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Direct Rambus DRAM (DRDRAM)

One of the two main competing standards to replace SDRAM is called Direct Rambus DRAM or DRDRAM (formerly called just "Rambus DRAM" or "RDRAM"). Unlike DDR SDRAM or SLDRAM, which are evolutionary designs based on regular SDRAM, DRDRAM is revolutionary design. It has received a lot of attention because of Intel's decision to pursue this technology for use in its future chipsets, in cooperation with its initial developer, a company unsurprisingly named Rambus.

DRDRAM works more like an internal bus than a conventional memory subsystem. It is based around what is called the Direct Rambus Channel, a high-speed 16-bit bus running at a clock rate of 400 MHz. As with DDR SDRAM, transfers are accomplished on the rising and falling edges of the clock, yielding an effective theoretical bandwidth of approximately 1.6 Gbytes/second. This is an entirely different approach to the way memory is currently accessed over a wide 64-bit memory bus. It may seem counterproductive to narrow the channel since that reduces bandwidth, however the channel is then capable of running at much higher speeds than would be possible if the bus were wide. As with SDRAM, DRDRAM makes use of a serial presence detect (SPD) chip to tell the motherboard certain characteristics of the DRDRAM module when the system is booted. DRDRAM is proprietary, and is being designed to use a special type of module called a Rambus Inline Memory Module, or RIMM.

Rambus memory may become the next standard for future PCs, but the jury is still out. As with all new technology competitions, often marketing wins out over engineering. There is some concern that DRDRAM may not even be the best solution for systems in the future. In particular, some folks are unhappy about the prospects of having to pay licensing fees to Intel and Rambus to use the technology; recall that this requirement was one reason why the MCA bus standard died. Furthermore, some say that SLDRAM is a solution that is less revolutionary, providing the same (or more) improvements in performance with fewer radical changes required to the system architecture. Meanwhile, Intel is proceeding with plans to use the technology, so we will have to see what happens in 1999 and beyond.

Next: Synchronous-Link DRAM (SLDRAM)

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