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J277 1.1.2 CPU Performance
Part 2 of 6 · J277 1.1 Systems Architecture
This worksheet explores the three key factors that affect CPU performance: clock speed, cache size and number of cores. Pupils learn to explain how each characteristic influences processing speed, and practise comparing processors based on these features. Tasks include cause-and-effect chains, multiple-choice questions and evaluation of realistic scenarios, requiring moderate reasoning.
Shared by Chris H.Verified teacher
- 15 cells
- About 50 minutes
- CC BY-SA 4.0
- Shared 29 Jul 2026
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Systems architecture 2: CPU performance
CPU performance is affected by three characteristics named in OCR J277: clock speed, cache size and number of cores. You need to understand each characteristic and explain the effects of changing them individually or in combination.
Processor tables can look complicated at first. You do not need to decide from every number at once. Learn one cause-and-effect chain for each characteristic, then combine them when a question gives you a scenario.
By the end of this worksheet, you should be able to:
- explain clock speed, cache size and number of cores
- build cause-and-effect explanations for each characteristic
- compare processor specifications
- apply performance knowledge to a scenario
- recognise why “more” does not always produce a proportional improvement
- justify a processor choice using several pieces of evidence
Before you begin, which component performs arithmetic calculations and logical comparisons?
- AControl unit
- BArithmetic logic unit
- CCache
- DRegister
1. Clock speed
The CPU's clock produces regular electronic pulses that coordinate processing. Clock speed is the number of clock cycles per second and is measured in hertz. Modern processor speeds are often expressed in gigahertz (GHz), where 1 GHz represents one billion cycles per second.
A higher clock speed can allow the processor to carry out more fetch-execute cycles, and therefore process more instructions, in the same time.
Build the explanation as a chain:
higher clock speed → more cycles per second → potentially more instructions processed per second → improved performance
Do not write only “the clock is faster”. Name the characteristic as clock speed and explain the effect.
______ measures the number of ______. It is often measured in ______. Increasing it can allow more ______ to be processed in the same time.- clock speed
- cache size
- cycles per second
- gigahertz
- cores
- instructions
2. Cache size
Cache is small, fast memory in or close to the CPU. It stores frequently or recently used data and instructions.
A larger cache can hold more of the items that the processor may need again. If the required item is found in cache, the CPU can access it more quickly than from RAM. This can reduce waiting and improve performance.
Build the chain:
larger cache → more frequently used data and instructions can be held close to the CPU → fewer or quicker accesses than repeatedly retrieving them from RAM → improved performance
The required characteristic is cache size, not just “cache” or “memory”.
Explain how increasing cache size can improve CPU performance.
Link capacity, frequently used data or instructions, access compared with RAM, and reduced waiting.
Students type their answer here.
3. Number of cores
A core is a processing unit within a CPU that can carry out its own fetch-execute cycle. A multi-core processor contains more than one core.
More cores can allow more instructions or tasks to be processed at the same time. This can improve multitasking and programs that divide their work into parts that can run in parallel.
Build the chain:
more cores → more instructions or tasks can be processed simultaneously → less time needed for suitable workloads → improved performance
The improvement depends on the software and task. If a program cannot divide its work between cores, additional cores may be unused. Twice as many cores does not guarantee twice the performance.
Which three statements about processor cores are accurate? Select all that apply.
- AEach core can carry out its own fetch-execute cycle
- BDoubling the number of cores always makes every program exactly twice as fast
- CMore cores can process more instructions or tasks simultaneously
- DA core is a permanent storage device for user files
- ESoftware must be able to divide suitable work between cores to gain the full benefit
Increasing ______ can increase the number of ______ completed each second. Increasing ______ can keep more ______ data and instructions close to the CPU. Increasing the ______ can allow suitable tasks to be processed ______.- clock speed
- cache size
- number of cores
- cycles
- frequently used
- simultaneously
4. Compare factors together
OCR can ask about characteristics individually or in combination. Read every specification rather than deciding from one number.
| Processor | Clock speed | Cache size | Cores |
|---|---|---|---|
| A | 3.8 GHz | 8 MB | 4 |
| B | 3.2 GHz | 16 MB | 8 |
Processor A has the higher clock speed. Processor B has the larger cache and more cores. There is no single correct choice without knowing the task.
For a program that mainly uses one core, A's higher clock speed may be more useful. For several parallel tasks, B's additional cores may be more useful. B's larger cache may also reduce slower accesses to RAM.
A sound recommendation follows this structure:
choice → relevant specification → how it affects processing → benefit in the scenario
Using the table above, compare processors A and B. Explain one possible performance advantage of each.
For each processor, quote the relevant characteristic and explain how it could affect processing.
Students type their answer here.
Which four statements would be sufficiently precise in an exam answer? Select all that apply.
- AThe processor is better because it has more clock
- BA higher clock speed allows more clock cycles to be completed per second
- CA larger cache can hold more frequently used data and instructions
- DMore cache always makes every program twice as fast
- EMore cores can allow suitable tasks to be processed simultaneously
- FAdditional cores may give little benefit if the software cannot use them
A video-editing application can divide rendering work across several cores. Processor C has 4 cores, a clock speed of 4.0 GHz and 8 MB cache. Processor D has 8 cores, a clock speed of 3.5 GHz and 16 MB cache.
Recommend one processor for the video editor and justify your choice.
State the choice first. Use at least two relevant characteristics, explain how they affect performance and acknowledge one trade-off. A well-supported choice matters more than simply choosing the processor with the largest numbers.
Students type their answer here.
Challenge: test the “more is always better” claim. Evaluate the statement, “The processor with the largest numbers in its specification will always run every program fastest.”
To evaluate, consider evidence and limitations before reaching a judgement. Explain all three OCR characteristics, how they can work in combination and why the workload or software matters.
Students type their answer here.
Closed-book review
Without looking back, write one cause-and-effect chain for each characteristic:
- higher clock speed → ...
- larger cache size → ...
- greater number of cores → ...
Then add one sentence explaining why more cores may not improve a particular program. Check that you used each full characteristic name.