A motherboard operates at a limited number of FSB speeds.
A computer Central Processing Unit is designed for a motherboard with a specific Front Side Bus speed such as 133 MHz. The FSB speed is one of two factors that determine the CPU's clock speed. Although it is possible to adjust a motherboard's FSB speed -- thus changing the speed of the CPU -- doing so can have negative effects.
FSB and Multiplier Speeds
A CPU has an internal multiplier that, along with the motherboard's FSB speed, determines the clock speed at which it runs. For example, the Intel Core i7-860 processor has a multiplier of 21x and is designed to run on a motherboard with an FSB speed of 133 MHz, giving it a clock speed of 2.8 GHz. The clock speed printed on a CPU or its box is not a fixed number; if you adjust the multiplier or FSB speed, the CPU clock speed changes.
Overclocking
Adjusting a motherboard to run at a higher FSB speed than the processor requires is called overclocking. For example, running the Intel Core i7-860 on a motherboard with an FSB speed of 150 MHz results in a CPU clock speed of 3.15 GHz. Overclocking a CPU causes a computer's performance to improve when running applications that depend heavily on CPU speed. Overclocking can also save money; after overclocking, a lower-speed CPU can potentially reach the same performance level as a more expensive CPU from the same product line.
Risks of Overclocking
All of a computer's components use the motherboard's FSB to communicate with one another. Therefore, although the most apparent result of overclocking is increased processor speed, overclocking actually forces all of the computer's components to work harder, causing added stress on the entire machine. The effects of overclocking are difficult to predict; the added heat and stress on the CPU and other components may cause the computer to fail prematurely, or it may not be possible to overclock the CPU at all. In other cases, the computer may operate in a completely normal fashion. Overclocking typically voids a CPU's warranty. It is generally only possible to overclock a computer built from off-the-shelf parts; pre-assembled computers made by manufacturers such as Dell or HP cannot be overclocked.
Underclocking
It is also possible to underclock a processor by lowering the motherboard's FSB speed. Because lower clock speed results in lower heat output, underclocking is useful in a scenario where cooling is difficult -- because the computer is placed in an enclosed environment, for example -- and the full power of the processor is not required.
Locked and Unlocked Multipliers
Changing a motherboard's FSB speed affects all of the computer's components, while changing the CPU's multiplier affects only the CPU. Therefore, attempting to overclock a CPU by changing the multiplier has a much higher chance of success. Unfortunately, most CPUs have locked multipliers that cannot be adjusted. Some premium CPUs made for the enthusiast community have unlocked multipliers.
Tags: clock speed, motherboard speed, computer components, motherboard with, changing multiplier, Core i7-860