Clock rate
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The clock rate is the fundamental rate in cycles per second, measured in hertz, at which a computer performs its most basic operations such as adding two numbers or transferring a value from one processor register to another. Different chips on the motherboard may have different clock rates. Usually when referring to a computer, the term "clock rate" is used to refer to the speed of the CPU.
The clock rate of a CPU is normally determined by the frequency of an oscillator crystal. The first commercial PC, the Altair (by MITS) used an Intel 8080 CPU with a clock rate of 2MHz. The original IBM PC, circa 1981, had a clock rate of 4.77 MHz (4,770,000 cycles/second). In 1995, Intel's Pentium chip ran at 100 MHz (100 million cycles/second), and in 2002, an Intel Pentium 4 model was introduced as the first CPU with a clock rate of 3 GHz (three billion 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 the speeds of entire computers, like the clock rate of the computer's front side bus, the clock rate of the RAM, the width in bits of the CPU's bus, and the amount of Level 1, Level 2, and Level 3 cache.
Clock rate should not be used when comparing different computers or different processor families. Rather, some software 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 superscalar processors can execute more than one instruction per cycle.
History
In the early 1990s, most computer companies advertised their computers' speed chiefly by referring to their CPUs' clock rates. This led to various marketing games, such as Apple Computer's decision to create and market the Power Macintosh 8100/110 with a clock rate of 110 MHz, so that Apple could advertise that its computer had the fastest clock speed available -- the fastest Intel processor available at the time ran at 100 MHz. This superiority in clock speed, however, was meaningless; the PowerPC and Pentium CPU architectures were completely different. The Power Mac was faster at some tasks, but slower at others.
In the 2000s, Intel's competitor AMD started using model numbers instead of clock rates to market its CPUs, saying the "megahertz myth" did not tell the whole story of the power of its CPUs. In 2004 Intel announced it would do the same, probably because of consumer confusion over its Pentium M mobile CPU, which reportedly ran at about half the clock rate of the roughly equivalent Pentium 4 CPU.
This article is based on material taken from the Free On-line Dictionary of Computing prior to 1 November 2008 and incorporated under the "relicensing" terms of the GFDL, version 1.3 or later.