---
title: "Isolated I/O vs memory-mapped I/O"
subject: "Computer Organization and Architecture"
chapter: 7
key: "semester_5/computer_organization_and_architecture:isolated-vs-mmio"
source: https://www.bctengineer.com/q/semester_5/computer_organization_and_architecture/isolated-vs-mmio
site: BCT Engineering
---

# Isolated I/O vs memory-mapped I/O

Computer Organization and Architecture · Chapter 7: Input-Output Organization

- Asked in: 2078 Bhadra, 4 marks; 2072 Chaitra, 4 marks (asked 2 times)
- Video: [L-4.2 Isolated I/O vs. Memory Mapped I/O | I/O Mapping | Computer Architecture | COA | CSA](https://www.youtube.com/watch?v=XLpvrI9JL8k) — Shanu Kuttan CSE Classes
- Answer checked by Claude Opus 5.5 (high effort) on 2026-10-14
- Page: https://www.bctengineer.com/q/semester_5/computer_organization_and_architecture/isolated-vs-mmio
- Whole chapter: https://www.bctengineer.com/md/semester_5/computer_organization_and_architecture/chapter-7

## Question

Differentiate between isolated I/O and memory-mapped I/O.

## Short answer

Isolated I/O uses separate address spaces and special IN/OUT instructions, while memory‑mapped I/O shares the common memory address space and uses regular load/store instructions. Isolated I/O reserves the full memory range but needs extra control lines; memory‑mapped I/O simplifies hardware at the cost of reduced memory address space.

## Answer

| Point | Isolated I/O | Memory-mapped I/O |
|---|---|---|
| Address space | Separate address spaces for memory and I/O | One common address space shared by memory and I/O |
| Control signals | Separate: IORD, IOWR and MEMRD, MEMWR | Same memory read/write signals |
| Instructions | Special instructions: IN, OUT | Ordinary memory instructions: LOAD, STORE, MOV |
| Address space for memory | Full address space available | Reduced by the I/O addresses |
| Hardware | More control lines and decoding for I/O | Simpler hardware |
| Programming | Less flexible, limited instructions | All memory instructions, including arithmetic, work on I/O |
| Example | Intel 8085/8086 with IN/OUT | Motorola 68000, ARM |

## Check yourself

The answer's key points as multiple-choice questions, with the correct option and why.

1. How do the address spaces differ between isolated I/O and memory-mapped I/O?
   A. Both use separate spaces
   B. Isolated I/O uses separate spaces; memory-mapped uses one common space
   C. Both use one common space
   D. Isolated I/O uses a common space; memory-mapped uses separate spaces
   Answer: B. Isolated I/O has separate memory and I/O address spaces; memory-mapped I/O shares one space. (Key point: Address space)

2. Which instructions access I/O in isolated I/O?
   A. Special IN and OUT instructions
   B. LOAD and STORE
   C. Only jump instructions
   D. MOV and ADD
   Answer: A. Isolated I/O uses special IN/OUT instructions with separate IORD/IOWR signals. (Key point: Instructions)

3. Which scheme reduces the address space available for memory?
   A. Neither
   B. Isolated I/O
   C. Both equally
   D. Memory-mapped I/O
   Answer: D. In memory-mapped I/O, addresses used by I/O ports cannot be used for memory. (Key point: Memory space cost)

4. Which pair is correctly matched?
   A. Isolated I/O with Motorola 68000
   B. Isolated I/O with 8085/8086; memory-mapped I/O with Motorola 68000/ARM
   C. Memory-mapped with 8085 IN/OUT
   D. Isolated I/O with ARM; memory-mapped with 8086
   Answer: B. 8085/8086 use IN/OUT (isolated); 68000 and ARM use memory-mapped I/O. (Key point: Examples)

## Related in Computer Organization and Architecture

- [Memory-mapped I/O reduces address space](https://www.bctengineer.com/question.md?key=semester_5%2Fcomputer_organization_and_architecture%3Ammio-address-space) — Computer Organization and Architecture, chapter 7
- [Control memory vs main memory](https://www.bctengineer.com/question.md?key=semester_5%2Fcomputer_organization_and_architecture%3Acontrol-vs-main-memory) — Computer Organization and Architecture, chapter 3
- [DMA vs CPU memory request](https://www.bctengineer.com/question.md?key=semester_5%2Fcomputer_organization_and_architecture%3Adma-vs-cpu-request) — Computer Organization and Architecture, chapter 7

## Related in other subjects

- [Memory mapped versus isolated I/O](https://www.bctengineer.com/question.md?key=semester_6%2Fembedded_system%3Amemory-mapped-vs-standard-io) — Embedded System, chapter 5, asked 2×
- [I/O mapped vs memory mapped I/O](https://www.bctengineer.com/question.md?key=semester_5%2Finstrumentation_ii%3Aiomapped-vs-memmapped) — Instrumentation II, chapter 2
- [Arbitration and memory-mapped vs standard I/O](https://www.bctengineer.com/question.md?key=semester_6%2Fembedded_system%3Aarbitration-memory-mapped-io) — Embedded System, chapter 5
- [Short note: I/O mapped and memory mapped I/O](https://www.bctengineer.com/question.md?key=semester_4%2Fmicroprocessor%3Aio-mapped-memory-mapped-io) — Microprocessor, chapter 4