Clock Speed
The clock speed of a CPU is defined as the frequency that a processor executes instructions or that data is processed. This clock speed is measured in millions of cycles per second or megahertz (MHz). The clock itself is actually a quartz crystal that vibrates at a certain frequency when electricity is passed through it. Each vibration sends out a pulse or beat, like a metronome, to each component that's synchronized with it.

By now, you've probably got a handle on the incomprehensible amount of data and information that is flowing around inside your computer. There are many different devices and components, each sending and receiving millions of bytes of information every second. Imagine the havoc if each of these components just sent their information where ever and whenever they wanted, or all at once. There has to be some sort of organization and cooperation between the different devices. That's where these little quartz crystals come in. Each device synchronized with the clock performs each instruction, transmits or receives data, transfers, reads, processes, etc. on each beat of the clock (clock cycle).

Early computers had a single system clock. The expansion bus and the CPU both ran at the same speed. It didn't take long before CPU technology had advanced to where they were faster than the expansion bus. This caused a real bottleneck. The CPU could process and send information faster than the expansion bus could possibly transfer it. The problem was to get each device to run at its own separate speed and still be in synch. What to do?

Well, first off, you give the expansion bus its own clock. Take a hint from music, and make that clock speed a sub-multiple of the system clock. In other words, if the expansion bus clock is running at 8 MHz, then the system clock would run at 16, or 32 etc. Of course, these are arbitrary numbers, the actual values can be pretty strange (like 14.31818 MHz) but you get the idea. Now, have the bus send a signal to the rest of the system that says, "don't send me anymore info until I'm through with this"! When the bus can handle more data, it sends a signal asking for more.

To begin with, the expansion bus was not really given a separate clock. The circuits and technology on the motherboard could split the clock signal or multiply it as needed by certain components. These circuits are called 'frequency dividers' or 'frequency multipliers'. Now, the memory system in your computer can be controlled by a 100 MHz clock. A frequency divider is used to reduce the speed by 1/3 to accommodate the 33 MHz PCI bus, and another divider to reduce the speed by 12 for the 8 MHz ISA bus. The CPU can have a frequency multiplier that will increase that 100 MHz speed to 500 MHz or 600 MHz for internal operations. So these devices are still maintaining synch even though they are running at different speeds.

Modern computer technology does allow for some asynchronous operation. Some motherboards now have separate clocks for the expansion bus, system bus, video, etc.. Although these devices run asynchronously, their individual clocks will still run at multiples or sub-multiples of each other which allows for the optimization of transfer rates and data exchange.