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Clock Rate

The clock rate of a CPU refers to the frequency at which the CPU is running. It uses the SI unit Hertz.

All actions of the processor are linked to a clock cycle; the shorter the period of the pulse - the higher the frequency of the chip. The use of the word "speed" (physical movement), should not be confused with frequency or its corresponding clock rate. Thus, the term "clock speed" or "processor speed" is a misnomer... and shouldn't be used.

The clock rate of a CPU is most useful for providing comparisons between CPUs in the same family. The clock rate is only one of several factors that can influence performance when comparing processors in different families.

For example, an IBM PC with an Intel 80486 CPU running at 50 MHz will be about twice as fast (internally only) as one with the same CPU and memory running at 25 MHz, while the same will not be true for MIPS R4000 running at the same clock rate as the two are different processors that implement different architectures and microarchitectures.

There are many factors to consider when comparing the performance of CPUs:

  • the clock rate and width of the CPU's data bus,
  • the latency of the memory, and the
  • cache architecture.

Clock rates alone should not be used when comparing different CPUs families. Software benchmarks are more useful. Clock rates can sometimes be misleading since the amount of work different CPUs can do in one cycle varies.

For example, superscalar processors can execute more than one instruction per cycle (on average), yet it is not uncommon for them to do "less" in a clock cycle. In addition, subscalar CPUs or use of parallelism can also affect the performance of the computer regardless of clock rate.

extracted from Wikipedia (May 2012)