Input/ Output
I/O vs. Memory mapped I/O
Heba Abdel-Nabi, Ph.D.
Introduction
❑ 8086 uses 16 address bits → A[15:0] to locate an I/O port
❑ 64k address space → 65,536 possible I/O ports
❑ Data transfer between ports and the processor is done over data bus
❑I/O devices serve two main purposes: To communicate with outside world
and to store data
Data bus
AL
AX
I/O I/O I/O
8086
Address bus A[15:0]
Based on Dr. Esam Slides
2
I/O address mapping
1. Memory-mapped I/O
• Uses the same shared memory of 8086
• Reading and writing are similar to memory read/write, e.g. MOV, XCHG
• Uses same memory read and write signals
2. Isolated I/O (AKA I/O mapped I/O)
• Separate I/O address space, thus, separate I/O read and write signals are needed
• uses dedicated I/O instructions (IN and OUT).
• 64 KB address space, can be any combination of 8- and 16-bit I/O ports
FFFFF FFFFF
Range of memory
Memory addresses
addressing I/O assigned for I/O transfers
space FFFF I/O
addressing Memory addressing
00000 space
00000 0000 space
Isolated I/O Memory-mapped I/O
Based on Dr. Esam Slides
3
I/O address mapping
I/O mapped I/O memory mapped I/O
Treated as an I/O device (has a Treated as a memory location (has a
Discussed later
separate 8- or 16- I/O address) 20-bit memory address)
Given an IOR’ and IOW’ signals Given an MEMR’ and MEMW’ signals
Can be accessed by IN/ OUT Can be accessed using any memory
instruction instruction
Only work with AL/AX registers Can work with any register
Example: Thermometer Example: Video Graphic arrays (VGA)
card →address of 0B8000H
4
Memory-mapped vs. Isolated I/O
Memory Space
00000
I/O Space
0000
1M 8
64K 8
FFFFF FFFF
Discussed later
IO/M = 0 IO/M = 1
MOV AX, DATA ; To Read from PORTA
MOV DS, AX MOV DX, PORTA
MOV BX, OFFSET IN AL, DX
; To Read from a Memory Location
MOV AL, [BX] ; To Write to PORTB
MOV DX, PORTB
MOV AL, VALUE
; To Write to a Memory Location
OUT DX, AL
MOV AL, VALUE
MOV [BX], AL
5
Input / Output Instructions
• For 8-bit port (data transferred size is 8 bits)
• Two possibilities (8-bit address or 16-bit address)
IN AL, Port # OUT Port #, AL → directly
MOV DX, Port # MOV DX, Port # → indirectly
IN AL, DX OUT DX, AL
• For 16-bit port (data transferred size is 16 bits)
• Two possibilities (8-bit address or 16-bit address)
IN AX, Port # OUT Port #, AX → directly
MOV DX, Port # MOV DX, Port # → indirectly
IN AX, DX OUT DX, AX
6
Example: thermometer control
again:
in al, 125 ; Data port Task: Control the thermometer
cmp al, 80 to maintain the temperature
jae off within the range of 60°C to
cmp al,60 80°C.
jb on •If the temperature drops
jmp again below 60°C, turn on the flame.
•If the temperature rises to or
above 80°C, turn off the flame.
off:
mov al,0
out 127,al ; Control port
jmp again
on:
mov al,1
out 127,al
jmp again
Example: VGA
• Print characters ‘a’ and ‘b’ directly without using DOS interrupt
mov ax, 0b800h
mov ds, ax
mov [0000],'a' ; each character takes two bytes
mov [0001],02h ; change the characteristics of the character
mov [0002],'b'
mov [0003],65h
