Approximately a month ago we took a look at SIS's answer to Intel's 865P, the 655FX. The Dual Channel performance of the 655FX was competitive to the 875P overall. We were impressed with the price/performance the 655FX offered over competing solutions. In our chipset preview we mentioned that the 655FX supported a feature called "Flexible Dual Channel." This feature allows users to "mix and match" memory sizes and odd number of sticks to operate in Dual Channel mode.

For those looking to upgrade from an 845x, 64x, or any other system loaded with three memory sticks or even mismatched sizes, this feature might seem appealing. The question is does the feature actually work or is it a gimmick used for marketing purposes. In today's article we'll be taking a look at the performance differences associated with the amount of DIMM's install on a 655xX board

To market the flexible dual channel feature, SIS has created a little comic featuring their very own "Dr. Gadget" talking to someone who bought a competing Dual Channel board. The comic, though a bit childish and written in Engrish, proves a point. Most end users who walk into stores such as PC Club or Fry's can be easily misinformed or uninformed on Dual Channel.

A common misconception among consumers is the more sticks installed, the better the performance due to an increase in memory. Since the inclination of Dual Channel memory architectures its advised that memory is installed in pairs for optimal performance, though the system will still post with one or three sticks installed, unlike old Rambus solutions.

Uninformed users might install three sticks of memory in a dual channel motherboard thinking their system is running optimally. Since Dual Channel DDR systems still post with an odd number of memory sticks, though the system performs in single channel mode.

For users who started out with 256MB and added more memory later on, issues might arise when upgrading to Dual Channel. Since Dual Channel requires two matched pairs of memory module, users who have an existing 256MB stick and added a 512MB stick later on might run into problems. If the memory sizes aren't matched, the motherboard refuses to operate in Dual Channel mode and reverts back to single channel.

To remedy this, SIS has come up with a feature they call Flexible Dual Channel to allow Dual channel performance with unmatched memory sizes and three sticks of memory.

How Dual Channel DDR works
Dual Channel DDR works by giving the north bridge two independent controllers to access two sets of memory. To put things in simpler terms, think of a stick of DDR as a two way bridge. DDR achieves double the data rate of normal SDR memory by allowing an independent data path for upstream and downstream travel; much like how a bridge or road works. The theoretical maximum amount of bandwidth for a stick of DDR400 is 3.2GB per second. Like a bridge with a speed limit, the amount of throughput achieved varies depending on the type of data it deals with.

With Dual Channel DDR, the data path is doubled once again by using two sticks of DDR memory. By giving the CPU/north bridge another data path to the memory, bandwidth is effectively doubled, giving Dual Channel DDR400 6.4GB/s. In simple terms, think of DCDDR as widening the bridge by adding two more lanes. This effectively allows more data to travel faster; much like traffic moving faster when there're more lanes to drive in. The downside to this is traditional DCDDR solutions require pairs of same sizes.

In more technical terms, DDR memory is 64bit and communicates with the CPU/north bridge at 64bit. Dual Channel DDR can operate in two modes. One mode is it accesses the memory in 128bit mode, which is how most DCDDR chipsets operate. The other mode is it communicates with the two memory controllers in 2x64bit mode, treating each controller individually.

How Flexible Dual Channel DDR works

When mismatched size and/or three DIMM's are installed, the 655FX/TX chipsets operate in a 2x64bit mode, rather than a 128bit mode. This allows dual channel operation with unmatched DIMM's and even three sticks. In easier terms, Flexible Dual Channel works like two independent two lanes bridges. Instead of having a large four lanes bridge, 128bit, a 2x64bit mode gives the memory two lane bridges that allows a variety of different memory configurations.

The question is what kind of performance sacrifices are made in Flexible Dual Channel Mode.

Test Setup
Intel Pentium 4 3 GHz 800 MHz FSB,
SIS 655TX Reference board,
2x256MB Corsair TwinX3200LL,
2x256MB Kingston HyperX3500,
1x512MB Corsair XMS3500,
Sapphiretech Radeon 9700 Pro,
Audiotrak Prodigy 192,
Western Digital 80GB 7200RPM 8MB Cache,
Windows XP Professional SP1,
ATI Catalyst 3.8,
SIS AGP Driver 1.17,
Audiotrak v1.97,

Comments
First off I'd like to thank Corsair and Kingston for providing me with the memory used in this article. For top end performance you can't go wrong with Corsair. For those looking for a memory with an excellent price/performance ratio, Kingston HyperX is an excellent choice. Both memory manufacturers produce excellent ram and you can't go wrong going with either choice. I'd also like to thank my buddy Chris Shock for loaning me the stick of 512MB PC3500 since I only have 256MB sticks.

Moving on I'd like to give props to SIS for producing an excellent performing chipset, the 655 series. We'll be pitting the 655TX against Intel's 875P in an upcoming article, stay tuned. We will also try to get a retail 655FX and 655TX board in as soon as possible.

