3-299
INSTRUCTION SET REFERENCE
INInput from Port
Description
This instruction copies the value from the I/O port specified with the second operand (source
operand) to the destination operand (first operand). The source operand can be a byte-immediate
or the DX register; the destination operand can be register AL, AX, or EAX, depending on the
size of the port being accessed (8, 16, or 32 bits, respectively). Using the DX register as a source
operand allows I/O port addresses from 0 to 65,535 to be accessed; using a byte immediate
allows I/O port addresses 0 to 255 to be accessed.
When accessing an 8-bit I/O port, the opcode determines the port size; when accessing a 16- and
32-bit I/O port, the operand-size attribute determines the port size.
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.
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; (* Reads from selected I/O port *)
FI;
ELSE (Real Mode or Protected Mode with CPL
?
IOPL *)
DEST
<
SRC; (* Reads from selected I/O port *)
FI;
Flags Affected
None.
Opcode
Instruction
Description
E4 ib
IN AL,imm8
Input byte from imm8 I/O port address into AL
E5 ib
IN AX,imm8
Input byte from imm8 I/O port address into AX
E5 ib
IN EAX,imm8
Input byte from imm8 I/O port address into EAX
EC
IN AL,DX
Input byte from I/O port in DX into AL
ED
IN AX,DX
Input word from I/O port in DX into AX
ED
IN EAX,DX
Input doubleword from I/O port in DX into EAX
