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

# Organization of control memory

Computer Organization and Architecture · Chapter 3: Control Unit

- Asked in: 2071 Shrawan, 4 marks
- Video: [Control Memory || Microprogrammed Control Organization || Computer Organization Architecture || Unit](https://www.youtube.com/watch?v=eC6pWLNPSQ4) — Sudhakar Atchala
- 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-memory-org
- Whole chapter: https://www.bctengineer.com/md/semester_5/computer_organization_and_architecture/chapter-3

## Question

Explain the organization of a control memory.

## Short answer

Control memory is a fast ROM inside the control unit, storing microprograms organized by routines (fetch, indirect, execute). The CAR addresses it; the fetched microinstruction is placed in CDR and its fields are decoded into control signals for the datapath.

## Answer

The control memory is a ROM (read-only memory) inside the control unit in which all the control information (the microprograms) is stored permanently. Each word is a microinstruction.

- **Size:** 2ⁿ words for an n-bit address; Mano's example has 128 words of 20 bits, with a 7-bit address.
- **Organization of routines:** it is divided into areas. In Mano's example, the fetch routine is at 64–66, the indirect routine at 67–68, and the execute routine of each instruction at the address given by 0 xxxx 00 (ADD = 0, BRANCH = 4, STORE = 8, EXCHANGE = 12), so each routine can have up to 4 microinstructions.
- **Addressing:** the control address register (CAR) gives the address of the microinstruction; the sequencer supplies the next address.
- **Output:** the microinstruction is placed in the control data (pipeline) register, whose fields give the control signals.
- **Access:** it is a very fast memory, so that it does not slow the control unit. A writable control memory (writable control store, WCS) can be used for flexible designs.

```text
 CAR --> [ Address decoder ]--> [ ROM array: micro-routines ]
                                      |
                              Control data register
                                      |
                               control signals
```

## Check yourself

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

1. What is control memory?
   A. A ROM inside the control unit storing microprograms, one microinstruction per word
   B. RAM storing user data
   C. Cache for operands
   D. Disk storage
   Answer: A. It stores control information permanently. (Key point: Nature)

2. Mano's control memory size?
   A. 64 words of 16 bits
   B. 128 words of 20 bits with a 7-bit address
   C. 256 words of 8 bits
   D. 128 words of 32 bits
   Answer: B. 2^7 = 128. (Key point: Mano size)

3. Where are execute routines placed?
   A. At 64-66
   B. At 67-68 only
   C. At addresses 0 xxxx 00, each up to 4 microinstructions
   D. At random addresses
   Answer: C. ADD=0, BRANCH=4, STORE=8, EXCHANGE=12. (Key point: Routine placement)

## Related in Computer Organization and Architecture

- [Multiprocessor classification by memory organization](https://www.bctengineer.com/question.md?key=semester_5%2Fcomputer_organization_and_architecture%3Amultiprocessor-classification) — Computer Organization and Architecture, chapter 8
- [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×
- [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
- [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
- [Microprogram control unit organization](https://www.bctengineer.com/question.md?key=semester_5%2Fcomputer_organization_and_architecture%3Amicroprogram-control-org) — Computer Organization and Architecture, chapter 3, asked 2×
- [Characteristics of memory](https://www.bctengineer.com/question.md?key=semester_5%2Fcomputer_organization_and_architecture%3Amemory-characteristics) — Computer Organization and Architecture, chapter 6, asked 4×

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- [8051 data memory organization](https://www.bctengineer.com/question.md?key=semester_6%2Fembedded_system%3A8051-data-memory-organization) — Embedded System, chapter 9
- [File system vs virtual memory; contiguous allocation](https://www.bctengineer.com/question.md?key=semester_6%2Foperating_system%3Afs-vs-virtual-memory-contiguous-tape) — Operating System, chapter 5
- [8255 at A0A0H: interface and control words](https://www.bctengineer.com/question.md?key=semester_5%2Finstrumentation_ii%3Appi-a0a0h-control-words) — Instrumentation II, chapter 2