This topic summary covers Knowledge Organiser: CPU Performance within CPU Performance for GCSE Computer Science. Revise CPU Performance in 3.4 Computer Systems for GCSE Computer Science with 15 exam-style questions and 12 flashcards. This topic appears regularly enough that it should still be part of a steady revision cycle. It is section 8 of 8 in this topic. Use this topic summary to connect the idea to the wider topic before moving on to questions and flashcards.
Knowledge Organiser: CPU Performance
Key Terms
- Clock speed: Number of FDE cycles the CPU can perform per second, measured in GHz
- GHz: Gigahertz — billions of cycles per second
- Core: Independent processing unit within a CPU; multiple cores allow parallel processing
- Cache: Small, very fast memory built into the CPU that stores frequently-used data
- Multi-threading: Programming technique that allows software to use multiple cores simultaneously
Must-Know Facts
- Three factors affecting CPU performance: clock speed, number of cores, cache size
- Higher clock speed = more FDE cycles per second = more instructions executed
- More cores = multiple tasks can run simultaneously (parallel processing)
- More cache = frequently-used data closer to CPU = less waiting for RAM
- Clock speed cannot increase forever — heat and power limits apply (~5 GHz max)
- Doubling cores does NOT double performance — programs must support multi-threading
- RAM is NOT a CPU performance factor — RAM affects system performance but not CPU speed
Key Concepts
- Cache hierarchy: L1 (fastest/smallest) → L2 → L3 (slowest/largest)
- Clock speed analogy: chef working faster; cores analogy: hiring more chefs
- Cache analogy: keeping ingredients on the counter rather than fetching from the fridge each time
- All three factors work together — modern CPUs balance all three for best performance
Common Mistakes
- Including RAM as a CPU performance factor: Exam questions ask specifically about CPU performance — RAM affects the whole system but is NOT one of the three CPU factors (clock speed, cores, cache)
- Thinking more cores always doubles speed: Performance only improves if the software is written to use multiple cores — not all programs are multi-threaded
- Confusing cache with RAM: Cache is inside (or very close to) the CPU and is much faster but smaller than RAM — it stores frequently used data to reduce waiting time
- Saying higher clock speed is always better: Higher clock speeds generate more heat — there is a practical limit, which is why manufacturers add more cores instead
Practice questions for CPU Performance
What does clock speed measure in a CPU?
Describe three factors that affect CPU performance.
Quick recall flashcards
What is clock speed measured in?
GHz (gigahertz) - billions of cycles per second
Three CPU performance factors?
Clock speed, cores, cache