SDRAM vs. RDRAM, Facts and Fantasy

The high per-pin and per-device bandwidths of RDRAMs have several system-level advantages. Providing high bandwidth from a small number of pins means fewer bus traces to route. In addition, the fact that the Rambus channel is routed in parallel means simpler routing and reduced complexity of motherboard designs. Often, simplified routing can result in fewer motherboard layers, reducing manufacturing costs as well.

The high per-device bandwidth of RDRAM, along with the narrow channel, allows memory systems to be composed of a small number of RDRAMs. For example, the Sony PlayStation2 uses just two Rambus channels in parallel, each with a single RDRAM, to achieve a peak memory bandwidth of 3.2 GB/sec. Using conventional memory technology, the same bandwidth requires using 32 8-bit PC100 SDRAMs in parallel. Two unwanted side effects of achieving bandwidth in this manner are that memory capacity and the number of DRAMs in the memory system increase. However, the PlayStation2 does not require high memory capacity, and the increased number of DRAMs increase overall system cost, motherboard size, and routing complexity. The high per-device bandwidth of RDRAMs allows critical system parameters like component cost, memory capacity, memory bandwidth, and motherboard size to be optimized to achieve high performance that is also cost-effective.

The Rambus channel design also has an advantage in minimum upgrade granularity. Conventional memory systems based on SDRAMs, discussed earlier, require that DIMMs have eight 8-bit (or four 16-bit) SDRAMs to span the 64-bit bus. However, because a single RDRAM spans all 16 bits of the data bus and can provide 1.6 GB/sec of bandwidth, RIMMs can have as few as one RDRAM. In fact, RIMMs can have any number of RDRAMs, up to the maximum 32 device capacity of one Rambus channel.

This granularity advantage allows upgrades of RDRAM-based memory systems to be tailored to different segments of the PC market. High-performance PCs have the highest capacity requirements, followed by mainstream desktops and finally value PCs. Price sensitivity is typically reversed for these three categories, with value PCs being the most cost-conscious, followed by mainstream desktops, and finally high-performance PCs. In traditional SDRAM memory systems, the minimum upgrade granularity is eight devices if 8-bit SDRAMs are used, or four devices if 16-bit SDRAMs are used, independent of the market segment addressed. Hence, the minimum upgrade cost is the same for all three market segments. RDRAMs offer a more cost-effective alternative. Since RIMMs can have anywhere from 1 to 32 RDRAMs on them, different RIMM capacities can be created to address different market segments.

As a result, performance PCs and desktops may choose to use RIMMs with 8 or 16 RDRAMs, while value PCs may choose to use four-device RIMMs. The single-device upgrade granularity inherent in the Rambus channel design allows OEMs and system manufacturers to tailor initial system capacity and minimum upgrade cost based on PC market segment and supply and demand constraints. In times of DRAM shortages, this can be especially important, as component costs can rise dramatically based on availability. For good examples of just how volatile DRAM prices can be, recall the dramatic rise in DRAM prices in the Fall of 1999 and the high DRAM prices of the mid 1990's.