uh-man, you are right, the CPU doesn't much care what kind of memory is sitting on the other side of the chipset, that statement is 90% true. It wants low latency and high bandwidth.
latency = time to get the first piece of data requested
bandwidth = data rate in GBytes/sec
There are other memory characteristics such as the ability to keep multiple "pages" of RAM open concurrently which RDRAM excelled at. But recently, to get RDRAM to be less expensive, the multi-page capability of RDRAM was degraded to near-DDR levels. Athlon doesn't work with Rambus' RDRAM not because of the CPU but because none of the chipsets support RDRAM. However, since only 1% of current ram production is RDRAM, Athlon won't be left in the dust anytime soon.
If RDRAM became popular, it would take six months to design a chipset. But it would take two years to convert even half of overall RAM production over to RDRAM.
Its dangerous to only have chipsets supporting one memory technology, but in reality all the new chipsets for DDR SDRAM also support standard SDRAM. It won't be possible to support both on the same motherboard, or very difficult at least.
Some specific comments:
<garden hose analogy>
His whole argument is about bandwidth under various conditions. The big problem with the RDRAM firehose is that it takes too long to turn it on! Furthermore, RDRAM is not like a firehose at all! It only transfers 16 bits at a time, but 4 times as fast. Kind of like a steel 1/2" pipe. The narrow spigot (16 bits) but high data rate leads to problems with overheating and synchronization. A big problem with designed RDRAM modules is that all the path lengths have to be absolutely identical. To use the hose/pipe analogy, designing a narrow pipe to handle a lot of water is not difficult, but the valves are a bitch!
The statement that RDRAM systems can use a single module but DDR systems will need 4 modules is TOTALLY BOGUS. In reality, all Pentium 4 systems have to have EITHER TWO OR FOUR RDRAM modules, but SDRAM systems can have 1,2 or 3 modules. I'm not sure DDR modules need to be used in pairs or not, but they certainly don't need to be installed in quads.
Hope this helped. Any claim you see by RDRAM fanatics should be taken with a big grain of salt, its good you are attempting to check out their claim of imminent doom for AMD.
Petz |
