← IGCSE Computer Science Paper 1

Buses

A set of wires that carries data, addresses or control signals between the CPU, RAM and other components. More wires means more bits can be carried at once.

Part of: IGCSE Computer Science 0478 — 3.1 Computer Architecture — Module 14

Understanding Buses in IGCSE Computer Science Paper 1

What it does: The control bus sends signals like memory read, memory write and clock pulses so components work together. A 32-bit bus has 32 wires and moves 32 bits at once. An 8-bit bus moves only 8 bits at once, so it is slower. A bus is a set of wires that links components such as the CPU and RAM. It is not a storage device. Buses connect the CPU to RAM and to other components so they can send information to each other. The three buses are the address bus, the data bus and the control bus. Each has its own job.

Common mistakes: The MAR holds a memory address, and addresses travel on the address bus, not the data bus. The line should be SEND MAR ON ADDRESS BUS. The address bus is one-way: the CPU sends addresses to RAM, and RAM never sends addresses back. It should say ONE WAY.

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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 is a bus in a computer?
A set of wires that carries data, addresses or signals between componentsA type of memory that stores programs permanently
Matches
The address bus carries data from RAM back to the CPU
↓
The address bus carries addresses, not data. Data travels on the data bus.
Applying
Meanings
Term
bus
Meaning
A set of wires that carries data, addresses or control signals between the CPU, RAM and other components. More wires means more bits can be carried at once.
Practice
bus = a set of wires
Testing
Gaps
= wires linking the CPU, RAM and I/O
Check Answer
Spots
2MAR ← PC
3SEND MAR ON DATA BUS

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Quick Questions

1. What is a bus in a computer?
A. A type of memory that stores programs permanently
B. A set of wires that carries data, addresses or signals between components ✓
A bus is a set of wires that links components such as the CPU and RAM. It is not a storage device.
2. Which component does a bus connect to the CPU?
A. RAM ✓
B. A bus never connects to the CPU
Buses connect the CPU to RAM and to other components so they can send information to each other.
3. How many kinds of bus are in the Von Neumann model?
A. Three: address bus, data bus and control bus ✓
B. One: a single bus does every job
The three buses are the address bus, the data bus and the control bus. Each has its own job.
4. Which bus carries the memory address of the location to be read or written?
A. Address bus ✓
B. Data bus
The address bus carries the location in memory. The data bus carries the actual data or instruction.
5. A bus has 32 wires and another has 64 wires. Which can carry more bits at once?
A. The 32-wire bus
B. The 64-wire bus ✓
Each wire carries one bit at a time, so more wires means more bits can be carried at once.

Match the Pairs

address bus → Carries the address of a memory location from the CPU to RAM, in one direction only
data bus → Carries data and instructions between the CPU, RAM and other components, in both directions
A bus carries control signals, such as read and write, to coordinate the components of a computer → control bus
A bus with more wires can carry more bits at the same time, so more data moves in each transfer → 32-bit data bus vs 8-bit data bus
The address bus carries data from RAM back to the CPU → The address bus carries addresses, not data. Data travels on the data bus.
A bus is a single wire that connects the CPU to the keyboard only → A bus is a set of wires, not a single wire, and it connects many components such as the CPU and RAM.

Key Term

bus
A set of wires that carries data, addresses or control signals between the CPU, RAM and other components. More wires means more bits can be carried at once.

Fill in the Blanks

1. bus = wires linking the CPU, RAM and I/O
Hint: Type the name for a set of wires that connects computer components
2. The address bus carries locations in RAM
Hint: Type the kind of bus that carries memory locations
3. The control bus carries read and write signals
Hint: Type the kind of bus that carries signals such as read and write
4. A bus with 16 wires carries 16 bits at once
Hint: Type the number of bits that match the number of wires
5. RAM -> data bus -> MDR
Hint: Type the kind of bus that carries the contents of a memory location back to the CPU

Spot the Error

Question 1
A PC ← 100
B MAR ← PC
C SEND MAR ON DATA BUS
D MDR ← DATA BUS
Line C — The MAR holds a memory address, and addresses travel on the address bus, not the data bus. The line should be SEND MAR ON ADDRESS BUS.
Question 2
A ADDRESS BUS: TWO WAYS
B DATA BUS: TWO WAYS
C CONTROL BUS: CARRIES SIGNALS
Line A — The address bus is one-way: the CPU sends addresses to RAM, and RAM never sends addresses back. It should say ONE WAY.
Question 3
A wires ← 32
B bits_at_once ← 1
C OUTPUT bits_at_once
Line B — Each wire carries one bit, so the number of bits carried at once equals the number of wires. 32 wires carry 32 bits, so the line should be bits_at_once ← wires.
Question 4
A CPU WRITES 7 TO RAM
B SEND ADDRESS ON ADDRESS BUS
C SEND VALUE ON ADDRESS BUS
D SEND WRITE ON CONTROL BUS
Line C — The value being written is data, so it travels on the data bus. The address bus only carries the location. The line should be SEND VALUE ON DATA BUS.
Question 5
A BUS 1 ← 'ADDRESS'
B BUS 2 ← 'PROGRAM'
C BUS 3 ← 'CONTROL'
Line B — The three kinds of bus are the address bus, the data bus and the control bus. There is no program bus, so line 2 should be BUS 2 ← 'DATA'.

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