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OCR J277 · Component 1 · Topic 1.1.2
CPU
Performance
Clock Speed · Number of Cores · Cache Size
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OCR GCSE Computer Science J277
Learning Objectives
By the end of this video you will be able to...
Name the three factors that affect CPU performance
Explain how clock speed affects the number of instructions processed
Explain how number of cores allows instructions to be processed simultaneously
Explain how cache size reduces the need to fetch from slower RAM
⚡ The exam will ask you to name all three AND explain how each affects performance — you need both
1.1.2 CPU Performance
Three factors affect CPU performance
⚡
Clock Speed
How fast the CPU runs its FDE cycles
🔲
Number of Cores
How many FDE cycles can run at the same time
💾
Cache Size
How much fast memory is available to the CPU
⚠ The mark scheme requires full phrases — "clock speed", "number of cores", "cache size" — single words like "clock" or "cores" alone are not enough
Factor 1 of 3
Clock Speed
Clock speed is the number of FDE cycles the CPU can perform per second.
It is measured in gigahertz (GHz). One gigahertz = one billion cycles per second.
2 GHz
2 billion FDE cycles per second
4 GHz
4 billion FDE cycles per second
Key idea:
The faster the clock, the more instructions the CPU can process in a given time
Clock Speed — Effect on Performance
How clock speed affects performance
Higher clock speed
→
More FDE cycles per second
→
More instructions processed per second
Lower clock speed
→
Fewer FDE cycles per second
→
Fewer instructions processed per second
"Increasing clock speed allows more instructions to be processed per second"
⚠ EXAM TRAP:
Don't just say "faster" — say more instructions processed per second or more FDE cycles per second
Factor 2 of 3
Number of Cores
A core is an independent processing unit within the CPU.
Each core can carry out its own FDE cycle independently.
Dual Core
2 cores — 2 FDE cycles simultaneously
Quad Core
4 cores — 4 FDE cycles simultaneously
Key idea:
More cores = more instructions carried out at the same time
Number of Cores — Effect on Performance
How number of cores affects performance
More cores
→
Multiple FDE cycles simultaneously
→
More instructions in a given time
Fewer cores
→
Fewer cycles at once
→
Fewer instructions in a given time
"Multiple cores can carry out FDE cycles simultaneously, resulting in more instructions being carried out in a given time"
⚠ EXAM TRAP:
Don't say "does more tasks at once" — say carries out FDE cycles simultaneously
Factor 3 of 3
Cache Size
Cache is a small, very fast memory built into or very close to the CPU.
It stores frequently used instructions and data so the CPU can access them without fetching from the slower main memory (RAM).
Much faster than RAM
CPU can access cache without waiting
Much less capacity
Smaller than RAM — only the most used data fits
Cache Size — Effect on Performance
How cache size affects performance
Larger cache
→
More instructions/data stored close to CPU
→
CPU fetches from slow RAM less often
Smaller cache
→
Fewer instructions stored close to CPU
→
CPU must wait for slower RAM more often
"A larger cache allows more instructions/data to be stored close to the CPU, so it does not need to fetch from RAM as often"
⚠ EXAM TRAP:
Insufficient cache slows the CPU down because it has to wait for RAM — make this link explicit
Exam Practice
Pause and have a go before the answer appears
Question 1
State three factors that can affect the performance of a CPU.
3 marks
✓ MARK SCHEME
⚠ Single words — "clock", "cores", "cache" — are not enough. Full phrases required.
Exam Practice
Question 2
Explain how increasing the clock speed of a CPU affects its performance.
2 marks
✓ MARK SCHEME
1 mark
Increasing clock speed means more FDE cycles are performed per second
1 mark
This means more instructions are processed per second, so performance increases
You need the cause (more cycles) and the effect (more instructions processed) for both marks
Exam Practice
Question 3
Explain how increasing the number of cores affects CPU performance.
2 marks
✓ MARK SCHEME
1 mark
Multiple cores can carry out FDE cycles simultaneously
1 mark
This results in more instructions being carried out in a given time, increasing performance
Key phrase: simultaneously — this is what makes multiple cores different from a faster single core
Exam Practice
Question 4
Explain how cache size affects the performance of a CPU.
2 marks
✓ MARK SCHEME
1 mark
A larger cache allows more instructions/data to be stored close to the CPU
1 mark
The CPU does not need to fetch from the slower RAM as often, so performance increases
You must make the link to RAM being slower — without this the explanation is incomplete
Common Mistakes
Don't lose easy marks
1
Writing "clock", "cores", or "cache" as single words for the name the three factors question. The mark scheme requires clock speed, number of cores, and cache size as full phrases.
2
Saying a faster clock speed just makes the computer "faster" without explaining why. You must say: more FDE cycles per second, therefore more instructions processed per second.
3
Explaining cache without mentioning RAM. The reason larger cache improves performance is that the CPU doesn't need to fetch from the slower RAM as often. That link must be in your answer.
Topic Summary — 1.1.2
What You Need to Know
| Factor | What it means | Effect on performance |
|---|
| Clock speed |
Number of FDE cycles per second (GHz) |
Higher = more instructions processed per second |
| Number of cores |
Independent processing units in the CPU |
More = FDE cycles carried out simultaneously |
| Cache size |
Fast memory built into or close to the CPU |
Larger = fewer fetches from slower RAM |
Remember:
Full phrases only — "clock speed", "number of cores", "cache size"
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1.2
Memory and Storage
RAM · ROM · Virtual Memory · Secondary Storage
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