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80386 Programmer's Reference Manual -- Opcode IN

IN -- Input from Port

Opcode	Instruction	Clocks	Description	Example
E4 ib	inb imm8,al	12,pm=6/26*	Input byte from immediate port into AL	`inb $0x7f,%al`
E5 ib	inw imm8,ax	12,pm=6/26*	Input word from immediate port into AX	`inw $0x7f,%ax`
E5 ib	inl imm8,eax	12,pm=6/26*	Input dword from immediate port into EAX	`inl $0x7f,%eax`
EC	inb dx,al	13,pm=7/27*	Input byte from port DX into AL	`inb %dx,%al`
ED	inw dx,ax	13,pm=7/27*	Input word from port DX into AX	`inw %dx,%ax`
ED	inl dx,eax	13,pm=7/27*	Input dword from port DX into EAX	`inl %dx,%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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