The system clock
All the circuits in the
CPU are synchronized by the system clock.
The clock is an input
to all sequential circuits; it changes from 1 to 0 and back again at regular
intervals (the clock cycle), and controls when the state of a circuit
Modern processors typically
complete instructions (i.e., fetch-execute cycles) at the rate of 1 per
clock cycle. In fact, many use multiple instruction ``pathways'' or
``pipelines'' in an attempt to complete more than one instruction per
The clock rate is measured
in "MHz" (megahertz), where 1 MHz is 1 million cycles per second.
Example. A 100
Mhz processor has a clock cycle time of 10 nanoseconds. (Do you see why?)
Last Updated 2001/10/10
L.Heath, 2000, extensively modified by J.A.N. Lee, 2001.