Articles: Memory

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The IT industry sees few revolutions for very simple reasons. The market is oriented onto effectiveness rather than performance while most break-throughs are expensive and at first usually don't bring even the same performance that the existing previous generation high-end solutions provide. Anyway, although the effectiveness and price-to-performance ratio of a technology may be the guiding factor for the market, the market also vitally needs going forward. That's why the industry is constantly making advances into new fields, if they are not unacceptably expensive.

The last few years of memory market development may be a good example. The natural on-flow of DRAM industry was interrupted by sudden emergence of Intel & Rambus factor. We won't dwell upon this story now: its details are well known to everyone (see our article called Rambus: the Story of One Company). Let's recall the outcome: the expensive, proprietary standard, which could boast no effectiveness, because had been developed by a single company, was rejected by the market in favor of DDR SDRAM. DDR SDRAM defeated RDRAM in an absolutely fair game (at least, on the DDR's part). So now there is a question: what standard should succeed it?

Considering the above said things, there could be no doubt about the choice the market is going to make. The market needed something that would be faster than DDR, but at the same time most compatible with it in the already built infrastructure, so that the transition would be most smooth and costless. In this case we have got a name that is showing the gist: DDR II. JEDEC (to be more exact, the Future DRAM Task Group formed by its JC-42 committee dealing with DRAM) started developing it as far back as April 1998 when there was not even DDR in the PC market. By now, all the major DRAM manufacturers have already rolled out their DDR II chips and the group includes over a hundred companies, whose interests range from CPU making to producing test equipment for memory modules.

This process is quite contrary to the process of RDRAM development, as it is as open as possible. The working group is open not only to JEDEC's members, and the standard itself was strictly positioned as open and free from the very beginning. Of course, it means lower prime cost and larger production volumes.

The keystones of the new specification are very logical. The main thing is not the memory as it is, but system (CPU and chipset) requirements to the memory. Of course, there are some advances made to reduce the cost and power consumption. Of course, they will also try to meet the desired requirements, such as: to make it backward compatible with DDR, make sure that there are no special, non-standard interfaces supported. And of course, they will do their best to improve the performance by, say increasing the bus efficiency.

In a separate line goes the encouragement to take all the best that is already here: like draft materials on SLDRAM, SRAM and so on. And this is not just wishful thinking. Back in the beginning of 1999, SLDRAM Inc. announced that it wound up its work on SLDRAM (by that time the research and development had consumed up to $4 billion) and would put its efforts (and all the results it had already got) into DDR II.

The first question on the agenda is the notorious backward compatibility of DDR II with DDR. If it had not been for that, there would have been no need to make up all the mess: they might have been better off developing some brand-new specification. But in many respects this backward compatibility will only be a thing for the engineers: the DIMM module changes its appearance again. The 184-pin DDR module will be replaced with a 232-pin DDR II.

On the whole, if we look into details, we will find that there has been plenty of work done for keeping present infrastructure alive. The controller's command set hasn't been replaced, but expanded, so that one and the same memory controller would be able to work with both DDR and DDR II. We have the same principle of data packets transfer along the data bus, the same four-bank structure and the same memory page size. We should admit that the industry seems to shun the word "revolution" after the Rambus affair.

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