The hertz (Hz) as a unit is defined as the number of cycles per second, making 1 megahertz (MHz) one million cycles per second, while 1 gigahertz is 10^9 hertz. Vibrations and electromagnetic radiation are measured in hertz units, but in computing it is the clock speed of the central processing unit that is referenced in terms of hertz.
Hertz (MHz, GHz)
In the broadest terminology, a CPU or processor is the element of a computer that carries out received programming. The CPU is usually an integrated circuit called a microprocessor which carries out the functions of retrieving, decoding, carrying out, and writing back (providing further instruction). This process is one cycle, measured in terms of the computer's clock speed.
A crystal oscillator, a vibrating metal crystal which is another circuit in a computer, provides a signal (clock signal) which determines the computer's clock speed (or rate), that is, the speed at which a CPU completes one cycle. Thus, a CPU at 1 GHz is completing 1 billion cycles per second (carrying out 1 billion programming instructions). In computing, it is very important that this signal remain precisely constant.
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Because of variances in the ways in which different machines and processors execute operations, it is not always possible for the casual user to tell ahead of time which type of machine and processor will perform a task the most quickly. This means that using the clock speed (Hz measurement) of a system will not always indicate how quickly that system will perform a task in relationship to other systems.