← IGCSE Computer Science Paper 1

The fetch-decode-execute cycle

The CPU repeats three stages for every instruction: fetch it from RAM, decode what it means, then execute it. The cycle then starts again with the next instruction.

Part of: IGCSE Computer Science 0478 — 3.1 Computer Architecture — Module 18, IGCSE Computer Science 0478 — 4.1 Types of Software and Interrupts — Module 42

Understanding The fetch-decode-execute cycle in IGCSE Computer Science Paper 1

What it does: The cycle order is always fetch, then decode, then execute. The cycle repeats for each instruction. A program with 1000 instructions needs the cycle to run 1000 times. Fetch means getting the instruction from RAM. Carrying it out is the execute stage. In decode, the control unit works out what the instruction is asking the CPU to do. Every instruction goes through all three stages, one after the other.

Common mistakes: The first stage fetches the instruction from RAM. Decoding happens after the fetch and works out what the instruction means. Line 2 should be ' FETCH instruction from RAM'. Instructions are fetched from RAM (main memory), not from the ALU. The ALU does calculations and comparisons. Line 1 should be 'FETCH instruction from RAM'.

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This module's key terms are also in a printable crossword (Modules 228–239) and word search (Modules 218–239).

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Learning

Duo

What is the correct order of the stages in the cycle?

Decode, then fetch, then executeFetch, then decode, then execute

Matches

decode -> fetch -> execute

↓

The stages are in the wrong order. An instruction must be fetched from RAM before it can be decoded.

Applying

Meanings

Term

fetch-decode-execute cycle

Meaning

The CPU repeats three stages for every instruction: fetch it from RAM, decode what it means, then execute it. The cycle then starts again with the next instruction.

Practice

REPEAT

Testing

Gaps

The CPU repeats: fetch, decode,

Check Answer

Spots

1REPEAT

2 DECODE instruction from RAM

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Quick Questions

1. What is the correct order of the stages in the cycle?

A. Fetch, then decode, then execute ✓

B. Decode, then fetch, then execute

The CPU must fetch an instruction from RAM before it can decode it, and it must decode it before it can execute it.

2. How often does the CPU run the fetch-decode-execute cycle?

A. Once for every instruction in the program ✓

B. Only once, at the start of the program

The cycle repeats for each instruction. A program with 1000 instructions needs the cycle to run 1000 times.

3. What happens in the fetch stage?

A. The instruction is carried out by the ALU

B. The instruction is copied from RAM into the CPU ✓

Fetch means getting the instruction from RAM. Carrying it out is the execute stage.

4. What happens in the decode stage?

A. The CPU saves the instruction to storage

B. The CPU works out what the instruction means ✓

In decode, the control unit works out what the instruction is asking the CPU to do.

5. What happens in the execute stage?

A. The CPU carries out the decoded instruction ✓

B. The CPU copies the next instruction from RAM

Execute means the instruction is actually performed, for example by the ALU. Getting the next instruction is the fetch stage of the next cycle.

Match the Pairs

fetch → The first stage: the CPU copies the next instruction from RAM into the CPU

decode → The second stage: the control unit works out what the instruction means

The three stages always happen in the same order for each instruction → fetch -> decode -> execute

After execute, the cycle starts again with the next instruction until the program ends → fetch -> decode -> execute -> fetch ...

decode -> fetch -> execute → The stages are in the wrong order. An instruction must be fetched from RAM before it can be decoded.

fetch -> decode -> execute -> STOP → The cycle does not stop after one instruction. It repeats for each instruction in the program.

Key Term

fetch-decode-execute cycle

The CPU repeats three stages for every instruction: fetch it from RAM, decode what it means, then execute it. The cycle then starts again with the next instruction.

Fill in the Blanks

1. The CPU repeats: fetch, decode, execute

Hint: Type the name of the third and final stage of the cycle.

2. fetch -> decode -> execute -> repeat

Hint: Type the name of the middle stage, which works out what the instruction means.

3. REPEAT FETCH; DECODE; EXECUTE UNTIL program ends

Hint: Type the pseudocode keyword that starts a loop checked at the end.

4. fetch: get the instruction from RAM

Hint: Type the name of the first stage, which brings the instruction into the CPU.

5. execute, then the cycle repeats

Hint: Type the verb for what the cycle does again for the next instruction.

Spot the Error

Question 1

A REPEAT

B DECODE instruction from RAM

C DECODE instruction

D EXECUTE instruction

E UNTIL program ends

Line B — The first stage fetches the instruction from RAM. Decoding happens after the fetch and works out what the instruction means. Line 2 should be ' FETCH instruction from RAM'.

Question 2

A FETCH instruction from the ALU

B DECODE instruction in the CU

C EXECUTE instruction

D REPEAT for next instruction

Line A — Instructions are fetched from RAM (main memory), not from the ALU. The ALU does calculations and comparisons. Line 1 should be 'FETCH instruction from RAM'.

Question 3

A FETCH: instruction from RAM

B DECODE: work out the meaning

C EXECUTE: meaning is found

D NEXT: start FETCH again

Line C — Working out the meaning of an instruction is the decode stage. The execute stage carries out the action. Line 3 should be 'EXECUTE: carry out the action'.

Question 4

A Order for one instruction:

B 1 DECODE

C 2 DECODE

D 3 EXECUTE

Line B — The cycle always starts with fetch, because the CPU must get the instruction before it can decode it. Line 2 should be '1 FETCH'.

Question 5

A WHILE program is running

B do FETCH, DECODE, EXECUTE

C stop after one instruction

D ENDWHILE

Line C — The cycle repeats for every instruction in the program. It does not stop after one instruction. Line 3 should be ' go to next instruction'.

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