What the heck does the front side bus do?

[telon2222]

What the heck does the front side bus do? I undertand that it is the speed at which data flows through the motherboard? I also understand that 800fsb is faster than 500fsb. Are these statements correct? Thank you!

 

[Steve]

Front Side Bus is the path between the CPU and the memory controller.

 

[Concillian]

In the Intel design, the Front Side Bus matches closely with memory speed.

memory ----> bridge chip (w/memory controller) -- FSB --> CPU
CPU -- FSB --> bridge chip (w/memory controller) ----> memory

The bridge chip also communicates with the PCI bus (hard drives, sound card, etc...), network controller, and graphics card. So any of these devices that need to communicate with the CPU will go through the FSB.

The Intel FSB is narrower than the dual channel memory bus... like the memory bus is a 4 lane highway each way, but the FSB is only 2 each direction.Note that 533MHz is really an 'effective' speed much as DDR400 is. a "533MHz FSB" is really only 133MHz, but transmits 4 times per cycle, similarly, an "800MHz FSB" is really 200 Mhz.

In this sense, it is the speed at which "data flows through the motherboard" but only data that needs to go back and forth with the CPU. There is data that doesn't need to go to the CPU, and thus never travels through the FSB.

The vast majority of the information that travels across the FSB is from the memory to the CPU (or the reverse... CPU to memory). 200 MHz FSB (800MHz in market-ese) is capable of 6.4 GB/sec, this matches well with DDR400 in Dual channel, which is 200 MHz DDR, but a wider bus width, and gives 6.4 GB/sec peak (theoretical) throughput.

Because of this relationship, typically FSB will trend with memory speed and while you can run say DDR2 533 with an 800 MHz FSB, you wouldn't necessarily gain anything over running DDR2 400 on the 800MHz FSB. For the most part, DDR 400 or DDR2 400 dual channel matches with "800 MHz FSB", DDR 266 dual channel with "533Mhz FSB" and DDR2 533 dual channel with "1066MHz FSB".

Edit:
If I were to assume that this question was posed because you are faced with a purchase decision and are being given the option of xxx FSB over yyy FSB speed, I would generally NOT pay a premium for a higher FSB unless you were also going with faster memory to match the FSB throughput improvement.