📚 Knowledge Library — Topic 3.5 — Hardware

CPU Performance

Understand the three main factors that affect CPU performance: clock speed, number of cores and cache size.

Start LearningWorked Example
1. Invitation

Why are some computers faster than others?

Two computers can both have a CPU, but one may run programs much faster.

CPU performance depends mainly on how quickly the CPU works, how many instructions it can handle at once, and how quickly it can access useful data.

💡 Remember: performance is about how efficiently the CPU can process instructions.
Figure 1.1
Performance Factors
Clock speed
Cores
Cache
↓
CPU performance
2. Big Idea

Three factors matter most.

Clock speed affects how many Fetch–Decode–Execute cycles happen each second.

Cores affect how many instructions can be processed simultaneously. Cache helps the CPU access frequently used data quickly.

💡 Clock speed = per second. Cores = at the same time. Cache = faster access.
Figure 2.1
Three Jobs
Speed
+
Simultaneous work
+
Fast nearby data
3. FutureLogic Bridge

Think of chefs in a kitchen.

Clock speed is how fast one chef works.

Cores are how many chefs are working at the same time. Cache is the workbench where frequently used ingredients are kept close by.

💡 Bridge: faster chef, more chefs, better workbench.
Figure 3.1
Kitchen Model
Chef speed
=
Clock speed

More chefs
=
More cores
4. Clock Speed

Clock speed controls cycles per second.

Clock speed is measured in gigahertz, or GHz.

A higher clock speed means more Fetch–Decode–Execute cycles can happen each second.

Important fact

1 GHz = 1 billion cycles per second
🎯 Exam Tip: 3.5 GHz means 3.5 billion FDE cycles per second, not million.
Figure 4.1
Clock Speed
1 GHz
↓
1 billion cycles
per second
Higher clock speed = more cycles each second.
5. Number of Cores

More cores can process more at once.

A core is an independent processing unit inside the CPU.

More cores can allow more instructions to be processed simultaneously, especially when multitasking or using software designed for multiple cores.

💡 Key word: simultaneously. That means at the same time.
Figure 5.1
More Cores
1 core
1 task

4 cores
4 tasks
at once
6. Cache Size

Cache keeps useful data close.

Cache is very fast memory close to the CPU.

It stores frequently used data and instructions so the CPU does not always need to fetch them from slower RAM.

💡 Cache is faster to access than RAM.
Figure 6.1
Cache vs RAM
CPU
↔
Cache
↓
RAM
Cache is closer and faster.
7. Worked Example

Choosing the best answer.

Different questions focus on different performance factors.

Which CPU handles more instructions simultaneously?

CPU A
4 cores
CPU B
2 cores
CPU C
1 core
CPU A, because more cores can process more instructions at the same time.
Figure 7.1
Simultaneous Work
More cores
↓
more instructions
at the same time
8. Exam Tip

Use the correct performance word.

Cambridge often tests the difference between clock speed and cores.

Question focusBest answer
More cycles per secondClock speed
More instructions at the same timeNumber of cores
Faster access than RAMCache
🎯 Exam Tip: clock speed = per second. Cores = simultaneously.
Figure 8.1
Exam Words
Per second
=
Clock speed

At same time
=
Cores
9. Common Mistake

More cores do not mean higher clock speed.

Students sometimes think adding more cores automatically makes each core faster.

That is not correct. Clock speed and number of cores are separate CPU performance factors.

⚠️ Common Mistake: a core does not control clock speed. A core is an independent processing unit.
Figure 9.1
Do Not Mix Them
More cores
≠
higher GHz

Cores
=
simultaneous work
10. Summary

CPU performance in one screen.

Clock speed affects how many FDE cycles happen each second.

More cores can allow more instructions to be processed simultaneously. More cache can improve performance because frequently used data is accessed faster than from RAM.

💡 Key idea: each factor improves performance in a different way.
Figure 10.1
Performance Summary
Clock speed
= cycles per second

Cores
= simultaneous work

Cache
= fast nearby data