← IGCSE Computer Science Paper 1

How cores, cache and clock affect CPU performance

How fast a CPU runs programs. More cores, a bigger cache or a faster clock speed can raise it, but each has limits: software must split tasks, cache is costly, and a faster clock makes more heat.

Part of: IGCSE Computer Science 0478 — 3.1 Computer Architecture — Module 28, ACARA Digital Technologies Years 7–8 — Hardware and Performance — Module 14

Understanding How cores, cache and clock affect CPU performance in IGCSE Computer Science Paper 1

What it does: A program written as one single task can only use one core, so the other cores sit idle and performance barely improves. CPU A has the faster clock, but CPU B has more cores and a bigger cache, so CPU B is likely to do better on programs that use several cores. Cache stores frequently used data close to the CPU, so it waits for RAM less often. It does not change the clock speed. A 3 GHz CPU with one core and a tiny cache can be beaten by a 2.5 GHz CPU with four cores and a large cache.

Common mistakes: On a cache miss the CPU makes a slower trip to RAM, not to the ALU. The ALU only does calculations and does not store data. It should be: ELSE fetch it from RAM. A bigger cache holds more data, so the CPU makes fewer slow trips to RAM. The trips should go down: ramTrips ← ramTrips - 5.

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Learning
Duo
A CPU has a larger cache. What is the main benefit?
Fewer slow trips to RAM to fetch data and instructionsThe clock ticks more times per second
Matches
A CPU with 8 cores always runs every program 8 times faster than a CPU with 1 core.
↓
Extra cores do not speed up every program. A program only gets faster if it can split its work into tasks that run on different cores.
Applying
Meanings
Term
CPU performance
Meaning
How fast a CPU runs programs. More cores, a bigger cache or a faster clock speed can raise it, but each has limits: software must split tasks, cache is costly, and a faster clock makes more heat.
Practice
IF taskCanSplit = TRUE THEN
Testing
Gaps
Clock speed = 3.5
Check Answer
Spots
2 use it quickly
3ELSE fetch it from the ALU

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Quick Questions

1. A CPU has a larger cache. What is the main benefit?
A. The clock ticks more times per second
B. Fewer slow trips to RAM to fetch data and instructions ✓
Cache stores frequently used data close to the CPU, so it waits for RAM less often. It does not change the clock speed.
2. A program can only run one task at a time and cannot be split up. What happens when you move it from a dual-core CPU to a quad-core CPU?
A. Little or no speed-up, because the extra cores sit unused ✓
B. It runs about twice as fast because there are twice as many cores
More cores only help when software can split work into tasks that run at the same time.
3. Which change means more fetch-decode-execute cycles per second?
A. Raising the clock speed from 2.5 GHz to 3.5 GHz ✓
B. Adding more RAM to the computer
Clock speed is measured in GHz, which is billions of cycles per second. More RAM does not change how many cycles the CPU runs per second.
4. What is a downside of making the clock speed much higher?
A. The CPU produces more heat and may need better cooling ✓
B. The CPU has fewer cores
A faster clock means more switching each second, which creates more heat. It does not change the number of cores.
5. CPU A: 4 cores, 3.0 GHz, 8 MB cache. CPU B: 2 cores, 3.0 GHz, 4 MB cache. Which is likely faster on software that uses all cores?
A. They are the same, because the clock speed is the same
B. CPU A, because it has more cores and a larger cache ✓
When comparing CPUs, look at cores, cache and clock together. Equal clock speeds do not make equal performance.

Match the Pairs

Clock speed: 3.5 GHz → The CPU completes 3.5 billion cycles per second, so it can run more instructions each second, but it also produces more heat.
Cache: 8 MB → A small, very fast memory inside the CPU that holds frequently used data and instructions, so the CPU makes fewer slow trips to RAM.
Extra cores only help when software can split its work into tasks that run at the same time. → Cores: 4 | Program: single task | Speed-up: little or none
To compare two CPUs, look at their cores, clock speed and cache together, not just one figure. → CPU A: 2 cores, 3.8 GHz, 4 MB cache | CPU B: 4 cores, 3.0 GHz, 8 MB cache
A CPU with 8 cores always runs every program 8 times faster than a CPU with 1 core. → Extra cores do not speed up every program. A program only gets faster if it can split its work into tasks that run on different cores.
Doubling the clock speed forever is the best way to improve performance because it has no downsides. → A faster clock speed makes the CPU produce more heat, which limits how high it can go without overheating or needing better cooling.

Key Term

CPU performance
How fast a CPU runs programs. More cores, a bigger cache or a faster clock speed can raise it, but each has limits: software must split tasks, cache is costly, and a faster clock makes more heat.

Fill in the Blanks

1. Clock speed = 3.5 GHz
Hint: Type the unit used to measure clock speed, which counts billions of cycles per second.
2. More cores = more tasks at the same time
Hint: Type the name of the CPU parts that let several tasks run at once.
3. Bigger cache = fewer trips to RAM
Hint: Type the slower main memory that the CPU visits less often when the cache is bigger.
4. Higher clock speed = more heat produced
Hint: Type the unwanted side effect of running a CPU faster.
5. IF coresA > coresB THEN OUTPUT "A can run more tasks" ENDIF
Hint: Type the comparison operator that checks whether the first value is greater than the second.

Spot the Error

Question 1
A IF data is in cache THEN
B use it quickly
C ELSE fetch it from the ALU
D END IF
Line C — On a cache miss the CPU makes a slower trip to RAM, not to the ALU. The ALU only does calculations and does not store data. It should be: ELSE fetch it from RAM.
Question 2
A cacheSize ← cacheSize + 4
B ramTrips ← ramTrips + 5
C OUTPUT ramTrips
Line B — A bigger cache holds more data, so the CPU makes fewer slow trips to RAM. The trips should go down: ramTrips ← ramTrips - 5.
Question 3
A clock ← 3.0 // GHz
B cycles ← clock / 1000000000
C OUTPUT cycles
Line B — 1 GHz means one billion cycles per second, so the clock speed in GHz must be multiplied by 1 000 000 000, not divided. It should be: cycles ← clock * 1000000000.
Question 4
A tasks ← 4
B cores ← 4
C IF software splits tasks THEN
D tasks run one by one
E END IF
Line D — When software can split its work, each core takes a task and they run at the same time (in parallel). One by one is what happens when the work cannot be split. It should be: tasks run at the same time.
Question 5
A IF clock speed decreases THEN
B heat produced increases
C END IF
Line A — A higher clock speed means more cycles per second, which makes more heat. The condition should say increases, to match the heat produced increasing.

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