I'm confused... DDR Ram and conventional SDRam on AMD systems

[Pakman]

Ok, I'm kinda confused on this whole DDR Ram thing. I always thought AMD Athlon systems were running at double the bus speed where data gets sent on the top and bottom cycle?...Can someone please explain?

 

[Noriaki]

There are two busses.

CPU connects to the North Bridge of the motherboard. This is called the Front Side Bus, and on Athlons/Durons runs at 100Mhz DDR or 133 in a few newer Athlons (200 or 266Mhz effective).

The North Bridge then connects to the Memory on a totally different bus.
In the AMD750 chipset this is 100Mhz bus. In Via's KX133, KT133 or KT133A this is either 100 or 133Mhz bus. In the AMD760 and ALi Magik1 this is a 100 or 133Mhz DDR Bus, just like the CPU's front side bus.

In a lot of cases (especially on the P2/P3/Celeron side) the FSB and Memory Bus run at the same speeds, but they don't have to, they are two different busses.

 

[Pakman]

OIC, cool thanx for the info... But now that leads me to think it's pretty pointless for the FSB to be running at double the speed if the bus leading to the memory is still running at 100mhz... I guess if you have a chipset that allows you to run the memory bus at 133mhz it would benefit a little. What was the whole purpose of the FSB running at 200mhz if you really couldn't take advantage of it?