SDRAM vs. RDRAM, Facts and Fantasy

Another contention is that RIMMs have high power consumption. A point worth repeating is that systems from major OEMs like Dell achieve some of the highest performance levels without special cooling. This contradicts the notion that RIMMs have high power dissipation. While it is true that an individual RDRAM can consume more power than an individual SDRAM, at a module level (DIMM versus RIMM), RIMM modules can consume much less power while providing higher bandwidth than equivalent-capacity SDRAM DIMM modules. The reason is straightforward, and relates to how bandwidth is provided by DIMM and RIMM modules. On a DIMM, eight SDRAMs respond to each request to provide the data requested by the CPU, so eight devices in the same rank are all dissipating the same amount of power. On an equivalent RIMM module with eight devices, only one RDRAM is providing data; the other seven devices are in lower-power states. On DIMM modules, power is thus evenly spread among all devices in the same rank. On RIMM modules, power can be localized in one RDRAM, with others at much lower power levels. At maximum module bandwidth the power dissipated by the eight SDRAMs exceeds the total power dissipated by the one active RDRAM and seven other RDRAMs in lower-power states. An additional difference is that SDRAMs dissipate I/O power whether transmitting 1's or 0's, while RDRAMs do so only when transmitting 1's.

Micron Technology recently presented a power analysis at the Platform 2000 conference. In this analysis, they computed maximum and typical power consumption for modules using PC133 SDRAM, DDR SDRAM, and RDRAM. Maximum power consumption is important for determining power delivery constraints and worst-case cooling requirements. This study concludes that PC133 DIMMs dissipate a maximum power of 11.6 Watts while providing 1.1 GB/sec of bandwidth, while DDR DIMMs dissipate 9.1 Watts while providing 2.1 GB/sec. The maximum power RIMMs dissipate is 4.6 Watts, while providing 1.6 GB/sec. These results clearly indicate that RDRAM provides more Bandwidth per Watt than the alternatives.

RIMM modules use an aluminum heatspreader to cover the RDRAMs, and some have argued this is evidence of the high power dissipation of RIMM modules. In reality, the heatspreader has two functions. The first, as its name implies, is to spread heat on the RIMM module when power is localized in one device across the entire surface of the module. The flat surface also indicates that the power dissipated by a RIMM module is not all that high, otherwise the heatspreader would need to look like more traditional heatsinks (i.e. lots of fins to maximize surface area for heat dissipation). The second function of the heatspreader is simply to provide mechanical protection for the RDRAMs during shipping and installation.

Skip To Page 1 Introduction 2 Rambus Direct RDRAM 3 Conventional Memory Systems 4 RDRAM Benefits 5 Reducing System Cost 6 RDRAM Pricing 7 RDRAM Pricing Continued 8 RDRAM Performance 9 RDRAM Performance Continued 10 System Performance • 11 RDRAM Power Consumption 12 Benchmark Applications 13 BAPCo SYSmark 2000 14 Benchmark Setup 15 Benchmark Results Intel 440BX 16 Benchmark Results VIA 694X Apollo Pro 133A 17 Benchmark Results Intel i820 18 Benchmark Evaluation 19 Conclusion

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