A modern computer will contain a large number of clocks, but they come in two basic types: calendar and logic.

The calendar clock is the one most people are used to. It indicates the date and the time of day. Granularity of the time measured varies from one second down to perhaps microseconds of precision. A computer system my have only one of these calendar clocks.

The other kind of clock is used to keep the internal logic of the computer synchronized, the logic clock. This is nothing but a regularly repeating pulse. The pulse rate depends on the function of this particular clock and may be in the gigahertz down to the megahertz. The clock circuit in a typical watch runs at 32KHz, much less than a megahertz. A computer system will have many of these logic clocks, possibly hundreds.

Why does a computer have these logic clocks? A computer's functions happen in small descrete time steps. These are the clock cycles. At the end of each clock cycle the data is stored, either to registers or to other places within the device. Breaking the operations into clock cycles allows the designer to ensure the data from operation A is ready before feeding those results to operation B. This means that when you overclock your computer (boost the clock rate above what the designer intended) you risk feeding bad data around inside the computer. The result could be a system crash, and possible loss to your files. It is also possible to smoke your computer.

Logic clocks will be explained more fully in a future lecture about logic.