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  DDR and RDRAM, an Ongoing Battle 
  Mar 14, 2002, 08:00am EST 


By: Sander Sassen

DDR is a valid solution for the desktop, anybody looking for a workstation platform however should definitely look towards Intelís i850 or i860 chipset and RDRAM. The various DDR chipsets weíve tested all seem to have matured since we did our initial round of testing over two months ago and the production version of these motherboards indeed have improved. Unfortunately they have yet to reach the same level of stability that the Intel i845D and i850 chipsets have to offer. None of the non-Intel chipsets was able to match performance with stability, even worse, they all seemed to sacrifice stability in order to gain performance. Total maximum memory size is a concern too, all of the DDR motherboards we tested have problems when more than two banks of DDR memory are used. A RDRAM chipset based motherboard does not have this problem and will accomodate up to 2GB of memory running at full speed.

As for stability, for someone at home typing out a quick email, playing a game or simply browsing the internet, an occasional lock-up, freeze or crash might be all right. However, for a PC that is used in a business environment, where time is money and efficiency is key, this is definitely not acceptable. Clearly DDR has potential as a low-cost solution for home-PCs when the non-Intel-chipsets are used, as the motherboards are somewhat cheaper. But from an end-userís perspective Iíd rather pay a little extra and get the added stability of a quality chipset rather than trying to cut a few corners and end up with a computer that occasionally crashes.

DDR SDRAM sockets

Fig 3. The 64-bit signaling interface on a DDR motherboard, the routing of these >64 traces and the termination can cause problems with the signal integrity at high speeds.

I mentioned it in the introduction already and Iíll mention it again here. The technical difficulties of maintaining signal integrity over a 64-bits parallel architecture such as DDR are hard to overcome. Furthermore the need to use DIMM sockets and the requirement to be compatible with a wide variety of DDR DIMMs available on the market, demands a lot from a design. The problems and instabilities weíve seen with these DDR motherboards for the Pentium 4 clearly show that maintaining signal integrity on a mass-produced product such as a PC-motherboard is tough hurdle to overcome. Intel choose wisely not to implement more than two DIMM sockets and a maximum memory size of 1GB, simply because they knew that that was within the design limits.

To summarize our findings we can safely say that from a price-performance point of view only Intelís i845D chipset and SiSí 645 chipset, both outfitted with no more than two DIMMs, are an affordable and overall stable solution. Anybody looking for a system that provides the best performance, stability and scalability, both in terms of memory bandwidth and maximum memory size, should look towards the i850 chipset and Rambusí RDRAM. Pricing of RDRAM, which used to be a concern, is no longer an issue as RDRAM modules are priced similar to DDR modules.

Sander Sassen.

1. Introduction
2. RDRAM, PC-1066 and PC-1200
3. DDR, PC2100 and more
4. Benchmarks and more
5. Conclusion

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