And this is where I say something about my buddy Ryszard over at Hexus.net, I have no idea why I'm mentioning him but he told me to and held a gun to my head. I'm joking by the way.

Oh and this is our first article utilizing PC Mark 2004. PC Mark 2004 will be gradually replacing our PC Mark 2002 benchmarks though a couple of reviews will still have PC Mark 2002 results. We should have a complete transition before the New Year. Later on we will be implementing Sysmark 2004 when that becomes available. We will also be looking at using Gordian Knot DIVX encoding, feel free to drop me an email with suggestions for additional benchmarks, I value everyone's opinion.

For our DVD9 to DVD5 test the Terminator 2 Extreme DVD was ripped to an ISO with DVD Decrypter to the hard drive to eliminate slow performance due to the DVD-Rom drive. An episode of Futurama from the Futurama Season 2 DVD was used for the MPEG2 Encoding benchmark. Later on we will be adding Family Guy for DIVX encoding.

Also when I mention "Traditional" or "Normal" Dual Channel, I'm referring to Dual Channel with matched sized/speed pair of DIMM's.

Onto more important things the labels for the graphs are as follows:

One = One stick of 256MB Corsair XMS3200LL
Two = Pair of 2x256MB Corsair TwinX3200LL
Three = Pair of 2x256MB Corsair TwinX3200LL + 1x256MB Kingston HyperX3500
Flex = One stick of Corsair XMS3200LL + 1x512MB Corsair XMS3500

Starting with the usual synthetic Sandra tests, SiSoft Sandra's CPU test, the results across different memory configurations are very similar. The slight differences are simply hiccups in the benchmarking program and nothing major. In the synthetic memory test the traditional Dual Channel configuration with matched DIMM's come out on top. When three sticks of memory are inserted, the performance drops behind by 34%. With mismatched size sticks, the 655TX offers 79% of traditional Dual Channel mode. Compared to Single Channel mode, the 655TX is 20-23% faster in Flexible Dual Channel.

In PC Mark 2004's memory test, the performance sacrifices made with three memory sticks decreases to 14%. When different sized DIMM's are inserted, the performance gap narrows to 11%. Whether or not we'll see the same performance in applications is another question.

In Cinebench 2003, the performance across the memory configurations, with the exception of Single Channel, is roughly the same. The time it took to render the image shows a fraction of a second difference. Since the difference is so small, they're nothing more then a mere hiccup in the benchmark. Either way compared to Single Channel, Dual Channel offers a 7% improvement in rendering speed. Though there's only roughly a 5 second difference in rendering time, the time increases as the image gets more complicated.

When it comes to MPEG2 Encoding there's an approximate 5-10 seconds difference in rendering time, with normal Dual Channel mode coming out on top.

For some reason when mismatched size DIMM's are inserted, there's an approximate 1.25 minute difference. After rerunning the benchmark about 3 times the numbers were still the same strangely. When three DIMM's are inserted, it takes about 22 seconds longer to re-encode the DVD. Compared to Single Channel, Flexible Dual Channel provides a good improvement.

When it comes to 3D Mark 2003's CPU test, Flexible Dual Channel is able to keep up with traditional Dual Channel. With three sticks of memory, the performance drops behind by 6% or so.

In 3D Mark 2001SE Build 330, the performance differences across the Dual Channel configurations is roughly 1-3%. Judging by the graph, Flexible Dual Channel is able to keep up with traditional Dual Channel.

When it comes to gaming, the difference between traditional Dual Channel and mismatched sized DIMM's is roughly 2%. With three sticks of memory, the difference increases to 5%. In Enemy Territory, the differences are less then 2%. This shows Flexible Dual Channel is able to keep up with normal Dual Channel when it comes to gaming.

Conclusion
When we first took a look at SIS's 655FX chipset, we neglected to take a look at the Flexible Dual Channel feature. To our regret, we should've examined the feature. After extensively testing the feature, the results were quite impressive. Not only does SIS make the migration to Dual Channel simple, the performance sacrifices are minimal. Users of existing DDR systems who have mismatched DIMM sizes or three sticks of memory should give SIS 655TX and FX solutions a look. As we mentioned in our 655FX preview, SIS's 655 solutions offer performance comparable to competing Intel solutions.

Unlike most marketing gimmicks, Flexible Dual Channel is a feature that works as advertised. The feature is also useful for users looking for a cost effective upgrade to Dual Channel and an 800 MHz FSB processor. When in Flexible Dual Channel mode, the system was completely stable and never crashed at all. With the recent price drops on 800 MHz P4 processors, the 655TX/FX is an attractive platform for users of existing Athlon or P4 processors looking to upgrade to the latest and greatest.

As for availability, 655FX solutions can be found for around $110 while 655TX solutions should start appearing soon. For those interested in how the new 655TX compares to Intel's 875P, stay tuned for our upcoming 655TX.