The clock rate of a computer is normally determined by the

frequency of a crystal. The original IBM PC, circa 1981,

had a clock rate of 4.77 MHz (almost five million

cycles/second). As of 1995, Intel's Pentium chip runs at

100 MHz (100 million cycles/second). The clock rate of a

computer is only useful for providing comparisons between

computer chips in the same processor family. An IBM PC with

an Intel 486 CPU running at 50 MHz will be about twice as

fast as one with the same CPU, memory and display running at

25 MHz. However, there are many other factors to consider

when comparing different computers. Clock rate should not be

used when comparing different computers or different processor

families. Rather, some benchmark should be used. Clock

rate can be very misleading, since the amount of work

different computer chips can do in one cycle varies. For

example, RISC CPUs tend to have simpler instructions than

CISC CPUs (but higher clock rates) and pipelined

processors execute more than one instruction per cycle.