---
title: "Control memory vs main memory"
subject: "Computer Organization and Architecture"
chapter: 3
key: "semester_5/computer_organization_and_architecture:control-vs-main-memory"
source: https://www.bctengineer.com/q/semester_5/computer_organization_and_architecture/control-vs-main-memory
site: BCT Engineering
---

# Control memory vs main memory

Computer Organization and Architecture · Chapter 3: Control Unit

- Asked in: 2079 Baisakh, 3 marks
- 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/control-vs-main-memory
- Whole chapter: https://www.bctengineer.com/md/semester_5/computer_organization_and_architecture/chapter-3

## Question

Differentiate between control memory and main memory.

## Short answer

Control memory holds microprograms, is ROM, very fast, small, inside the CPU, and invisible to the programmer. Main memory stores user programs and data, is RAM, larger, slower, external on the system bus, and programmer-visible.

## Answer

| Basis | Control memory | Main memory |
|---|---|---|
| Contents | Microprograms (microinstructions) | User programs and data |
| Type | ROM (read only; sometimes writable control store) | RAM (read and write) |
| Location | Inside the control unit of the CPU | Outside the CPU, on the system bus |
| Used by | Control unit, to produce control signals | CPU, I/O and the programmer |
| Speed | Very fast | Slower |
| Size | Small (for example 128 to a few K words) | Large (MB to GB) |
| Word width | Wide (microinstruction, e.g. 20 bits or more) | Equals word length of the machine |
| Visible to programmer | No | Yes |
| Addressed by | Control address register (CAR) | Address register (AR / MAR) |

## Check yourself

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

1. What does each memory hold?
   A. Control: microprograms; main: programs and data
   B. Control: data; main: microprograms
   C. Both: programs
   D. Control: stack; main: ROM
   Answer: A. Basic difference. (Key point: Contents)

2. Which type is control memory?
   A. Always RAM
   B. ROM (sometimes writable control store)
   C. Magnetic disk
   D. DRAM
   Answer: B. Main memory is RAM. (Key point: Type)

3. What addresses each?
   A. Control: AR; main: CAR
   B. Both PC
   C. Control: CAR; main: AR/MAR
   D. Control: SBR; main: DR
   Answer: C. CAR belongs to control unit. (Key point: Addressing)

## Related in Computer Organization and Architecture

- [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×
- [Organization of control memory](https://www.bctengineer.com/question.md?key=semester_5%2Fcomputer_organization_and_architecture%3Acontrol-memory-org) — Computer Organization and Architecture, chapter 3
- [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
- [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×
- [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
- [Direct mapping with 32-block memory, 8-block cache](https://www.bctengineer.com/question.md?key=semester_5%2Fcomputer_organization_and_architecture%3Adirect-mapping-example) — Computer Organization and Architecture, chapter 6, asked 3×

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