Front-Side-Bus (FSB) is a term describing a Processor (CPU)-to-system memory data bus. It has also known as CPU bus speed, external CPU speed, memory bus and system bus. It is the speed that the CPU communicates with RAM (memory). The front side bus on a computer connects the processor to the north side bridge, which comprises the memory bus, PCI bus and AGP bus. In general, a faster frontside bus means higher processing speeds and a faster computer.
For example the Pentium 4 has a “400Mhz Front Side Bus”, but it is in fact 100Mhz ‘quad pumped’. This means that data is transferred twice per clock cycle, on the rising and falling edge (like DDR memory which will be mentioned later), and also transfers two bytes of data at a time to effectively give four times the throughput of a 100Mhz front side bus. 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. Athlon processors are available in multiples of 100MHz, 133Mhz, 266MHz 333MHz and now with the AMD K8 3200+ and AMD Athlon64 FX-51 1600MHz FSB.
Some motherboards offer the ability for the user to manually set the clock multiplier and FSB settings by changing jumpers. CPU and especially the "Big Name" computer manufacturers usually "lock" the preset multiplier setting into the chip or locked in the motherboard BIOS meaning manually-set multiplier settings are ignored in favor of the preset multiplier. 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. 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. CPUs work with front side bus speeds ranging from 133 MHz dual channel (266 MHz effective) to 200 MHz quad channel (800 MHz effective).
