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Front side bus

Front-Side-Bus (FSB) is an Intel term describing a CPU-to-system memory data bus. It has been formerly known as CPU bus speed, '\external CPU speed, memory bus and system bus', too. It is the speed at which the CPU communicates with RAM (memory).

Many system components - including the PCI and AGP buses - usually run at speeds derived from the frontside bus's speed. In general, a faster frontside bus means higher processing speeds and a faster computer, for a number of resons which are outlined below.

How fast your processor runs at is determined by applying a clock multiplier to the frontside bus speed. For example, a processor running at 550MHz might be using a 100 MHz FSB; this means there is a clock multiplier setting of 5.5, thus the CPU is set to run at 5.5 times the MHz speed of the front side bus: basically equating to 100 MHz x 5.5 = 550 MHz.

Most motherboards offer the ability for the user to manually set the clock multiplier and FSB settings by changing jumpers. Although many CPU manufacturers now usually "lock" an unchangable preset multiplier setting into the chip, meaning manually-set multiplier settings are ignored in favour of the preset multiplier. It is possible to unlock some locked CPUs (namely those from AMD) through a complicated process of connecting electrical currents across points on the CPU's surface.

For some processors, the FSB speed can be increased to boost processing speed (called overclocking). This overclocking can take different forms; such as overclocking the front side bus higher than the motherboard was designed to go, or overclocking the front side bus for the purposes of overclocking a (usually, locked) CPU.

The PCI and AGP buses, which usually run much slower than the frontside bus, use dividers to reduce the clock speed. Typically the PCI bus runs at 33 MHz, and the AGP bus runs at twice the PCI bus's speed. The speed at which the PCI bus is set at is almost always a division of the front side bus's speed: eg, a 166 MHz front side bus will mean the PCI bus is set to run at 1/5th of the speed of the FSB. Usually a motherboard will have FSB speed increments at which these PCI dividers are designed with: typically 66 MHz, 100 MHz, 133 MHz, 166 MHz and 200 MHz; meaning dividers of one half, one third, one forth and so on, to go with the standard bus speeds. When a person overclocks or just increases the front side bus speed away from one of the normal speed increments, the PCI bus (and AGP bus + the hard drive controller) will therefore be overclocked too: eg if Joe increases the front side bus from 100 MHz to 105 MHz, the PCI bus will then be at 35 MHz and the AGP at 70 MHz), because it is still retaining the ratio meant for the 100 MHz bus. Changing the FSB all the way to 133 MHz will swap over to a 1/4th ratio for the PCI bus, and therefore it will be similarly overclocked if the FSB is moved to 140 MHz.

It is technically possible for the PCI and AGP buses to run at speeds not derived from the front side bus's clock. However, in practice, few motherboards have ever supported asynchronous PCI. Examples do include some socket 7 motherboards which supported an 83 MHZ FSB for clock-doubled 166 MHz Cyrix 6x86s, but wished to be able to run the PCI at a safe speed; 41 MHz was, and still is, beyond the ability of many PCI components to cope with.

When choosing a FSB speed for the CPU you chose, be aware that you'll need to purchase memory capable of this faster speed. Pushing the front-side bus to 110 MHz means you are also pushing your memory, be it PC100 (or higher, i.e. PC133), to 110 MHz. Some PCI devices (such as sound cards) won't handle the PCI bus going too far above the default speed. Sometimes hard-drive controllers will behave incorrectly in such environments too.

In a typical image processing application where the data set (acquired images) is large, FSB speed becomes a major performance issue. A slow FSB will cause the CPU to spend significant amounts of time waiting for data to arrive from system memory.

One issue of confusion is the labeling of front side bus speeds. Typically a FSB today is dual or quad channel, meaning a FSB speed advertised as being "333 MHz" may actually be 166 MHz dual channel, effectively meaning 333 MHz of speed. Nowadays, PC (X86) CPUs work with front side bus speeds ranging from 133 MHz dual channel (266 MHz effective) to 200 MHz quad channel (800 MHz effective).

The back side bus goes to the L2 RAM cache and has always been faster than the front side bus which goes to main RAM and the rest of the system.

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