Spec & Goals 3 min
AQA Spec 3.4.5 · Systems architecture — CPU performance
By the end of this lesson you can:
- State what is meant by clock speed and the unit it is measured in.
- Explain how the number of cores affects CPU performance.
- Explain how cache size affects CPU performance.
Warm-Up 5 min
Last lesson you learned the CPU repeats the fetch–decode–execute cycle. The faster it runs that cycle, the more work it gets done.
Quick starter
A shop advertises a laptop as "3.2 GHz, quad-core". What do you think those two numbers tell a buyer?
Reveal the answer
3.2 GHz is the clock speed (cycles per second) and quad-core means it has four cores. Both affect how fast it works.
Key Concept — three factors that affect performance 14 min
Three things change how much work a CPU can do: its clock speed, its number of cores, and its cache size.
1. Clock speed
Clock speed is how many fetch–decode–execute cycles the CPU runs each second. It is measured in hertz, usually gigahertz (GHz).
A higher clock speed means more instructions per second, so the CPU does more work in the same time.
2. Number of cores
A core is a processing unit inside the CPU. Each core can run instructions on its own.
More cores let the CPU run several instructions at once — in parallel — so it can handle more tasks at the same time.
3. Cache size
Cache is fast memory built very close to the CPU. It holds frequently-used data and instructions.
A larger cache means more useful data is held nearby, so the CPU makes fewer slow trips to RAM and waits less.
Worked Example — what each factor does and why 12 min
For each factor, the exam wants the effect and the reason. This table shows both.
| Factor | Effect on performance | Why (the reason that earns the mark) |
|---|---|---|
| Clock speed | Higher speed = more instructions per second. | Each cycle runs one step of the fetch–decode–execute cycle, so more cycles per second means more work done. |
| Number of cores | More cores = more tasks handled at once. | Each core can fetch and execute instructions independently, so several instructions run in parallel. |
| Cache size | Larger cache = less waiting for data. | More frequently-used data is kept near the CPU, so it makes fewer slow trips to RAM. |
For example, upgrading a school laptop from a dual-core to a quad-core CPU lets it edit video while a virus scan runs, because two extra cores share the work in parallel.
Try It Yourself 12 min
Goal: State the unit that clock speed is measured in.
Hint: CPUs are advertised in GHz — what does the "Hz" stand for?
Goal: Explain, in one sentence, why a higher clock speed improves performance.
Hint: link it to the number of fetch–decode–execute cycles per second.
Goal: A gamer is choosing between a CPU with a larger cache and one with more cores. Explain a situation where each would be the better choice.
Hint: cache helps with repeated data; cores help with running many tasks at once.
📝 Exam Practice 10 min
Answer the way the examiner expects — the command word and the marks tell you how much to write.
State what is meant by clock speed.
Mark scheme
- The number of (fetch–decode–execute) cycles / instructions the CPU performs per second (1).
Explain how increasing the number of cores can improve performance.
Mark scheme
- Each core can fetch and execute instructions independently (1).
- So more instructions / tasks can be processed at the same time / in parallel (1).
Explain how a larger cache improves performance.
Mark scheme
- More frequently-used data / instructions can be stored close to the CPU (1).
- So fewer / slower accesses to RAM are needed, reducing waiting time (1).
Recap & Key Terms 3 min
Three factors raise CPU performance. A higher clock speed runs more cycles per second. More cores process instructions in parallel. A larger cache keeps useful data near the CPU, cutting slow trips to RAM. In the exam, always give the reason, not just the effect.
- Clock speed
- The number of fetch–decode–execute cycles a CPU performs per second, measured in hertz (Hz).
- Core
- A processing unit inside the CPU that can fetch and execute instructions independently.
- Cache
- Small, fast memory near the CPU that stores frequently-used data and instructions.
- Performance
- How much work a CPU can do in a given time, affected by clock speed, cores and cache.
Homework 1 min
Task (≤ 15 min): A shop sells two laptops. Laptop A is 3.5 GHz dual-core; Laptop B is 2.8 GHz quad-core. Explain one reason a buyer might choose each one.
Model answer
Laptop A: a higher clock speed runs each cycle faster, so single tasks finish sooner.
Laptop B: more cores run instructions in parallel, so it handles many tasks at once better.
Award marks for: a correct reason for A linked to clock speed (1), a correct reason for B linked to cores / parallel processing (1).