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.