IGCSE · Practice questions · Cores, cache and clock speed

Faster, why?

Six original Cambridge-style questions on CPU performance, with mark-scheme style solutions.

Original questions All questions on this page are original work, written in the Cambridge IGCSE style. They are not from past papers. They test the same concepts and skills the syllabus rewards.

Keep these straight

Factor, then effect.

Clock: faster clock, more FDE cycles per second.
Cache: bigger cache, more frequently used data near the CPU, fewer slow trips to RAM.
Cores: more cores, more instructions processed at the same time.
Limits: heat at high clock speeds; not all tasks split across cores; cache is expensive.

Explain

[2 marks]

Explain how increasing the clock speed of a CPU can improve its performance.

The clock sends more pulses per second. ✓
So more fetch-decode-execute cycles (instructions) are carried out per second. ✓

Explain

[3 marks]

Describe what is meant by cache and explain how its size affects performance.

Cache is a small amount of very fast memory in or near the CPU. ✓
It stores frequently used instructions and data. ✓
A larger cache stores more of them, so the CPU makes fewer slower fetches from RAM. ✓

Explain

[2 marks]

Explain how having more cores can affect the performance of a CPU.

Each core can process instructions independently. ✓
So more instructions can be processed at the same time (simultaneously). ✓

Apply

[3 marks]

A school compares CPU X (2 cores, 4.0 GHz, 6 MiB cache) and CPU Y (6 cores, 3.2 GHz, 12 MiB cache) for running several programs at once. Recommend one and justify your choice.

CPU Y. ✓
It has more cores, so more programs/instructions can be processed at the same time. ✓
It has a larger cache, so fewer slower fetches from RAM (even though X has a faster clock). ✓

Recall

[2 marks]

Give two reasons why doubling the number of cores may not double the performance of a computer.

Some tasks/programs cannot be split up to run on several cores. ✓
Time is spent communicating and sharing data between cores (or the software is not written to use multiple cores). ✓

Analysis

[2 marks]

A shop assistant says a computer with a 3 GHz CPU "can store 3 gigabytes". Explain the mistake.

3 GHz is the clock speed: 3 billion clock cycles per second. ✓
It describes processing speed, not storage capacity. ✓

Mark this once you have attempted all six and checked your working. It records a Practiced badge on the topic and adds a one-time bonus. Revealing the solutions alone does not count.