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


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 9, Input/Output, in the Intel Architecture Software Developer's Manual, Vo l u me 1 , for more information on accessing I/O ports in the I/O address space.


Operands        Bytes                Clocks

al, imm8             2                    7   NP

ax, imm8             2                   7   NP

eax, imm8            2                   7   NP

al, dx               1                  7   NP

ax, dx               1                  7   NP

eax, dx              1                 7   NP


Protected mode

acc, imm              2                    4/21/19 NP

acc, dx               1                       4/21/19 NP


cycles for: CPL <= IOPL / CPL > IOPL / V86


Flags

None.

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