GCSE · Computer Science · AQA · Spec 8525

Fetch-Execute cycle

Computer Science · CPU

Fetch · Decode · Execute

Step through the cycle — watch the CPU fetch the next instruction from main memory, decode it, and carry it out, then begin again.

Fetch
Decode
Execute

Main Memory

100
ADD 5
101
ADD 3
102
SUB 2
103
OUT
address
data
CPU

Control Unit

Directs & coordinates every stage

ALU

Does the maths & logic

InstructionADD 5
Working value0
Output—
Fetch·ADD 5 @ 100

The Fetch-Execute cycle, stage by stage

The CPU repeats these three stages continuously while the computer is running.

Predict, then check

Think about the order of the three stages.

A CPU has just finished the decode stage for an instruction. What happens next?

Why does the CPU repeat this cycle continuously?

?

Reason it through

Why can't the CPU simply carry out a whole program in one step?

Link 1 of 3

First link · your turn

Where is a program stored while the computer is running?

2
Locked — reveal the link above first
3
Locked — reveal the link above first

Decode vs Execute — two different jobs

DecodevsExecute

Both happen inside the CPU, but they do different things and always happen in this order.

Focus

What happens

Decode

The instruction is decoded to work out what it is

Execute

The instruction is carried out

The insight

Working out what an instruction is, and doing it, are two separate steps.

Order in the cycle

Decode

Second — after the instruction has been fetched

Execute

Third — after the instruction has been decoded

Which component leads

Decode

The control unit works out what the instruction is

Execute

The ALU performs any calculation or comparison the instruction needs

What comes next

Decode

The instruction moves on to be executed

Execute

The CPU fetches the next instruction and the cycle repeats

AQA 8525 — Computer systems

Model answer — describe the Fetch-Execute cycle

Read the question, study the model answer, then compare it with your own.

Question

Describe the stages of the Fetch-Execute cycle carried out by the CPU.

Student answer

The CPU continually reads instructions stored in main memory and executes them. First the next instruction is fetched to the CPU from main memory. The instruction is then decoded to work out what it is. Finally the instruction is executed — carried out — with the ALU performing any calculation or comparison required. The CPU then begins the cycle again with the next instruction.

Method marks0/4

WHAT YOU'VE LEARNED

A quick recap of today's lesson.

How your CPU fetches, decodes, and executes every instruction — billions of times a second.

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