One DDR channel requires at least 180 pins. One RDRAM channel requires about 80 pins, I think. No, pins do not come for free. It's one of the reasons why multiple RDRAM channels can be grouped together rather easily. In contrast, an upcoming chipset supporting dual Athlons and (I believe) dual DDR channels will require over 1,000 pins on the north bridge. (The 840 north bridge requires half the number of pins.)
I suspect the cost of the additional module is much more significant than the cost of the controller pins. It may cost next to nothing to add a second Rambus channel to a north bridge, but the additional memory module PCB, assembly, test, etc. does cost something. I repeat myself, but Rambus would occupy a much more attractive design point if it offered a fast AND wide module/channel. Fast and narrow (Rambus as it is) drives up the cost without adding performance. [Of course, fast and narrow is cool for many NON-PC applications...I'm talking only about PCs here]
Granuality is still an issue with DDR. To go with less than eight chips per DIMM, DDR will require higher pin counts per chip, and that isn't trivial. With Rambus, the minimum number of chips per channel is one. Of course, this isn't much of an issue with current memory densities, but at those densities increase, the granularity will become a bigger issue.
I don't know where you got 8 chips - that should be 4 (x16 chips, which fit in the standard package). |
