I/O-Mapped vs Memory-Mapped I/O

Real-world analogy

What is it?

I/O-mapped (isolated) I/O uses separate IN/OUT instructions and a dedicated 64KB address space to access peripherals, signaled by M/IO=0. Memory-mapped I/O places device registers in the normal memory address space and uses standard MOV/ADD/CMP instructions, signaled by M/IO=1. The 8086 supports both methods, and most real systems use a combination of both.

Real-world relevance

Your PC still uses both methods today. When your OS reads the keyboard controller, it uses IN/OUT instructions (I/O-mapped). When your GPU driver writes pixels, it writes to memory-mapped video RAM. The PCI Express bus on modern computers is entirely memory-mapped — every PCIe device gets a range of physical addresses.

Key points

• Two Approaches to Device Communication — The CPU must read from and write to peripheral devices (keyboard, display, timer). There are two fundamental approaches: (1) treat devices as memory locations (memory-mapped), or (2) give devices their own separate address space (I/O-mapped / isolated I/O). The 8086 supports BOTH methods — it has a separate M/IO pin to distinguish them.
• I/O-Mapped (Isolated) I/O — IN and OUT — In isolated I/O, peripheral devices have their own address space (0000h-FFFFh for 8086, 64KB). Access uses special instructions: IN reads a port, OUT writes a port. The CPU asserts M/IO=0 to signal an I/O operation. This keeps the memory address space completely separate from I/O — devices cannot accidentally shadow RAM or ROM.
• Memory-Mapped I/O — MOV to Device — In memory-mapped I/O, device registers are assigned addresses within the normal memory space. You read/write them with ordinary MOV, ADD, or any memory instruction — no special I/O instructions needed. The CPU asserts M/IO=1 (same as a normal memory access). The downside: these addresses are 'stolen' from memory space, reducing available RAM/ROM capacity.
• The M/IO Signal — The 8086 M/IO pin tells external hardware whether the current bus cycle is a memory access (M/IO=1) or an I/O access (M/IO=0). Address decoders use this signal along with address lines to generate the correct chip select. For memory chips, CS requires M/IO=1. For I/O devices, CS requires M/IO=0. This one pin is what makes the two separate address spaces possible.
• Address Space Comparison — With I/O-mapped I/O, the 8086 has two separate spaces: 1MB for memory and 64KB for I/O. With memory-mapped I/O, there is only one 1MB space shared by both memory and devices. I/O-mapped preserves the full memory space but requires special instructions. Memory-mapped sacrifices some memory addresses but allows richer instruction use on device registers.
• Pros and Cons Summary — I/O-mapped: clear separation, smaller address (simpler decoding), faster for simple reads — but limited to IN/OUT/AL/AX only. Memory-mapped: use any instruction (MOV, ADD, AND, CMP), no special instructions needed — but uses up memory addresses and address decoding is more complex. Most real systems use BOTH: I/O-mapped for simple control ports, memory-mapped for large buffers (like video RAM).
• IBM PC Example: Both Methods in Action — The original IBM PC used both methods simultaneously. Simple devices like the keyboard controller (port 60h), timer (port 40h), and interrupt controller (port 20h) used I/O-mapped I/O with IN/OUT. The video display buffer (B8000h) and ROM BIOS (F0000h) used memory-mapped addresses. This hybrid approach gave the best of both worlds.
• Interfacing a Device to Both Spaces — When designing a system, you choose which I/O method to use for each device. For the 8255 PPI, you typically use I/O-mapped (ports 80h-83h). For video memory, memory-mapped (segment B800h). The address decoding circuitry must include M/IO in the chip select logic to prevent a memory access from accidentally activating an I/O device or vice versa.

Code example

; =============================================
; I/O-Mapped vs Memory-Mapped I/O Comparison
; =============================================
;
; === I/O-Mapped (Isolated) I/O ===
; Access a device at I/O port 60h:
;
  IN AL, 60h           ; read byte from port 60h
                        ; M/IO = 0 (I/O cycle)
                        ; address bus = 0060h
  OR AL, 80h           ; process in register
  OUT 60h, AL          ; write back to port 60h
;
; === Memory-Mapped I/O ===
; Access a device at memory address B8000h:
;
  MOV AX, 0B800h
  MOV ES, AX           ; point ES to device
  MOV AL, ES:[0]       ; read from device
                        ; M/IO = 1 (memory cycle)
                        ; address bus = B8000h
  OR ES:[0], 80h       ; modify device register DIRECTLY
                        ; (can't do this with I/O-mapped!)
;
; Key difference: I/O-mapped uses IN/OUT only
; Memory-mapped can use ANY memory instruction

Line-by-line walkthrough

1. 1. Title comment for the I/O comparison
2. 2. Separator line
3. 3. Blank line
4. 4. Section header: I/O-Mapped (Isolated) I/O
5. 5. Comment: accessing a device at I/O port 60h
6. 6. Blank line
7. 7. IN AL, 60h — special instruction reads one byte from port 60h into AL
8. 8. Comment: M/IO pin goes LOW (0) telling hardware this is an I/O cycle
9. 9. Comment: only the lower 16 address bits carry the port address
10. 10. OR AL, 80h — manipulate the data in a register (set bit 7)
11. 11. OUT 60h, AL — write the modified byte back to port 60h
12. 12. Blank line
13. 13. Section header: Memory-Mapped I/O
14. 14. Comment: accessing a device at memory address B8000h
15. 15. Blank line
16. 16. Load segment value 0B800h into AX
17. 17. Move AX into ES segment register (ES now points to B8000h)
18. 18. Read a byte from ES:[0] which is physical address B8000h — the device register
19. 19. Comment: M/IO pin goes HIGH (1) because this is a normal memory cycle
20. 20. Comment: the full 20-bit address appears on the bus
21. 21. OR ES:[0], 80h — directly modify the device register in place using OR instruction
22. 22. Comment: this direct-modify trick is impossible with I/O-mapped I/O
23. 23. Blank line
24. 24. Summary: I/O-mapped is limited to IN/OUT instructions
25. 25. Memory-mapped can leverage the entire instruction set on device registers

Spot the bug

; Read from I/O port 300h
;
  IN AL, 300h         ; read byte from port 300h
;
; Write to memory-mapped device at C0000h
;
  MOV AX, 0C000h
  MOV DS, AX
  OUT [0], AL          ; write to device

Explain like I'm 5

Fun fact

Hands-on challenge

More resources

• I/O Mapped vs Memory Mapped I/O (Neso Academy)
• Memory Mapped I/O and Isolated I/O (GeeksforGeeks)
• 8086 I/O Interfacing (Education 4u)
• x86 IN/OUT Instructions Reference (x86 Reference)