Front Side Bus
FSB is the path between the CPU and the motherboard chipset (north bridge chip, to more specific). It is also known as ”external bus“.
Usually this term appears when the CPU external clock is mentioned. For example ”100 MHz FSB“ means ”100 MHz external clock“. All CPUs from 486DX2 on use a scheme called clock multiplication, where the CPU internal clock is higher than its external clock (i.e. FSB clock). For example, a 3.2 GHz Pentium 4 works internally at 3.2 GHz but externally it runs at 200 MHz (i.e. its FSB is of 200 MHz).
Notice that Intel CPUs like Pentium 4 transfer four data per clock cycle (see QDR term). Because of that you can read that Pentium 4 FSB is of 400 MHz, 533 MHz or 800 MHz, while in fact it works at 100 MHz, 133 MHz or 200 MHz, respectively.
The same occurs with Athlon and Duron processors from AMD, which work transferring two data per clock cycle (see DDR term). Because of that you can read that Athlon FSB is of 200 MHz, 266 MHz, 333 MHz or 400 MHz, while in fact it works at 100 MHz, 133 MHz, 166 MHz or 200 MHz, respectively.
To make our texts easier to understand here on Hardware Secrets we do not use the term FSB but ”external bus“ or ”external clock“.