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The fundamental rate in cycles persecond at which a computer performs its most basic operationssuch as adding two numbers or transfering a value from oneregister to another.The clock rate of a computer is normally determined by thefrequency of a crystal. The original IBM PC, circa 1981,had a clock rate of 4.77 MHz (almost five millioncycles/second). As of 1995, Intel's Pentium chip runs at100 MHz (100 million cycles/second). The clock rate of acomputer is only useful for providing comparisons betweencomputer chips in the same processor family. An IBM PCwith an Intel 486 CPU running at 50 MHz will be abouttwice as fast as one with the same CPU, memory and displayrunning at 25 MHz. However, there are many other factors toconsider when comparing different computers. Clock rateshould not be used when comparing different computers ordifferent processor families. Rather, some benchmark shouldbe used. Clock rate can be very misleading, since the amountof work different computer chips can do in one cycle varies.For example, RISC CPUs tend to have simpler instructionsthan CISC CPUs (but higher clock rates) and pipelinedprocessors execute more than one instruction per cycle.