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Two DRAM Sides Are Better Than One

In Search of True DDR2 Bleeding Edge Memory

As PC enthusiasts know, many modules of the same size come in both one- and two-sided versions. It's a good idea to buy double-sized modules whenever possible, because they have a built-in speed advantage over their single-sided brethren. In addition, there are now numerous Intel chipsets with DDR2 support under development that will treat both sides of a memory module together in a kind of RAID relationship. The resulting speed advantage is on the order of 2%, and was measured in the THG labs. In fact, using four single-sided memory modules doesn't help, and doesn't produce the same effect. By comparison with double-sided memory modules, measured performance actually decreases when using single-sided modules.

The chipset memory control works with two single-sided modules in dual-channel use.

With two double-sided modules, the Intel chipset can actually run four sides (memory banks) in parallel.

An overview of speed improvements based on chip size and placement:

Sides Speed (CL 5.0-5-5-15) FSB 1066
Sides Read Write Latency
4 (2 Module) 7320 MB/sec 2157 MB/sec 83.9 ns
2 (2 Module) 7225 MB/sec 2118 MB/sec 83.5 ns
4 (4 Module) 7058 MB/sec 2093 MB/sec 89.3 ns
3 (3 Module) 5130 MB/sec 1698 MB/sec 84.2 ns
1 (1 Module) 5123 MB/sec 1672 MB/sec 84.4 ns
Sides Read Write Latency
4 (2 Module) 142.9% 129.0% 99.4%
2 (2 Module) 141.0% 126.7% 98.9%
4 (4 module) 137.8% 125.2% 105.8%
3 (4 module) 100.1% 101.6% 99.8%
1 (1 module) 100% 100% 100%

It was also interesting to observe that double-sided 512 MB modules were slightly thinner although they were produced with 110-nm fab technology. However, this also results in a somewhat lower potential for overclocking. In any case, our measurements show that speed appears to be independent of the number of memory modules paired up on a motherboard. A PC enthusiast can therefore populate a motherboard with either 2 or 4 GB of DRAM, without really impacting performance.

Some DDR2 modules come with heatsinks attached. These only look good, because their memory chips use either 110-nm or 90-nm processes and don't need additional cooling.
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