---
title: "Memory-mapped I/O reduces address space"
subject: "Computer Organization and Architecture"
chapter: 7
key: "semester_5/computer_organization_and_architecture:mmio-address-space"
source: https://www.bctengineer.com/q/semester_5/computer_organization_and_architecture/mmio-address-space
site: BCT Engineering
---

# Memory-mapped I/O reduces address space

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

- Asked in: 2075 Chaitra, 2 marks
- Video: [Memory Mapped IO || Lesson 78 || Computer Organization || Learning Monkey ||](https://www.youtube.com/watch?v=7qVWVCaNCLw) — Wisdomers - Computer Science and Engineering 
- 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/mmio-address-space
- Whole chapter: https://www.bctengineer.com/md/semester_5/computer_organization_and_architecture/chapter-7

## Question

Why are memory address spaces reduced in memory mapped I/O?

## Short answer

In memory-mapped I/O, device registers share the memory address space, so each I/O address takes a location otherwise available for memory. Thus, the memory address space is reduced by the amount reserved for I/O; for example, reserving 4K in a 64K space leaves 60K for memory.

## Answer

In memory-mapped I/O, the I/O device registers (ports) are given addresses from the **same address space** as memory, and the CPU uses ordinary memory instructions to access them. Each address given to an I/O port can no longer be used for a memory location. Therefore the address space available for memory is reduced by the amount reserved for I/O. For example, a CPU with a 16-bit address (64K) that reserves 4K addresses for devices has only 60K left for memory.

## Check yourself

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

1. Why does the memory address space shrink in memory-mapped I/O?
   A. Ports share the same address space, so addresses given to I/O cannot be used for memory
   B. I/O ports use extra pins
   C. Instructions get longer
   D. Memory is physically removed
   Answer: A. I/O registers occupy addresses from the common space. (Key point: Why reduced)

2. A CPU with 16-bit addresses (64K) reserves 4K addresses for devices. How much remains for memory?
   A. 60K
   B. 64K
   C. 68K
   D. 4K
   Answer: A. 64K - 4K = 60K. (Key point: Calculation)

3. Which instructions access I/O ports in memory-mapped I/O?
   A. Ordinary memory instructions such as LOAD and STORE
   B. Only IN and OUT
   C. Only jump instructions
   D. Special I/O instructions only
   Answer: A. Ports have memory addresses, so ordinary memory instructions are used. (Key point: Instruction type)

## Related in Computer Organization and Architecture

- [Isolated I/O vs memory-mapped I/O](https://www.bctengineer.com/question.md?key=semester_5%2Fcomputer_organization_and_architecture%3Aisolated-vs-mmio) — Computer Organization and Architecture, chapter 7, asked 2×
- [Address sequencing for control memory](https://www.bctengineer.com/question.md?key=semester_5%2Fcomputer_organization_and_architecture%3Aaddress-sequencer) — Computer Organization and Architecture, chapter 3, asked 7×
- [Address sequencing capabilities of control memory](https://www.bctengineer.com/question.md?key=semester_5%2Fcomputer_organization_and_architecture%3Aaddress-sequencing-capabilities) — Computer Organization and Architecture, chapter 3
- [Instruction fields and address formats for X=(E+F)*(G+H)](https://www.bctengineer.com/question.md?key=semester_5%2Fcomputer_organization_and_architecture%3Acode-fields-e-f-g-h) — Computer Organization and Architecture, chapter 2

## Related in other subjects

- [Memory-mapped I/O design for 8085](https://www.bctengineer.com/question.md?key=semester_5%2Finstrumentation_ii%3Amemmapped-io-design-1000h) — Instrumentation II, chapter 2
- [Unique/non-unique decoding, memory mapped I/O](https://www.bctengineer.com/question.md?key=semester_4%2Fmicroprocessor%3Aunique-nonunique-decoding-memory-mapped-io) — Microprocessor, chapter 4
- [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×
- [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