Dear Jim:
How many electronic circuits have you designed? What is your education level? Have you heard of what IBM did with Microchannel? Did you realize that almost no one wanted RDRAM even from the start?
It has been over hyped. It was a solution for embedded electronics. Here you want low granularity, low pin count, and high transfer rate. In the computer world, high bandwidth is usually achieved through parallization. That is why Cray, IBM, and even Intel built massively parallel processor and memory arrays. The memory is interleaved to allow high bandwidth. Alpha (the current processing champ) uses 256 bit wide memory busses using 4 way interleaving of PC-100 memory (in the highest configurations) yielding 12.8GB/sec transfers. It is not unusual to have 16GB memory which takes 512 256Mbit SDRAM chips arranged in DIMMS of 16 chips each in banks of 16 DIMMS to a Memory Subsystem Module.
It was Intel who insisted that RDRAM be used in workstations and PCs. The only other place you see RDRAM used is in embedded applications like video games, set top boxes and the like. At current prices, RDRAM is as expensive as SRAM. And if you have any historical knowledge, you would know that SRAM is faster than any other current memory type. It has an access time of 3.5nsec where your vaunted RDRAM PC800 from Samsung has an access time of 45nsec, PC-133 SDRAM has the same 45nsec access time, and PC-2100 DDRDRAM has an access time of 40nsec. All of these timings are from Samsung, so you can compare. Unless RDRAM prices fall soon, they will not be used.
When any company forces you to pay much more for the same benefit, you will change your business away from it. This happened with IBM pushing Microchannel, Circuit City with Divx, and Sony with Mini Disc. Now it appears to happen with Intel and RDRAM. RDRAM is a square peg that Intel has been trying to fit in everyones round holes of SDRAM in PCs.
If you would check Samsung PDF on RDRAM RIMMs, you would notice that every other pin is either Power or Ground. They use 29 traces in and 29 traces out of each RIMM not including power. SDRAM uses 64 for data (I am not counting ECC traces in either case), and 22 for control and addressing. That is a savings of only 28 traces. Also not included were SPDs and other trace types. RDRAM does not save that many pins. Too bad!
All in all, RDRAM has too low of a Benefit given its costs. At $560 per 128MB, SRAM would be cheaper (it usually costs about 3x DRAM cost but has higher performance) and better.
Pete |
