The fetch-decode-execute cycle

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Quick Questions

Shade in the A or B circle for each question.

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

A Fetch, then decode, then execute

B Decode, then fetch, then execute

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

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

4. What happens in the decode stage?

A The CPU saves the instruction to storage

B The CPU works out what the instruction means

5. What happens in the execute stage?

A The CPU carries out the decoded instruction

B The CPU copies the next instruction from RAM

Match the Pairs

Draw a line connecting each item on the left to its match on the right.

Left column		Right column
1fetch		AThe first stage: the CPU copies the next instruction from RAM into the CPU
2decode		Bfetch -> decode -> execute -> fetch ...
3The three stages always happen in the same order for each instruction		Cfetch -> decode -> execute
4After execute, the cycle starts again with the next instruction until the program ends		DThe second stage: the control unit works out what the instruction means
5decode -> fetch -> execute		EThe stages are in the wrong order. An instruction must be fetched from RAM before it can be decoded.
6fetch -> decode -> execute -> STOP		FThe cycle does not stop after one instruction. It repeats for each instruction in the program.

Definitions

fetch-decode-execute cycle

Fill in the Blanks

Complete each line of code by filling in the missing parts.

The CPU repeats: fetch, decode, _______

fetch -> ______ -> execute -> repeat

______ FETCH; DECODE; EXECUTE UNTIL program ends

_____: get the instruction from RAM

execute, then the cycle _______

Spot the Error

Shade in the letter of the line that contains an error.

Question 1

A REPEAT

B DECODE instruction from RAM

C DECODE instruction

D EXECUTE instruction

E UNTIL program ends

Question 2

A FETCH instruction from the ALU

B DECODE instruction in the CU

C EXECUTE instruction

D REPEAT for next instruction

Question 3

A FETCH: instruction from RAM

B DECODE: work out the meaning

C EXECUTE: meaning is found

D NEXT: start FETCH again

Question 4

A Order for one instruction:

B 1 DECODE

C 2 DECODE

D 3 EXECUTE

Question 5

A WHILE program is running

B do FETCH, DECODE, EXECUTE

C stop after one instruction

D ENDWHILE

Answer Key

The fetch-decode-execute cycle

Section A: Quick Questions

A — Fetch, then decode, then execute

A — Once for every instruction in the program

B — The instruction is copied from RAM into the CPU

B — The CPU works out what the instruction means

A — The CPU carries out the decoded instruction

Section B: 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.

Section C: Definitions

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.

Section D: Fill in the Blanks

The CPU repeats: fetch, decode, execute

fetch -> decode -> execute -> repeat

REPEAT
  FETCH; DECODE; EXECUTE
UNTIL program ends

fetch: get the instruction from RAM

execute, then the cycle repeats

Section E: Spot the Error

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'.

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'.

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'.

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'.

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'.