GCSE · Computer Science · AQA · Spec 8525
Fetch-Execute cycle
Every instruction your CPU ever runs follows this exact three-step loop — billions of times per second.
What you need to know
- The three stages are Fetch, Decode and Execute — in that order, every time.
- Fetch: the next instruction is fetched to the CPU from main memory.
- Decode: the instruction is decoded to work out what it is.
- Execute: the instruction is carried out; the CPU then begins the cycle again with the next instruction.
The big picture
The Fetch-Execute cycle (also called the Fetch-Decode-Execute cycle) is the continuous process by which a CPU carries out the instructions stored in main memory. The next instruction is fetched to the CPU from main memory, decoded to work out what it is, then executed — carried out, with the ALU performing any calculation or comparison the instruction needs. The CPU then begins the cycle again with the next instruction, and repeats it for as long as the computer is running.
TONIGHT'S REVISION
Fetch-Execute Cycle
How your CPU fetches, decodes, and executes every instruction — billions of times a second.
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.
Main Memory
Control Unit
Directs & coordinates every stage
ALU
Does the maths & logic
The next instruction is fetched to the CPU from main memory.
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?
First link · your turn
Where is a program stored while the computer is running?
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.
Key points
Worked example
Problem
A program is stored in main memory. Describe what the CPU does to carry out the next instruction, using the stages of the Fetch-Execute cycle.
Memory hook
Think of the CPU as a chef following recipe cards: FETCH = pick up the next card; DECODE = read what it says; EXECUTE = cook the dish. The chef works through the cards one after another, continuously.
★ Exam tip
Describe the cycle in the order it happens and say what each stage does: the next instruction is fetched from main memory, it is decoded to work out what it is, then it is executed. Naming the stages without saying what each one does leaves the description incomplete.
⚠ Watch out
Giving the three stages out of order, or merging decode into execute. The instruction has to be fetched before it can be decoded, and decoded before it can be carried out.
Check yourself
Without looking — name the three stages of the cycle in order, and say in one sentence what happens in each.
Flashcards
(23)What are the three stages of the Fetch-Execute cycle?
What happens during the FETCH stage?
What happens during the DECODE stage?
What happens during the EXECUTE stage?
Where are the instructions the CPU fetches stored?
Which part of the CPU decodes the instruction?
Which part of the CPU carries out arithmetic and logical operations during execute?
What does the CPU do after executing an instruction?
Why must decode come before execute?
Why must fetch come before decode?
Which CPU component's speed affects how many cycles complete each second?
Name three factors that affect CPU performance.
What carries data and instructions between main memory and the CPU?
Does the CPU stop after one instruction?
What is main memory's role in the cycle?
True or False: the instruction is executed before it is decoded.
In one sentence, describe the Fetch-Execute cycle.
What might happen to the result of an executed instruction?
Which components does the control unit direct during the cycle?
Going further: do you need to name specific registers for AQA 8525?
What kind of operation would the ALU perform during execute?
How does a faster clock speed affect the cycle?
What is the last thing that happens in one pass of the cycle?
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 Fetch-Execute cycle properly — interactive practice, marked questions and flashcards.
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