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J277 1.1.1 Introducing the CPU

Part 1 of 6 · J277 1.1 · Systems architecture

The opening worksheet of OCR J277 section 1.1, introducing what the CPU is for and the components inside it.

Students will:

  • state what a CPU does and why a computer needs one
  • identify the control unit, arithmetic logic unit, cache and registers
  • describe the job each component performs
  • recognise the fetch, decode and execute stages
  • apply component knowledge to familiar devices

Inside: 6 explanation cells, 3 multiple-choice questions, 1 fill-in-the-blanks cell and 4 written answers. 29 marks, about 60 minutes.

Series: J277 1.1 · Systems architecture, part 1 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

Preview

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

Systems architecture 1: introducing the CPU

The central processing unit (CPU) is the processor at the centre of a computer system. It processes data and instructions so that programs can run and the computer can respond to input.

The component names may be new, but each one has a clear job. Work through one section at a time. The first questions ask you to recognise ideas before you explain and apply them.

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

  • state what CPU stands for
  • explain the purpose of the CPU
  • identify the control unit, arithmetic logic unit, cache and registers
  • describe the basic function of each component
  • recognise the fetch, decode and execute stages
  • apply component knowledge to familiar devices

This worksheet gives you a complete introductory model of the CPU. The performance, register and fetch-execute ideas are introduced here and can be explored in more detail separately.

1. What does a CPU do?

A program is a sequence of instructions. An instruction tells the computer to perform an action, such as add two values, compare them or move data. The CPU repeatedly:

  1. fetches an instruction that is needed
  2. decodes it to work out what action is required
  3. executes it by carrying out the action

This repeating process is called the fetch-execute cycle, often described as fetch, decode and execute.

A precise statement of purpose is:

The CPU processes data and instructions by repeatedly carrying out the fetch-execute cycle.

Avoid saying only that the CPU is the “brain of the computer”. That analogy does not explain what it does.

Multiple choice1 mark

Which statement most accurately describes the purpose of the CPU?

  • AIt permanently stores every file created by the user
  • BIt provides the physical screen used to display output
  • CIt processes data and instructions by repeatedly carrying out the fetch-execute cycle
  • DIt connects every computer directly to the Internet
Written answer3 marks

State what CPU stands for and explain its purpose.

Write the full name, then use the terms data, instructions and fetch-execute cycle.

Students type their answer here.

2. Four common CPU components

Control unit (CU)

The control unit coordinates the operation of the CPU. It manages the fetch-execute cycle, decodes instructions and sends control signals so that other components carry out the required actions.

Arithmetic logic unit (ALU)

The arithmetic logic unit performs arithmetic calculations, such as addition, and logical operations, such as comparisons.

Cache

Cache is small, fast memory located in or close to the CPU. It stores frequently or recently used data and instructions so that the processor can access them more quickly than if it had to fetch them from RAM. RAM is the computer's main working memory. For this comparison, remember that cache is smaller, faster and closer to the CPU than RAM.

Registers

Registers are very small, fast storage locations inside the CPU. They temporarily hold a particular item of data, an instruction or an address while the CPU is processing it. Different registers have different purposes.

Main CPU components and nearby memory A CPU contains a control unit, arithmetic logic unit and registers. Cache is shown inside or close to the CPU. RAM is outside the CPU and connected to it. CPU Control unit coordinates and decodes ALU calculations and logic Registers specific temporary values Cache frequently used items RAM main working memory
Fill in the blanks4 marks
The label 1 coordinates the CPU and decodes instructions. The label 2 performs calculations and logical operations. label 3 stores frequently used data and instructions close to the processor. label 4 are small storage locations that temporarily hold data, instructions or addresses inside the CPU.
  • arithmetic logic unit
  • secondary storage
  • cache
  • router
  • control unit
  • registers
Multiple choice, several answers4 marks

Which four component-to-function matches are correct? Select all that apply.

  • AControl unit: coordinates the CPU and decodes instructions
  • BControl unit: permanently stores the user's files
  • CALU: performs arithmetic calculations and logical operations
  • DALU: provides the graphical user interface
  • ECache: holds frequently used data and instructions for quick access
  • FRegisters: temporarily hold data, instructions or addresses inside the CPU

3. Use precise verbs

Exam questions often ask for the function of a component. Your answer needs to say what the component actually does.

  • Weak: “The control unit controls the computer.”

  • Stronger: “The control unit coordinates the fetch-execute cycle, decodes instructions and sends control signals to other components.”

  • Weak: “The ALU does maths.”

  • Stronger: “The ALU performs arithmetic calculations and logical operations.”

The phrase logical operations includes decisions or comparisons such as checking whether one value is greater than another. It does not mean that the ALU thinks like a person.

Written answer2 marks

Describe one function of the control unit and one function of the arithmetic logic unit.

Write two sentences. For the CU, use coordinate, decode or send control signals for a stated purpose. For the ALU, use arithmetic calculations or logical operations.

Students type their answer here.

4. Cache and registers are not the same

Both cache and registers are fast storage used by the CPU, but they have different roles.

  • Cache can hold frequently or recently used data and instructions so that they can be accessed quickly.
  • A register temporarily holds a specific item needed during processing. Each named register has a particular purpose.

Registers are smaller and are accessed directly by the CPU. Cache holds more than the registers but is still much smaller and faster than RAM.

OCR names four registers with distinct purposes. For this introductory task, secure the general idea that registers are temporary storage locations inside the CPU.

Multiple choice, several answers3 marks

Which three statements are accurate? Select all that apply.

  • AThe ALU permanently stores all installed applications
  • BA register temporarily stores a specific item needed during processing
  • CCache is an output device that displays processed data
  • DCache can reduce the need to wait for frequently used data or instructions from RAM
  • EThe control unit can decode an instruction
Written answer6 marks

A digital thermostat reads a temperature sensor, compares the reading with the chosen temperature and sends a signal to switch the heating on or off. Explain how the CPU and two of its components help carry out this task.

Start with the CPU processing instructions. Then select two from the CU, ALU, cache and registers. Link each component to an action in the thermostat.

Students type their answer here.

Written answer6 marks

Challenge: bring the four components together. A student says, “The CPU is only the ALU because calculations are the only processing a computer performs.” Evaluate this statement.

Build a connected response. Decide how accurate the claim is, explain how the ALU, control unit, cache and registers contribute to processing, then finish with a reasoned conclusion.

Students type their answer here.

Closed-book review

Hide the explanations above and complete these prompts from memory:

  1. CPU stands for...
  2. The purpose of the CPU is...
  3. The CU...
  4. The ALU...
  5. Cache...
  6. Registers...
  7. The three repeated stages are...

Check each answer. If you used a vague verb such as “does”, “controls” or “stores things”, replace it with a precise action.