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Summary

The first wave of DDR chipset-boards has hit the market and caused some concerns in the industry as well as with the consumers as to availablity and compatibility of DDR SDRAM memory modules. We have looked at some of the DIMMs currently available and further analyzed the BIOS parameters that, under certain circumstances, can cause compatibility problems or performance abnormalities. This article is meant to provide some introduction to the present DDR platforms by describing some currently available modules, the technology behind them and the optimal BIOS parameters as a function of memory bus frequency.

The year 2000 has seen the introduction of the first double data rate (DDR) platforms as a means to alleviate the bottleneck constituted by the increasing mismatch between processor speed and memory bus frequency. However, the introduction of the new platform did not sail as smoothly as its creators had hoped for. While there has been a substantial increase in bandwidth and, more importantly, in memory dependent applications, differences in the three major platforms, that is, ALi with the ALiMAGiK1 chipset, AMD with the 761 North Bridge, and VIA with their VT8633 have caused quite a few compatibility problems with the DDR memory modules.

Unfortunately, the situation is not as simple as it used to be in the case of SDRAM where basically every quality DIMM runs on any supporting platform. On the other hand, let's not forget that it wasn't until the introduction of the PC100 standards by Intel that PC100 DIMMs became generally interchangeable between different mainboards. In other words, the current situation is some kind of deja vue of the scenario some 4 years ago.

The chipset and DRAM industry certainly have learned some of the lessons from the past and tried to coordinate the development of DDR discrete chips suitable for bus use (main memory as opposed to direct mapped graphics chips) with the development of suitable substrates in form of PCB gerbers (reference designs) and the efforts of the chipset makers to find commonly acceptable timing settings amongst the different architectures. Nonetheless, one of the main reasons for the delay of DDR as the new industry standard has been the fact that, for quite awhile, DIMMs that would run on the AMD platform wouldn't function correctly on the ALi chipset, not to mention the late-comer VIA. The probably best description of the situation came from Micron Technologies, where, some three months ago, a spokesman of Crucial Technology hinted that out of 10 DDR DIMM designs, about 4 would run on the ALi M1647, and 6 on the AMD 761 North Bridge but that there was basically no overlap.

In the DRAM industry, this has caused certain uncertainties since, until a common modus vivendi was found, there was not much use in speculative development and production of DDR DIMMs. The reason for this caution is that the timing margins are substantially tighter than in SDRAM, making it even necessary to forward a clock signal from the chipset to the DIMMs to ensure optimal accuracy. In other words, it is more complicated than with SDRAM and not just a little bit.

The consequence of this dilemma is, as Eric Chang, Marketing Director of VIA put it, that, all of a sudden, there is an abundance of chipsets and mainboards (at least projected) but there is no memory available on the market. This, of course, causes a major problem for the mainboard industry since, without memory, the best boards cannot run and, without a DDR supply, mainboards cannot be sold. To quote a few facts and numbers:

• Tyan had their first AMD 760-based mainboard ready in September but finally cancelled since there was no market that would even cover the production costs
• Transcend's show-off at Comdex as one of the main DDR supplier of the show was based on 50 DDR chips, enabling them to build 11 DDR DIMMs, that, fortunately enough, worked.

It does sound a bit grimmer than it actually is, there has been some turnaround, the chipsets are better tuned and the DRAM industry has finally gotten into gear. Even though the field is still in rapid motion, there are good DDR DIMMs available from several manufacturers. This is the first part of a series of DDR articles and deals mostly with the technical limitations of DDR DIMMs and compatibility issues with the ALiMAGiK1 and AMD 760 chipset. In addition, some of the crucial BIOS parameters are explained as to what they mean and further to shed some light on the trade-offs between stability and performance.

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