Different FSB's

[IsaacInsoll]

Hi, I was just wondering why the original computers up until a few years ago always had "true" FSBs of 66,100,133 etc.

Why does intel now have a "quad-pumped" 100mhz FSB instead of a real 400 mhz fsb? and same with the athlon/duron, or more specifically DDR, why not just make 266mhz RAM rather than 133x2.

Even if i do get an answer, what would be the difference between a 400mhz FSB and a quad pumped 100mhz?

Thanks

 

[AndyHui]

The FSB is quad pumped because of the difficulty in getting other components the cope with the full speed.

Using the Pentium 4 as an example, the other components apart from the CPU will still run at 100MHz, with dividers from there. RAM, PCI bus, AGP bus, other motherboard components all need to cope...

Designing a true 400MHz FSB board is not an easy thing to do. Motherboard design will be expensive as timings will be very tight, and you will need to minimise interference and cross talk issues. You also have to take into account clock skew, and there is very little margin with traces as well.

In short, expensive and difficult to maintain signal integrity.

In terms of the difference between a QDR 100MHz bus and a true 400MHz FSB, the difference is close to negligible at the moment.