CPU performance factors

OCR J277 Paper 1 regularly sets 3–6 mark questions asking students to explain how different factors affect CPU performance. You need to know three key factors and be able to explain the mechanism — not just "more = faster".

1. Clock speed

The clock speed (measured in GHz — gigahertz) determines how many fetch–decode–execute cycles the CPU can complete per second.

• 1 GHz = 1,000,000,000 (one billion) clock cycles per second.
• A 3.6 GHz CPU can carry out up to 3.6 billion cycles per second.
• Higher clock speed → more instructions processed per second → faster performance.

Limits

• Higher clock speeds produce more heat and consume more power.
• There is a physical limit to how fast individual transistors can switch.
• Modern CPUs use multiple cores rather than purely increasing single-core speed.

2. Cache size

Cache is extremely fast, small memory located inside or very close to the CPU.

• The CPU checks cache before RAM; cache access takes ~1–5 ns vs RAM's ~60–100 ns.
• L1 cache: smallest (32–512 KB), fastest, inside the CPU core.
• L2 cache: larger (256 KB–4 MB), slightly slower, per-core.
• L3 cache: largest (4–64 MB), shared between cores, slowest of the three.

How it affects performance

• If frequently used data/instructions are in cache: cache hit → very fast.
• If not in cache: cache miss → CPU must wait for RAM → slower.
• Larger cache → more data can be stored → fewer cache misses → faster overall performance.

3. Number of cores

A core is a complete processing unit within a CPU that can independently fetch, decode and execute instructions.

• Dual-core: 2 cores; Quad-core: 4 cores; Octa-core: 8 cores.
• Multiple cores allow parallel processing — different cores handle different tasks simultaneously.
• More cores → more tasks can run simultaneously → better multi-tasking and multi-threaded performance.

Limits

• Not all software is written to use multiple cores (single-threaded software only uses one core).
• The improvement is not perfectly linear — inter-core communication adds some overhead.

Summary comparison

Factor	How it works	Effect of increasing
Clock speed	Cycles per second → instructions per second	Faster execution of each task
Cache size	Reduces RAM access → fewer waits	Fewer cache misses → less CPU idle time
Number of cores	Parallel processing	More tasks simultaneously

Common OCR exam mistakes

1. Saying "faster clock speed = more cores" — these are independent factors.
2. Claiming all software benefits from multiple cores — only multi-threaded software does.
3. Mixing up cache and RAM — cache is inside the CPU, orders of magnitude faster.
4. Forgetting to explain the mechanism (not just "more cores = faster") — OCR mark schemes want the reason.