From jc-news.com
Info about Rambus DRAM:
People seem to be a little confused about the MHz info, so I did a little checking. The reason why the world isn't all drooling over 400MHz DRDRAM (Rambus) is because each cycle DRDRAM transmits a fourth as much information as with SDRAM or DDR SDRAM. Rambus is 16-bits wide, and SDRAM is 64-bits wide (not 32...EDO DRAM is 32-bits). So, basically, 400MHz RDRAM is equivalent to 100MHz SDRAM.
This means that if you had 400MHz RDRAM, you could only run 100MHz bus, and there's a question as to whether or not this would actually be *slower* than the equivalent SDRAMs, because of latency issues (taking longer to return a request).
However, it may happify you to know that there is no 400MHz DRDRAM. It's currently spec'd for 600MHz to 800MHz (I think 600, 700, 800). This means that it can support 150, 175, and 200MHz equivalent SDRAM speeds. 600MHz DRDRAM can be used for the 133MHz bus speed. Unfortunately, yields are really, really bad, and it looks like DRDRAM for 133MHz bus will not be ready until very late in the year. The higher speeds will not become really available until some unknown time next year.
DDR SDRAM is the major competitor, and at the recent Computex in Taiwan it utterly destroyed Rambus in terms of support. Why? It's faster, cheaper, easier to update to from regular SDRAM because of similar technology, and it's not controlled by a single, monopolistic company.
DDR SDRAM runs at regular SDRAM clocks of today, but delivers twice the data by popping out two bursts of bits each clock instead of one. 100MHz DDR SDRAM is equivalent to 200MHz SDRAM or 800MHz DRDRAM (Rambus). Currently, DDR SDRAM is yielding as high as 155MHz, so it can hit speeds of up to the equivalent of 310MHz SDRAM, while Rambus will be able to hit equivalent of 150MHz SDRAM by the end of the year. This is an *astounding* difference.
The Comparison:
"(name):
(clock) x (width) x (burst-per-tick) = (datarate)"
PC100 SDRAM = 100MHz x 64b x 1 = 6.4gbps
PC133 SDRAM = 133MHz x 64b x 1 = 8.5gbps
PC266 DDR SDRAM = 133MHz x 64b x 2 = 17gbps
"310" DDR SDRAM = 155MHz x 64b x 2 = 19.8gbps
600MHz DRDRAM = 600MHz x 16b x 1 = 9.6gbps
I put 310 in quotes because while it is the upper limiter for DDR at the moment, the official spec will be PC266. PC266 will be ratified probably before 600MHz DRDRAMs start coming out in volume, and it's possible that we'll see "333" or "350" DDR SDRAMs popping out from places like Micron or Verite by then (late this year). DDR is a fairly proven "technology" in that it is already being produced and it is going to be used for graphics cards and such.
In short: Rambus is a really horrible performer that costs a lot more and has far less availability than DDR SDRAM. Plus, DDR is so close technically to SDRAM (compared to Rambus) that it is much easier for manufacturers to make the switch to DDR than it is to move over to an entirely new memory platform (that happens to be really crappy).
-JC
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