what is front side bus?

In personal computers, front side Bus, FSB is also known as the Processor Bus, Memory Bus, or System Bus and connects CPU and Northbridge. Depending on the processor used, some computers may also have a back-side bus that connects the CPU to the cache. This bus and the cache connected to it are faster than accessing the system memory, RAM via the front-side bus. The FSB speed is an important thing to consider when buying a new Motherboard and Processor. It is a very important factor considering the speed of a computer. The higher the speed of the front side bus, the better is the performance of your computer.
The FSB speed is generally equal to the clock frequency of your processor and can usually be set by either using the system BIOS or with jumpers located on the computer motherboard. For most motherboards, you can set the FSB to any setting to ensure that the FSB is properly set unless you plan to Overclock the computer. And the frequency at which the processor operates is determined by multiplying the FSB with a clock multiplier.

The speed at which the System Memory should operate depends on the FSB speed. Often the memory and the FSB bus must operate at same speed for the better performance, that is, if a CPU having a FSB speed of 400 MHz, the memory which is connected must also have a speed of 400 MHz.

If a bus speed is slow, the processor needs to wait for a longer time for the data from the RAM to be processed and if the data is too large, even more time will be taken by the processor to process it.