A front side bus is the term referring to the bus which connects the processor to the motherboard. For PCs this bus is of great interest, since all of the program code going into the processor, and all of the data going out must pass through this bus. The processor inside runs much more quickly than the FSB.

Of the many reasons the bus is so slow, the major one is that at such high speeds the wiring on the motherboard is too long. The speed of electricity is slightly slower than the speed of light. On a 1GHz computer, light travels less than 30cm per clock cycle. You may notice on your motherboard tracks that are squigly, rather than straight. The designers are essentially slowing down the signal on the line by making a longer path for it to travel (it's more complex than that, but it suffices for this discussion). Added to that is the fact that electricity, like a wave, bounces around on the wire, and the wires act like antennae (both receiving and transmitting) and you have a whole bunch of little details which have to be accounted for when you're speeding up the motherboard.

I was excited by intel's announcement of the timna processor precisely for the reason that if the major motherboard functions and peripherals were included on the same die (or even seperate die on the same chip) then the speeds could go up for the interconnecting system... Not that the timna would be fast itself, but that later chips would ride its coattails. (Yes, I know. The timna was cancelled. Chips such as the timna have failed in the past, but they could do well in the future. Not everyone is after extreme performance)

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