In a personal computer, a "bus" is a pathway for transferring information from one component, such as the CPU or memory, to another. The bus speed refers to the frequency with which data is passed from one component to another.

The most common use of the term "bus speed" refers to the "front-side bus" which connects the CPU to the chipset, which manages communication between memory (handled by the "Northbridge" portion of the chipset) and other components (keyboard, mouse, I/O devices, which are handled through the "Southbridge" portion of the chipset). Northbridge also manages the connection to today's high-speed, high-bandwidth video cards. Northbridge and Southbridge are specifically designed to manage their particular areas of expertise to minimize the amount of data that has to be passed over the busy front-side bus to the CPU.

Computer performance is very dependant on the amount of information that can be passed over the front side bus. Anything your computer does basically depends on your CPU getting the information it needs to make computations, and then being able to pass back out instructions to the components to let you see the results. Because both input and output must travel over this same pathway, the front side bus has the potential to throttle down your entire system, by starving the CPU for data or restricting the amount of information the CPU can pass back out as output.

Strictly speaking, "front side bus speed" is only the frequency, usually in megahertz (one megahertz = 1 million cycles per second). Frequency is only part of the equation, though, when it comes to passing information back and forth from the CPU to the chipset the "width" of the bus is also critical. A modern bus, running at 266Mhz, with a width of 64 bits, is able to pass over 17 billion bits of information per second. Finally, different CPU's can manage different numbers of transfers per cycle. Some modern CPU's can perform as many as 4 transfers per cycle; with the bus frequency and width used in our example, that results in a total bandwidth of over 8 GB/s:

266,000,000 x 64 x 4 = 68,096,000,000 bits/s.
A byte = 8 bits, so 68 Gb/s divided by 8 = approx. 8 GB/s

When looking at bus speed, it is important to look at the total bandwidth, not just the frequency, in order to really understand how well a given front side bus will be able to support the operation of your CPU.

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