IN

Input from Port

Opcodes

Hex Mnemonic Encoding Long Mode Legacy Mode Description
ED IN EAX,DX B Valid Valid Input doubleword from I/O port in DX into EAX.
ED IN AX,DX B Valid Valid Input word from I/O port in DX into AX.
EC IN AL,DX B Valid Valid Input byte from I/O port in DX into AL.
E5 ib IN EAX, imm8 A Valid Valid Input dword from imm8 I/O port address into EAX.
E5 ib IN AX, imm8 A Valid Valid Input word from imm8 I/O port address into AX.
E4 ib IN AL, imm8 A Valid Valid Input byte from imm8 I/O port address into AL.

Instruction Operand Encoding

Op/En Operand 0 Operand 1 Operand 2 Operand 3
A NA NA NA imm8
B NA NA NA NA

Description

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 processor's I/O address space. See Chapter 13, "Input/Output," in theIntel®64 and IA-32 Architectures Software Developer's Manual, Volume 1, for more information on accessing I/O ports in the I/O address space.

This instruction's operation is the same in non-64-bit modes and 64-bit mode.

Pseudo Code

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

Flags Affected

None.

Exceptions

64-Bit Mode Exceptions

Exception Description
#UD If the LOCK prefix is used.
#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.

Compatibility Mode Exceptions

Same exceptions as in protected mode.

Virtual-8086 Mode Exceptions

Exception Description
#UD If the LOCK prefix is used.
#PF(fault-code) If a page fault occurs.
#GP(0) If any of the I/O permission bits in the TSS for the I/O port being accessed is 1.

Real-Address Mode Exceptions

Exception Description
#UD If the LOCK prefix is used.

Protected Mode Exceptions

Exception Description
#UD If the LOCK prefix is used.
#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.