Ace's Hardware: "RDRAM could make a surprising comeback, thanks to SIS. SIS and Rambus launched today the SISR659, a chipset that is capable of using 4 (16 bit) 1200 MHz RDRAM channels. When you do the math you'll see that those four RDRAM channels can deliver no less than 9.6 GB/s (4 x 2 bytes x 1200) to the chipset. Of course, this awesome amount of bandwidth is going to be bottlenecked by the 800 MHz FSB of the Pentium 4.
To get better performance out of the otherwise wasted bandwidth, a Dynamic Look-ahead Cache and Adaptive Page Management can be found within the R659 chipset. The Dynamic Look-ahead Cache Intelligently prefetches data into an on-chip chipset cache, and lowers the latency of the memory subsystem. Normally, a pagehit on a well-designed DDR 400 controller like the i875p results in a 5 cycle latency: 2 cycles through the controller, 2 cycles for the CAS latency (assuming CAS = 2) and 1 cycle back through the controller. The look ahead cache reduces this to two cycles. If a page hit does not occur, PC1200 RDRAM should have a latency that is very similar to DDR400. Rambus and SIS claim that the latency of the RDRAM channels vary between 10 (cache) and 35 ns, while DDR400 memory latency should vary between 25 and 35 ns. "