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J277 1.1.1 Fetch-execute cycle

Part 4 of 6 · J277 1.1 · Systems architecture

How the CPU actually runs a program, taught as the repeating cycle OCR describes as fetch, decode and execute.

Students will:

  • describe what happens at each stage of the cycle
  • explain how the components and registers support each stage
  • distinguish an instruction from the data it uses
  • apply the cycle to an unfamiliar instruction
  • correct the misconceptions examiners see most often

Inside: 6 explanation cells, 3 multiple-choice questions, 1 fill-in-the-blanks cell and 4 written answers. 37 marks, about 60 minutes.

Series: J277 1.1 · Systems architecture, part 4 of 6.

Shared by Coding PathwayVerified teacher

  • 14 cells
  • About 60 minutes
  • CC BY-SA 4.0
  • Shared 17 Aug 2026
  • Updated 9 Sept 2026

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The whole resource, exactly as a class sees it. Answers and marking are held back.

Systems architecture 4: the fetch-execute cycle

The CPU processes instructions by repeating the fetch-execute cycle. The three actions are commonly described as fetch, decode and execute.

If the cycle feels technical, hold on to three everyday verbs: get it, understand it, do it. You will turn those into the precise exam terms fetch, decode and execute.

By the end of this worksheet, you should be able to:

  • describe what happens during fetch, decode and execute
  • explain how the CPU components support the cycle
  • distinguish an instruction from the data it uses
  • apply the cycle to an unfamiliar instruction
  • correct common misconceptions

OCR requires the actions at each stage and the purpose of the components and registers. It does not require a memorised sequence showing every transfer between registers.

Multiple choice, several answers2 marks

Which two registers store addresses? Select both.

  • AMemory Data Register
  • BProgram Counter
  • CAccumulator
  • DMemory Address Register

1. The repeating cycle

Fetch

The CPU retrieves the next instruction from memory. The processor uses the address of the required instruction, and the instruction is transferred from memory into the CPU.

Decode

The control unit interprets the fetched instruction. It works out the operation to perform and any data or address involved.

Execute

The CPU carries out the instruction. The ALU may perform an arithmetic calculation or logical comparison, or the control unit may coordinate another required action. A result can be stored for further processing.

After execution, the cycle repeats for the next instruction. Programs work because the CPU performs this cycle continuously and very quickly.

The repeating fetch, decode and execute cycle Three boxes form a cycle. Fetch obtains the next instruction, decode interprets it and execute carries it out. An arrow returns from execute to fetch. FETCHobtain the instruction DECODEinterpret the instruction EXECUTEcarry out the instruction The CPU repeats the cycle for the next instruction.
Fill in the blanks6 marks
During label 1, the next instruction is retrieved from label 2. During label 3, the label 4 interprets the instruction and identifies the required label 5. During label 6, the CPU carries out the instruction.
  • memory
  • operation
  • secondary storage
  • fetch
  • execute
  • input
  • decode
  • control unit

2. Fetch in more detail

The CPU needs to know where the next instruction is stored and then obtain what is stored there.

  • The PC stores the address of the next instruction to be fetched.
  • The MAR stores the address in memory currently being accessed.
  • The MDR stores the instruction transferred from memory to the CPU.

These roles reinforce the difference between an address and content. The PC and MAR hold locations; the MDR holds the instruction found at a location.

You can use these register purposes to explain the fetch stage, but you do not need to reproduce every register-to-register transfer in an exact order.

Written answer4 marks

Describe the fetch stage of the fetch-execute cycle. Include what the PC, MAR and MDR store during this stage.

Explain the overall action first. Then state the relevant stored address or instruction for each named register. You do not need to give an exact sequence of transfers.

Students type their answer here.

3. Decode and execute

An instruction contains an operation and may also identify data or a location to use. For example, ADD 7 tells the CPU to perform addition using the value 7.

During decode, the control unit interprets the instruction so that the required operation and any associated value or address are understood. Decoding is not the same as carrying out the operation.

During execute, the operation happens. For an arithmetic or logical instruction, the ALU performs the work and the accumulator can store the result. Other instructions may move data or cause a different component to act.

The stages have distinct purposes:

  • fetch obtains the instruction
  • decode interprets the instruction
  • execute carries out the instruction
Multiple choice, several answers3 marks

Which three situation-to-stage matches are correct? Select all that apply.

  • AThe next instruction is retrieved from memory: fetch
  • BThe instruction is interpreted: fetch
  • CThe control unit works out that an addition is required: decode
  • DThe instruction is obtained from memory: execute
  • EThe ALU adds two values: execute

4. Components working together

The fetch-execute cycle is a whole-CPU process:

  • the control unit coordinates the cycle, decodes instructions and sends control signals so that other components carry out the required actions
  • the ALU performs arithmetic calculations and logical operations when required
  • registers hold the particular addresses, data and instructions needed during processing
  • cache holds frequently used data and instructions close to the CPU, allowing faster access than RAM when the required item is present

An exam question may ask for a component's role during the cycle. Link its general function to the relevant action rather than listing disconnected definitions.

Multiple choice, several answers4 marks

Which four statements accurately link a component to the fetch-execute cycle? Select all that apply.

  • AThe accumulator performs every calculation instead of the ALU
  • BThe control unit decodes instructions and coordinates other components
  • CThe ALU performs a required arithmetic or logical operation
  • DThe PC counts the number of completed programs
  • ERegisters temporarily hold addresses, data or instructions used during processing
  • FCache can provide frequently used data or instructions more quickly than RAM
Written answer6 marks

A program contains an instruction that tells the CPU to add 6 to a stored value. Explain how the CPU processes this instruction through fetch, decode and execute.

Write one developed point per stage. Include the control unit during decode and the ALU during execute.

Students type their answer here.

Written answer6 marks

A student writes: “First the ALU decodes the instruction. Next the CPU fetches it from the hard drive. Finally the control unit executes the calculation.” Identify and correct three errors.

Check the order, where the instruction is fetched from and the roles of the CU and ALU.

Students type their answer here.

Written answer6 marks

Challenge: apply the whole cycle. A smart door controller has an instruction meaning “compare the entered code with the stored code”. Explain how the CPU processes this instruction using fetch, decode and execute and at least three relevant CPU components or registers.

Build a connected response. Write a connected sequence in stage order. Each time you name a component or register, explain what it contributes to this instruction.

Students type their answer here.

Closed-book review

Without looking back:

  1. Write the three stages in order.
  2. Add one precise action beside each stage.
  3. Add the component most closely associated with decoding.
  4. Add the component that performs calculations and logical comparisons.
  5. Explain the difference between the values in the MAR and MDR.

Check that fetch means obtaining, decode means interpreting and execute means carrying out.