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J277 1.1.1 Architecture of the CPU 3

Part 4 of 6 · J277 1.1 Systems Architecture

This worksheet guides pupils through the fetch-execute cycle in CPU architecture. Learners identify key registers and components involved in fetching, decoding, and executing instructions, and apply their understanding to explain CPU processes with support questions. The material provides structured reinforcement suitable for learners familiarising themselves with fundamental CPU operations at a moderate difficulty level.

Shared by Chris H.Verified teacher

  • 14 cells
  • About 50 minutes
  • CC BY-SA 4.0
  • Shared 29 Jul 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.

Fill in the blanks6 marks
During ______, the next instruction is retrieved from ______. During ______, the ______ interprets the instruction and identifies the required ______. During ______, the CPU carries out the instruction.
  • fetch
  • decode
  • execute
  • memory
  • control unit
  • operation

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.

Explain the overall action first. Then use the PC, MAR or MDR to add accurate detail about an address and the instruction.

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
  • 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 answer8 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.

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