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
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.
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