Clock Speed & CPU Performance

Key Terms

  • Clock speed / clock rate
  • Hertz (Hz) – cycles per second
  • Fetch–Decode–Execute (FDE) cycle
  • Cores, Cache (other CPU performance factors)

Clock Speed Fundamentals

  • Every CPU has an internal clock that “ticks” at a fixed rate.
  • With each tick, the CPU completes one FDE cycle.
  • Clock speed = number of FDE cycles per second\text{number of FDE cycles per second}.

Measuring Clock Speed

  • Unit: Hz\text{Hz} (cycles · s−1^{-1}).
  • Common prefixes:
    • 1 MHz=106 Hz1\,\text{MHz} = 10^{6}\,\text{Hz} (~1 million cycles/s)
    • 1 GHz=109 Hz1\,\text{GHz} = 10^{9}\,\text{Hz} (~1 billion cycles/s)
  • Examples:
    • 1.2 GHz⇒1.2×1091.2\,\text{GHz} \Rightarrow 1.2\times10^{9} cycles/s.
    • 2.4 MHz⇒2.4×1062.4\,\text{MHz} \Rightarrow 2.4\times10^{6} cycles/s.

Impact on CPU Performance

  • Higher clock speed ⇒ more FDE cycles each second ⇒ more instructions executed per second.
  • Comparison example:
    • 2.3 GHz2.3\,\text{GHz} CPU ≈ 2.32.3 billion instructions/s.
    • 1.6 GHz1.6\,\text{GHz} CPU ≈ 1.61.6 billion instructions/s.
    • Difference ≈ 0.70.7 billion instructions/s faster for the 2.3 GHz2.3\,\text{GHz} chip.

Related CPU Performance Factors (besides clock speed)

  • Cache size – faster on-chip memory reduces fetch time.
  • Number of cores – multiple cores allow parallel execution of instruction streams.
  • These factors + clock speed collectively determine overall CPU performance.