---
title: "Why microprogrammed control is more flexible"
subject: "Computer Organization and Architecture"
chapter: 3
key: "semester_5/computer_organization_and_architecture:micro-vs-hardwired-flex"
source: https://www.bctengineer.com/q/semester_5/computer_organization_and_architecture/micro-vs-hardwired-flex
site: BCT Engineering
---

# Why microprogrammed control is more flexible

Computer Organization and Architecture · Chapter 3: Control Unit

- Asked in: 2073 Chaitra, 10 marks
- Video: [Introduction of Control Unit | Hardwired & Microprogrammed Unit | Computer System & Architecture](https://www.youtube.com/watch?v=kt-OdnKBbwA) — Vaibhav Talks - Coding
- 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/micro-vs-hardwired-flex
- Whole chapter: https://www.bctengineer.com/md/semester_5/computer_organization_and_architecture/chapter-3

## Question

Why is a micro-programmed control unit more flexible as compared to a hardwired control unit? Explain the sequencing technique used in control memory.

## Short answer

Microprogrammed control is flexible because behavior resides in writable microcode; adding instructions needs only microcode changes, not hardware rewiring. The sequencer picks the next address from CAR+1, branch field AD, opcode mapping, or subroutine return from SBR.

## Answer

A microprogrammed control unit is more flexible because its behaviour is defined by a microprogram stored in the control memory, not by fixed wiring.
- To change or add an instruction, only the contents of the control memory are rewritten; no hardware is rebuilt.
- Errors can be corrected by changing the microcode.
- With a writable control store, the same hardware can emulate a different instruction set.
- New instructions can easily be added to a family of machines, and the design is systematic.

In a hardwired unit, the control signals come from gates and flip-flops, so any change needs a redesign of the logic and new hardware.

#### Sequencing technique used in control memory
The sequencer selects the address of the next microinstruction from:
1. **CAR + 1** (normal sequential execution),
2. **Branch address (AD)** for conditional or unconditional jumps (the CD field tests a status bit and the BR field selects JMP),
3. **Mapped address** from the instruction opcode (0 xxxx 00) at the end of the fetch routine,
4. **Return address** from the subroutine register (SBR) on RET, saved there by CALL.

```text
 opcode -> map ----+
 AD field ---------+--> MUX --> CAR --> Control memory
 SBR --------------+      ^                 |
 CAR+1 ----------+--------+                 v
                 ^                      Control data
 status, CD, BR -+ (branch logic)        register
```
CALL stores CAR + 1 in SBR, and RET returns it to CAR. Initially, CAR is loaded with the address of the fetch routine.

## Check yourself

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

1. Why is microprogrammed control more flexible?
   A. It uses faster gates
   B. Its behaviour is defined by control memory contents, which can be rewritten without rebuilding hardware
   C. It has more registers
   D. It has no clock
   Answer: B. A change is only a microprogram change. (Key point: Flexibility)

2. What does a writable control store allow?
   A. Faster clock
   B. Removal of CAR
   C. The same hardware can emulate a different instruction set
   D. Hardwired timing
   Answer: C. Rewriting microcode changes the instruction set. (Key point: Emulation)

3. What is needed to change a hardwired unit?
   A. Rewrite ROM
   B. Change AD field
   C. Nothing
   D. Redesign of the logic and new hardware
   Answer: D. Gates and flip-flops are fixed. (Key point: Hardwired change)

4. Which source supplies the next address after RET?
   A. SBR
   B. Mapping logic
   C. AD field
   D. CAR+1
   Answer: A. CALL stored CAR+1 in SBR. (Key point: Sequencing sources)

## Related in Computer Organization and Architecture

- [Hardwired vs microprogrammed; sequencer diagram](https://www.bctengineer.com/question.md?key=semester_5%2Fcomputer_organization_and_architecture%3Ahardwired-vs-micro-sequencer) — Computer Organization and Architecture, chapter 3
- [Hardwired vs microprogrammed control](https://www.bctengineer.com/question.md?key=semester_5%2Fcomputer_organization_and_architecture%3Ahardwired-vs-micro) — Computer Organization and Architecture, chapter 3, asked 4×
- [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
- [Direct mapping with 32-block memory, 10 blocks used](https://www.bctengineer.com/question.md?key=semester_5%2Fcomputer_organization_and_architecture%3Adirect-mapping-example-10) — Computer Organization and Architecture, chapter 6, asked 3×
- [Direct mapping with 64-block memory, 10 blocks used](https://www.bctengineer.com/question.md?key=semester_5%2Fcomputer_organization_and_architecture%3Adirect-mapping-example-64) — Computer Organization and Architecture, chapter 6, asked 3×

## Related in other subjects

- [Hardwired vs microprogrammed control unit](https://www.bctengineer.com/question.md?key=semester_4%2Fmicroprocessor%3Ahardwired-microprogrammed-cu) — Microprocessor, chapter 1, asked 3×