Clock Speed · Cores · Cache · Bus Width · How They Affect Performance
CSZoneEdexcel GCSE Computer Science 1CP2
Clock Speed
How Fast the CPU Ticks
The clock speed determines how many fetch-decode-execute cycles can be completed per second. Measured in GHz (gigahertz). 1 GHz = 1 billion cycles per second.
Higher clock speed → more instructions processed per second → faster performance
Drawback: higher speed generates more heat, requires more power
Overclocking: running the CPU above its rated speed — can cause instability/overheating
Cores
Multiple Processing Units
A core is an independent processing unit within the CPU. A quad-core CPU has 4 cores, each capable of running instructions simultaneously.
More cores → multiple tasks can run in parallel → better multitasking
Limitation: some programs cannot be split across cores (not all software is multi-threaded)
Hyper-threading: one physical core acts as two logical cores for further efficiency
Cache Memory
Fast Storage Near the CPU
Cache is small, very fast memory built into or near the CPU. It stores frequently used instructions and data so the CPU doesn't have to wait for slower RAM.
L1 cache: fastest and smallest (KB); built into each core
L2 cache: slightly larger, slightly slower; per-core or shared
L3 cache: largest cache (MB), shared between all cores — slowest of the three but still far faster than RAM
Exam Practice
Have a go at this question
Edexcel-style question
Explain how increasing the number of cores in a CPU can improve performance. State one limitation of this approach.
3 marks
More cores allow multiple tasks (threads) to be processed simultaneously [1], so overall throughput increases [1]. Limitation: if a program is single-threaded, it cannot use additional cores and there is no benefit [1].
Key Takeaways
What to Remember
Clock speed (GHz): faster = more cycles/sec; generates more heat
Cores: more cores = parallel processing; limited by software design