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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 text sections, 2 multiple-choice activities, 2 completion activities, 6 written answers. 18 marks. Suggested teaching time: 40–55 minutes. 1 diagram.
Includes a teacher guide with suggested sections, timings and common misconceptions.
Series: J277 1.1 · Systems architecture, part 1 of 6.
Shared by Coding PathwayVerified teacher
- 16 cells
- About 45 minutes
- CC BY-SA 4.0
- Shared 17 Aug 2026
- Updated 27 Oct 2026
Preview
The whole resource, exactly as a class sees it. Answers and marking are held back.
Introducing the CPU
The central processing unit (CPU) processes data and carries out program instructions. It makes actions such as comparing a temperature or calculating a total possible.
You will learn how its main components work together, then apply their roles to a heating controller.
1. What does the CPU do?
A program is a sequence of instructions: actions such as adding values, comparing them or moving data. The CPU repeatedly:
- fetches an instruction from memory;
- decodes it to work out the action required;
- executes it by carrying out that action.
This repeating process is the fetch-execute cycle. A CPU processes data by carrying out these instructions.
Which statement describes the CPU's main purpose?
- AKeeping user files when the power is off
- BProcessing data and carrying out instructions
- CHolding every installed program ready to run
- DStoring the start-up program permanently
Describe the purpose of the CPU.
Students type their answer here.
2. Components with different jobs
| Component | Job |
|---|---|
| Control unit (CU) | Coordinates processing, decodes instructions and sends control signals. |
| Arithmetic logic unit (ALU) | Performs calculations and logical operations, including comparisons. |
| Cache | Holds frequently used data and instructions for quicker access than RAM. |
| Registers | Hold particular addresses, data or instructions temporarily during processing. |
RAM is the computer's main working memory. Cache is smaller and faster to access. Registers hold specific current values; cache keeps items the CPU is likely to need again.
The diagram groups the components; it does not show their physical layout.
- RAM
- ALU
- Registers
- control unit
- Cache
Describe one function of the control unit.
Students type their answer here.
Which statement correctly distinguishes a register from cache?
- AA register holds a current value; cache holds frequently used items
- BA register carries out calculations; cache holds their results
- CA register holds every open program; cache holds its memory addresses
- DA register keeps its contents without power; cache loses them
3. Use the role in a situation
A checkout program adds the price of an item to a running total. The ALU performs the addition, while a register can hold the result. This links each component's job to an action in the program.
Now use that approach in a different system.
Heating controller
A thermostat measures room temperature using a sensor. Its program compares the reading with a chosen temperature and switches the heating on or off. Answer each part about this controller.
(a) Explain how the CPU runs the thermostat's program.
Students type their answer here.
(b) Explain how the ALU contributes to the thermostat's operation.
Students type their answer here.
4. Check from memory
Answer the final questions without looking back. Then use your answers to decide which component needs another look.
(a) What does cache store?
Students type their answer here.
(b) What is a register?
Students type their answer here.