IN -- Input from Port

Opcode    Instruction   Clocks            Description

E4  ib    IN AL,imm8    12,pm=6*/26**     Input byte from immediate port
                                          into AL
E5  ib    IN AX,imm8    12,pm=6*/26**     Input word from immediate port
                                          into AX
E5  ib    IN EAX,imm8   12,pm=6*/26**     Input dword from immediate port
                                          into EAX
EC        IN AL,DX      13,pm=7*/27**     Input byte from port DX into AL
ED        IN AX,DX      13,pm=7*/27**     Input word from port DX into AX
ED        IN EAX,DX     13,pm=7*/27**     Input dword from port DX into
                                          EAX

Notes

*If CPL <= IOPL **If CPL > IOPL or if in virtual 8086 mode

Operation

IF (PE = 1) AND ((VM = 1) OR (CPL > IOPL))
THEN (* Virtual 8086 mode, or protected mode with CPL > IOPL *)
   IF NOT I-O-Permission (SRC, width(SRC))
   THEN #GP(0);
   FI;
FI;
DEST := [SRC]; (* Reads from I/O address space *)

Description

IN transfers a data byte or data word from the port numbered by the second operand into the register (AL, AX, or EAX) specified by the first operand. Access any port from 0 to 65535 by placing the port number in the DX register and using an IN instruction with DX as the second parameter. These I/O instructions can be shortened by using an 8-bit port I/O in the instruction. The upper eight bits of the port address will be 0 when 8-bit port I/O is used.

Flags Affected

None

Protected Mode Exceptions

#GP(0) if the current privilege level is larger (has less privilege) than IOPL and any of the corresponding I/O permission bits in TSS equals 1

Real Address Mode Exceptions

None

Virtual 8086 Mode Exceptions

#GP(0) fault if any of the corresponding I/O permission bits in TSS equals 1

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