8086 mode *)
IF (Any I/O Permission Bit for I/O port being accessed 1)
THEN (* I/O operation is not allowed *)
#GP(0);
ELSE ( * I/O operation is allowed *)
DEST SRC; (* Writes to selected I/O port *)
FI;
ELSE (Real Mode or Protected Mode with CPL IOPL *)
DEST SRC; (* Writes to selected I/O port *)
FI;

Flags Affected

None.

Protected Mode Exceptions

#GP(0) - If the CPL is greater than (has less privilege) the I/O privilege level (IOPL) and any of the corresponding I/O permission bits in TSS for the I/O port being accessed is 1.

Real-Address Mode Exceptions

None.

Virtual-8086 Mode Exceptions

#GP(0) - If any of the I/O permission bits in the TSS for the I/O port being accessed is 1.

 

 

 

 

 

 

 

For details, see Volume 2A and Volume 2B of the Intel(R) 64 and IA-32 Intel Architecture Software Developer's Manual. For the latest updates on the instruction set information, go to the web site.