Re: the price disparity is small enough between an RDRAM and DDR system
But RDRAM wasn't really supposed to be faster (and it isn't) it was designed to be cheaper.
RDRAM was conceived under the premise that pins were very expensive and silicon was very cheap and getting cheaper. Silicon, as expected, is indeed very cheap and getting cheaper fast, but pin costs have seen similar cost reductions, which was not expected.
RDRAM isn't fast memory at all, it's actually slower than SDRAM 133 - what it does is to compress a very wide bus onto relatively few pins. Almost all current Rambus systems use two RDRAM channels which are, on the chip, 128 bits wide, resulting in a 256 bit path to slow memory. In fact, it actually acts more like two interleaved 256 bit busses. The problem for RDRAM is that squeezing 8 bits from each read onto a single pin takes time: the bits must be collected, parity generated, sent, decoded, and parity checked at the other end. This adds a delay for each read that becomes more and more of a problem as chips get faster and a delay of a given length wastes more CPU cycles.
DDR and SDRAM are just plain old memory, but fast memory, that is directly connected to the memory controller. Each time cells are read the data just goes straight to the CPU, so a 64 bit read requires 64 data lines, and their associated pins. But the CPU sees the results of that 64 bit read right away, without waiting for the serialize/deserialize step. A dual channel DDR motherboard needs 128 data paths to the memory controller, which sounds much more expensive than the Rambus approach - but pins and traces on boards have gotten much cheaper, and this has come as something of a surprise to Intel.
It would really be more accurate to say that the DDR / SDRAM approach is the high cost / high performance memory connection - not RDRAM. But that the performance of DDR / SDRAM is good enough so that cheap versions of it can be used where only high end (dual channel) RDRAM is adequate. Single channel 64 bit DDR is outperforming dual channel 256 bit RDRAM in many applications.
Regards,
Dan |
