← IGCSE Computer Science Paper 1

Control bus

A set of wires that carries control signals, such as read and write, from the control unit to other components so they work together. It carries signals, not data or addresses.

Part of: IGCSE Computer Science 0478 — 3.1 Computer Architecture — Module 17

Understanding Control bus in IGCSE Computer Science Paper 1

What it does: The control unit sends the write signal along the control bus to RAM, telling it to store the data. Each bus has its own job. The control bus carries signals, the data bus carries data and the address bus carries addresses. Control signals coordinate the components. Storing instructions is the job of memory, not a bus. The control bus is the pathway for control signals, so the control unit can manage the other parts of the computer.

Common mistakes: The control bus only carries control signals. The data being read goes on the data bus, so the line should be 'RAM puts data on data bus'. A READ signal tells RAM to copy the data out without changing it. Overwriting a location is what a WRITE signal does. The line should be 'RAM copies data to data bus'.

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This module's key terms are also in a printable crossword (Modules 218–227) and word search (Modules 218–239).

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Learning
Duo
What does the control bus carry?
Control signals such as read and writeThe data being processed by the CPU
Matches
The control bus carries the data being read from RAM to the CPU
↓
The data bus carries data, not the control bus. The control bus only carries control signals such as read and write.
Applying
Meanings
Term
control bus
Meaning
A set of wires that carries control signals, such as read and write, from the control unit to other components so they work together. It carries signals, not data or addresses.
Practice
CONTROL_BUS <- "READ"
Testing
Gaps
The bus carries read/write signals
Check Answer
Spots
2RAM reads the address bus
3RAM puts data on control bus

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Quick Questions

1. What does the control bus carry?
A. The data being processed by the CPU
B. Control signals such as read and write ✓
The control bus carries control signals. The data bus carries data.
2. Which component sends signals along the control bus to the other components?
A. Control unit (CU) ✓
B. Arithmetic logic unit (ALU)
The control unit sends control signals so that components work together. The ALU only does calculations and logic.
3. The control bus carries the memory address of the next instruction.
A. False ✓
B. True
Memory addresses travel along the address bus. The control bus carries control signals only.
4. The CPU needs to fetch an instruction from RAM. Which signal does the control bus send to RAM?
A. Read signal ✓
B. Write signal
A read signal tells RAM to send the contents of a location to the CPU. A write signal tells RAM to store data.
5. The CPU needs to store a result in RAM. Which signal is sent along the control bus?
A. Read signal
B. Write signal ✓
A write signal tells RAM to store the data being sent to it.

Match the Pairs

control bus → A set of wires that carries control signals from the control unit to other components
read signal → A control signal telling memory to send the data at an address back to the CPU
The control bus carries write signals, which tell memory to store data → CU -> control bus -> RAM: write
The control bus carries signals only, not data or addresses → control bus = signals; data bus = data; address bus = addresses
The control bus carries the data being read from RAM to the CPU → The data bus carries data, not the control bus. The control bus only carries control signals such as read and write.
The control bus carries the memory address of the next instruction → The address bus carries addresses. The control bus carries control signals sent from the control unit.

Key Term

control bus
A set of wires that carries control signals, such as read and write, from the control unit to other components so they work together. It carries signals, not data or addresses.

Fill in the Blanks

1. The control bus carries read/write signals
Hint: Type the name of the bus that carries signals from the CU so components work together.
2. CU sends a read signal on the control bus
Hint: Type the signal that tells RAM to send its contents out.
3. A write signal tells RAM to store data
Hint: Type the signal that tells memory to save incoming data.
4. The CU sends signals along the control bus
Hint: Type the abbreviation of the component that creates the control signals.
5. Control bus carries no data or addresses
Hint: Type the other thing, besides addresses, that this bus does not carry.

Spot the Error

Question 1
A CU sends READ on control bus
B RAM reads the address bus
C RAM puts data on control bus
Line C — The control bus only carries control signals. The data being read goes on the data bus, so the line should be 'RAM puts data on data bus'.
Question 2
A CU sends READ signal
B Address bus carries address
C RAM finds that location
D RAM overwrites that location
Line D — A READ signal tells RAM to copy the data out without changing it. Overwriting a location is what a WRITE signal does. The line should be 'RAM copies data to data bus'.
Question 3
A Control bus carries signals
B Signals go from CU to others
C Example signals: address, data
Line C — Addresses and data travel on the address bus and data bus. Examples of control signals are READ and WRITE.
Question 4
A Control bus starts at the RAM
B Signals travel to RAM and I/O
C This keeps components in sync
Line A — Control signals come from the control unit (CU), not from RAM. The line should be 'Control bus starts at the CU'.
Question 5
A Data bus carries data
B Control bus carries data too
C Address bus carries addresses
Line B — The control bus carries control signals only. Data is carried by the data bus. The line should be 'Control bus carries signals'.

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