In a computer, clock speed refers to the number of pulses per second generated by an oscillator that sets the tempo for the processor. Clock speed is usually measured in MHz (megahertz, or millions of pulses per second) or GHz (gigahertz, or billions of pulses per second). Today's personal computers run at a clock speed in the hundreds of megahertz and some exceed one gigahertz. The clock speed is determined by a quartz-crystal circuit, similar to those used in radio communications equipment.
Computer clock speed has been roughly doubling every year. The Intel 8088, common in computers around the year 1980, ran at 4.77 MHz. The 1 GHz mark was passed in the year 2000.
Clock speed is one measure of computer "power," but it is not always directly proportional to the performance level. If you double the speed of the clock, leaving all other hardware unchanged, you will not necessarily double the processing speed. The type of microprocessor, the bus architecture, and the nature of the instruction set all make a difference. In some applications, the amount of random access memory (RAM) is important, too.
Some processors execute only one instruction per clock pulse. More advanced processors can perform more than one instruction per clock pulse. The latter type of processor will work faster at a given clock speed than the former type. Similarly, a computer with a 32-bit bus will work faster at a given clock speed than a computer with a 16-bit bus. For these reasons, there is no simplistic, universal relation among clock speed, "bus speed," and millions of instructions per second (MIPS).
Excessive clock speed can be detrimental to the operation of a computer. As the clock speed in a computer rises without upgrades in any of the other components, a point will be reached beyond which a further increase in frequency will render the processor unstable. Some computer users deliberately increase the clock speed, hoping this alone will result in a proportional improvement in performance, and are disappointed when things don't work out that way.