System clock

A system clock is used to synchronise all internal components, the clock outputs a steady high-frequency signal.

• A fixed number of clock cycles are required to carry each data movement or computational operation.

• Clock frequency

  The clock frequency, measured in Hertz, determines the speed with which all operations are carried out. Clock cycle time is the reciprocal of frequency.

  $$t_c=\frac{1}{f_c}$$

  A $800\text{ MHz}$ clock has a cycle time of $t_c=1.25\text{ ns}$.

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• The CPU time required to run a program is given by the general performance equation: $$T=\frac{\text{seconds}}{\text{program}}=\frac{\text{instructions}}{\text{program}}\times \frac{\text{avg. cycles}}{\text{instruction}}\times \frac{\text{seconds}}{\text{cycle}}$$ We can improve CPU throughput if we reduce:
  1. Number of instructions per program.
  2. Number of cycles per instruction.
  3. Number of seconds per clock cycle.