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Control unit, ALU and processor organisation

Computers follow instructions to process data. The Central Processing Unit (CPU) processes these instructions, but it is not one component performing every task. Specialised parts of the CPU have different roles and work together. This first worksheet introduces how the CPU is organised and the precise language used to describe its parts.

By the end, you will be able to

distinguish the Control Unit from the Arithmetic Logic Unit;
describe how registers and buses support processing;
explain a component's role using precise examination language.

Starting point: an instruction tells a processor what operation is required.

A useful division of labour

The Control Unit (CU) coordinates processing. It interprets instructions and sends control signals so that the required transfers and operations occur. The Arithmetic Logic Unit (ALU) carries out arithmetic, Boolean logic, comparisons and shifts. Registers are very small, fast stores inside the CPU. Buses are communication paths that carry addresses, data, instructions or control signals.

Use this language rule: a component acts; a register holds; a bus carries. This avoids vague phrases such as ‘the CPU processes it’.

Model the roles before naming the parts

A ticket costs £18 and a £5 discount must be applied.

The CU interprets the request and sends control signals to coordinate the operation.
The ALU performs the subtraction: 18 − 5.
A register holds the result, 13, ready for the next step.

The CU does not subtract. The register does not subtract. Each statement names the correct action for that part.

Multiple choice1 mark

Which statement describes the control unit most precisely?

AIt decodes instructions and sends control signals to coordinate operations
BIt performs every arithmetic calculation
CIt permanently stores programs
DIt is the bus that carries addresses

Fill in the blanks3 marks

The gap 1 coordinates operations, the gap 2 performs arithmetic and logical operations, and a gap 3 temporarily holds a working value.

control unit
ALU
register
address bus

Guided explanation

For spaces_left = capacity - attendees, build two linked points: CU interprets and signals → ALU subtracts and a register holds the result. Name each action; avoid saying only that a component ‘processes data’.

Written answer4 marks

A supplied instruction compares a scanned age of 17 with a minimum age of 18 and produces the Boolean result false. Explain the roles of the CU and ALU, and state where the result may be held.

Transfer the model to a comparison: write two distinct component → action statements, then identify storage for the result.

Students type their answer here.

Checkpoint

Complete each statement from memory. Write the action or role precisely; there is no answer bank.

Fill in the blanks4 marks

To calculate a new ticket total, the CU first checkpoint gap 1 the instruction and coordinates the other parts. The ALU then checkpoint gap 2 the values. The result is temporarily checkpoint gap 3 in a register. A bus may checkpoint gap 4 a value between components.

Consolidate your understanding

Check that every processor action is assigned to a component and every temporary value is assigned to a register.