Twice The Power: SiS655 With Dual DDR333

Wider, Not Faster: Dual Channel DDR Instead Of Speed Increase

It was clear that the dramatic speed increases of the Pentium 4 needed to be accompanied by increases in the FSB speed if the full potential of the faster processors was to be harnessed. It is hardly surprising that Intel is planning to increase the FSB to an impressive 800 MHz. DDR 400 was the natural successor to DDR266 and DDR333, but these double data rate chips running at 200 MHz have not demonstrated any performance advantage over DDR333 in single channel systems.

There is a clear trend in PC hardware towards parallel processes. There is a good reason why small RAID arrays have become so popular with power users, and that is simply because two hard disks are faster than one. Intel is heading in the right direction with its Hyper-Threading technology. Tackling several tasks at the same time is often preferable to carrying out a single task quickly - especially as modern thread-based software is very well placed to handle distribution to more than one processing unit.

The same thinking applies to main memory: with two memory controllers you get double the bandwidth. The 5.4 GB/s (2.7 GB/s per channel) achieved by the SiS655 establishes a new record. However, our benchmarks show that a highly theoretical figure does not automatically translate into top performance in practice.

As You Like It: Concurrent Mode Or Dual Channel Mode

Irrespective of performance, the SiS655 allows the user to choose what kind of memory he wants to use, including the option of using a single module of DDR DIMM. Performance is therefore at the same level as the SiS648.

Fitting several modules is interesting, because activating the dual channel mode requires two or four identical DDR DIMMS, as does Intel's Granite Bay chipset. Here, the SiS655 works in 128 bit mode to achieve the highest possible performance. Nevertheless, if you have different DDR DIMMs you can still in use them; the chipset then works in 64 bit concurrent mode. The SiS website does not give too much away about the various possible combinations. Our test system ran marginally slower with 1 x 256 MB and 1 x 512 MB than it did with 2 x 256 or 1 x 512 MB. This option is therefore only of any interest to those for whom memory size is more important than memory performance.

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