x86doc › IN - Input From Port

IN—Input From Port

Opcode	Instruction	Op/En	64-Bit Mode	Compat/Leg Mode	Description
E4 ib	IN AL, imm8	I	Valid	Valid	Input byte from imm8 I/O port address into AL.
E5 ib	IN AX, imm8	I	Valid	Valid	Input word from imm8 I/O port address into AX.
E5 ib	IN EAX, imm8	I	Valid	Valid	Input dword from imm8 I/O port address into EAX.
EC	IN AL,DX	ZO	Valid	Valid	Input byte from I/O port in DX into AL.
ED	IN AX,DX	ZO	Valid	Valid	Input word from I/O port in DX into AX.
ED	IN EAX,DX	ZO	Valid	Valid	Input doubleword from I/O port in DX into EAX.

Instruction Operand Encoding

Op/En	Operand 1	Operand 2	Operand 3	Operand 4
I	imm8	N/A	N/A	N/A
ZO	N/A	N/A	N/A	N/A

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 19, “Input/Output,” in the Intel^® 64 and IA-32 Architectures Software Developer’s Manual, Volume 1, for more infor-mation 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.

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; (* 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.

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.
#PF(fault-code)	If a page fault occurs.
#UD	If the LOCK prefix is used.

Real-Address Mode Exceptions

#UD	If the LOCK prefix is used.

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.
#PF(fault-code)	If a page fault occurs.
#UD	If the LOCK prefix is used.

Compatibility Mode Exceptions

Same exceptions as in protected mode.

64-Bit 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.
#PF(fault-code)	If a page fault occurs.
#UD	If the LOCK prefix is used.