GCSE · Computer Science · AQA · Spec 8525

Arithmetic logic unit (ALU)

The ALU is the CPU's calculator — every sum, comparison, and logic decision flows through it.

What you need to know

  • The ALU performs arithmetic operations (add, subtract) and logical operations (AND, OR, NOT, comparisons).
  • The specification names five major CPU components: the arithmetic logic unit, the control unit, the clock, registers and buses.
  • A bus is a collection of wires through which data and signals travel from one component to another.
  • The Control Unit sends control signals to tell the ALU which operation to perform — the ALU never decides for itself.

The big picture

The Arithmetic Logic Unit (ALU) is a component inside the CPU responsible for arithmetic operations (such as addition and subtraction) and logical operations (such as AND, OR and NOT, and comparisons). The control unit directs it, telling it which operation to carry out. Without the ALU the CPU could not process any instruction that involves a calculation or a comparison.

TONIGHT'S REVISION

Arithmetic Logic Unit (ALU)

The component inside the CPU that carries out all arithmetic and logical operations

Computer Science · Structure

Inside the CPU — the ALU in context

Tap each component to see what it does and how it connects to the ALU

CPUControl signalsOperands inResult storedFed backData loadedResult written backControl Unit (CU)ALURegisterGeneral RegistersMain Memory (RAM)

Tap any part of the diagram to see what it does.

Think before you reveal

Read carefully — only one of these is performed by the ALU itself.

A program runs the instruction: IF score > 100 THEN bonus = TRUE. Which part of the CPU carries out the comparison 'score > 100'?

How an ADD instruction flows through the CPU

Follow one instruction from memory through the CPU — each stage involves a different component

ALU vs Control Unit — what each one actually does

ALUvsControl Unit (CU)

These two are the most commonly confused CPU components in exams.

Focus

Primary role

ALU

Performs calculations and logical decisions

Control Unit (CU)

Manages and coordinates all CPU activity

The insight

The ALU executes; the CU directs — one computes, the other commands.

Types of operation

ALU

Arithmetic (add, subtract) and logical (AND, OR, NOT, comparisons)

Control Unit (CU)

Fetch, decode, issue control signals — no arithmetic

What it receives as input

ALU

Operands from registers + a control signal specifying the operation

Control Unit (CU)

Raw binary instructions fetched from memory

Output destination

ALU

Sends results to registers

Control Unit (CU)

Sends control signals to ALU, registers, and memory

Role in a comparison (IF x > y)

ALU

Performs the 'greater than' comparison — a logical operation

Control Unit (CU)

Decodes the IF instruction and signals the ALU to compare

AQA 8525 — CPU Components

Model answer — describe the role of the ALU

Does your answer match what the examiner is looking for?

Question

Describe the role of the Arithmetic Logic Unit (ALU) in a CPU.

Student answer

The ALU performs arithmetic operations such as addition and subtraction. It also performs logical operations such as AND, OR, and comparisons.

Method marks0/2

Key points

1ALU = Arithmetic + Logic: it handles maths AND Boolean decisions.
2The CU tells the ALU what operation to perform — the ALU executes it.
3Registers feed the ALU with operands and store its results temporarily.
4The result of an ALU operation is placed in a register or written back to memory.
5ALU operations underpin every program instruction — loops, comparisons, and calculations all pass through it.

Worked example

Problem

A CPU carries out the instruction ADD 5. A register currently holds the value 3. Describe what the ALU does and what result is produced.

🧠

Memory hook

Think of the ALU as a pocket calculator bolted inside the CPU: the CU presses the buttons, the registers hold the numbers, and the ALU does the maths.

★ Exam tip

If you are asked to describe the role of the ALU, cover both sides of its name: arithmetic operations (such as addition and subtraction) and logical operations (such as AND, OR, NOT and comparisons). Giving a brief example of each makes the answer clearer.

⚠ Watch out

Confusing the ALU with the Control Unit — the CU directs traffic and decodes instructions; only the ALU actually performs calculations and comparisons.

✓

Check yourself

Without looking — name two types of arithmetic operation AND two types of logical operation the ALU can carry out.

Flashcards

(23)
What does ALU stand for?
Arithmetic Logic Unit
What are the two categories of operation the ALU performs?
Arithmetic operations (e.g. add, subtract) and logical operations (e.g. AND, OR, NOT, comparisons)
Give two examples of arithmetic operations the ALU performs.
Addition and subtraction (also multiplication and division in some implementations)
Give two examples of logical operations the ALU performs.
AND, OR (also NOT, XOR, and comparisons such as equal to or greater than)
Which CPU component tells the ALU what operation to perform?
The Control Unit (CU)
Where are operands held before being sent to the ALU?
In registers
Where does the ALU get the values it works on?
From registers inside the CPU.
How does data reach the ALU from memory?
On a bus
Name the major components of the CPU given in the specification.
Arithmetic logic unit, control unit, clock, register and bus (together with main memory).
What is the role of the Control Unit in relation to the ALU?
The CU decodes instructions and sends control signals that direct the ALU to perform the correct operation
What happens to the ALU's result after a calculation?
It is placed in a register or written back to memory
Does the ALU or the CU fetch instructions from memory?
The Control Unit fetches instructions; the ALU processes data only
What carries the result out of the ALU?
A bus
Can the ALU perform comparisons (e.g. is A > B)?
Yes — comparisons are logical operations performed by the ALU
What is the difference between an arithmetic and a logical operation?
Arithmetic operations manipulate numerical values (add, subtract); logical operations evaluate Boolean conditions (AND, OR, NOT, comparisons)
In the fetch-decode-execute cycle, at which stage does the ALU act?
The execute stage — after the CU has fetched and decoded the instruction
Why does the ALU use registers rather than fetching directly from RAM each time?
Registers are much faster to access than RAM, keeping the ALU from being slowed down by memory latency
Name one logical comparison operation the ALU can perform.
Equal to (=), greater than (>), or less than (<) — any one is correct
What is the von Neumann architecture component that contains the ALU?
The CPU (Central Processing Unit)
A register holds 10 and the ALU performs SUB 4. What is the result?
6 — the ALU subtracts 4 from 10 and the result is placed in a register.
What type of operation is AND?
A logical operation — it returns TRUE only if both inputs are TRUE
Which bus carries control signals from the CU to the ALU?
The control bus
What is the difference between the ALU and the CU?
The ALU performs calculations and logical operations; the CU fetches, decodes, and coordinates — it never performs arithmetic itself

Tap any card to flip it, or use Study as deck to go through them one at a time. In the full lesson these run as a spaced-repetition deck — you rate each card Hard, Good or Easy and the tricky ones keep coming back until they stick.

Learn Arithmetic logic unit (ALU) properly — interactive practice, marked questions and flashcards.

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