GCSE Computer Science (CCEA)

Topic Summaries

COMPUTER SYSTEMS: Systems architecture

Factors affecting CPU performance

Factor Description Effects of changing characteristics
Clock speed
  • The CPU’s clock (part of the CU) pulses at fixed intervals to trigger the next action in the CPU.
  • Clock speed is typically measured in GHz (gigahertz) where 1 GHz = 1 billion cycles per second
  • The faster the clock, the higher the speed at which fetchexecute cycles can take place, therefore more instructions are processed per second
Cache size
  • Cache memory is a small, very fast memory in the CPU that holds frequently used data/instructions as it is quicker to access than data/instructions in RAM.
  • Cache memory is very expensive, so the average CPU may only have 4MB or 8MB of cache, which is 1000× smaller than 4GB or 8GB of RAM.
  • Having a larger cache allows CPUs to hold a larger number of frequently used instructions. This reduces the time it takes to fetch by reducing the time lost whilst waiting to retrieve from RAM.
Number of cores
  • Modern CPUs typically have multiple processing cores, each with their own control unit and ALU. They share cache memory and access to other memory.
  • Common types include dual-core (2 cores) and quad-core (4 cores).
  • Multi-core CPUs process multiple instructions simultaneously so can increase CPU performance in compatible programs.
  • A multi-core CPU with lower clock speed may complete tasks faster (e.g. analysing big data sets like weather patterns, or mining cryptocurrency) than a single-core CPU with a higher clock speed.
  • However, an increased number of cores can only benefit programs written to utilise multiple cores simultaneously.

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