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J277 1.1.2 CPU performance
Part 2 of 6 · J277 1.1 · Systems architecture
The three characteristics OCR J277 uses to explain CPU performance: clock speed, cache size and number of cores.
Students will:
- explain clock speed, cache size and number of cores
- build a cause-and-effect chain for each characteristic
- compare processor specifications
- justify a processor choice from several pieces of evidence
- explain why more of one characteristic is not always a proportional gain
Inside: 8 text sections, 2 multiple-choice activities, 2 completion activities, 6 written answers. 18 marks. Suggested teaching time: 40–60 minutes. 1 diagram.
Includes a teacher guide with suggested sections, timings and common misconceptions.
Series: J277 1.1 · Systems architecture, part 2 of 6.
Shared by Coding PathwayVerified teacher
- 18 cells
- About 45 minutes
- CC BY-SA 4.0
- Shared 17 Aug 2026
- Updated 27 Oct 2026
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CPU performance
Clock speed, cache size and number of cores can affect how quickly a CPU processes instructions. Learn what each characteristic changes, then use them to choose a processor for a stated task.
Which CPU component performs arithmetic calculations and logical comparisons?
- AControl unit
- BProgram Counter
- CArithmetic logic unit
- DCache
1. Clock speed
The clock produces regular electrical pulses. The control unit uses these pulses to coordinate operations. Clock speed is the number of clock cycles each second.
Clock speed is measured in hertz (Hz). One gigahertz (GHz) means one billion cycles per second. A higher clock speed can allow more instructions to be processed in the same time, when other characteristics are comparable.
- million
- clock speed
- cache size
- billion
2. Cache size
Cache is small, fast memory in or close to the CPU. It holds frequently used data and instructions.
A larger cache can hold more of these items. When a required item is already in cache, the CPU can access it more quickly than from RAM. This can reduce waiting. Increasing capacity does not mean each individual cache access becomes faster.
Explain how increasing cache size can improve CPU performance.
Explain what changes and how that affects processing.
Students type their answer here.
3. Number of cores
A core is a processing unit within a CPU. Each core can carry out its own fetch-execute cycle. More cores can process instructions or tasks at the same time.
The benefit depends on the program. Some tasks can be divided between cores; others must mainly run on one core. Doubling the number of cores does not guarantee that every program finishes in half the time.
A program can divide its work evenly between several cores. What could extra cores allow?
- AEach core to receive a higher clock speed automatically
- BThe program to stop using RAM
- CEach instruction to be stored permanently
- DMore parts of the work to run at the same time
Keep the mechanisms separate
Follow each row from a changed characteristic to its possible effect.
4. Compare processors for a task
| Processor | Clock speed | Cache size | Cores |
|---|---|---|---|
| A | 3.8 GHz | 8 MB | 4 |
| B | 3.2 GHz | 16 MB | 8 |
Here are short explanations using the figures:
- A has a higher clock speed. It has more clock cycles each second, which can help a program that mainly uses one core.
- B has more cores. A program that shares work between cores may process more parts at the same time.
- B has a larger cache. It can hold more frequently used items, which may reduce slower accesses to RAM.
A recommendation needs a characteristic, its effect and a link to the task. The largest number alone is not a reason.
Your comparison
A school makes videos. Its editing program creates the final video frames by dividing the work between several cores. The processors are otherwise comparable.
| Processor | Clock speed | Cache size | Cores |
|---|---|---|---|
| C | 4.0 GHz | 8 MB | 4 |
| D | 3.5 GHz | 16 MB | 8 |
(a) Recommend processor C or D. Give two developed reasons for your choice using different CPU characteristics.
Students type their answer here.
(b) Processor C has the higher clock speed. Explain one possible advantage this gives C when processing instructions.
Students type their answer here.
5. Check from memory
Use a separate answer for each characteristic. Explain its effect rather than writing only 'faster'.
(a) Explain one possible effect of increasing clock speed.
Students type their answer here.
(b) What can a larger cache hold more of?
Students type their answer here.
(c) What can more cores allow a suitable program to do?
Students type their answer here.