DDR Dual Channel

Most of today’s mainstream chipsets are using some form of dual channel to supply processors with bandwidth. The nForce and nForce2 are, at this time, the only two chipsets to supply dual-channel goodness for the Athlon XP. The original nForce was not on the same performance and stability level as the competitor VIA’s chipset was, but the new and improved dual-channel DDR400 nForce2 has been a smash success — in fact, is today’s de facto choice for performance-minded / overclocker AMD desktop buyers. VIA is now about to release a Dual Channel chipset for the Athlon XP/Duron family called the KT880.

Take note that the memory isn’t dual channel, the platform is. In fact there is no such thing as dual channel memory. Rather, it is most often a memory interface composed of two (or more) normal memory modules coordinated by the chipset on the motherboard, or in the case of the AMD64 processors, coordinated by the integrated memory controller. But for the sake of simplicity, we refer to DDR dual channel architecture as dual channel memory.

The nforce2 platform has two 64 bit memory controllers (which are independent of each other) instead of just a single controller like other chipsets. These two controllers are able to access “two channels” of memory simultaneously. The two channels, together, handle memory operations more efficiently than one module by utilizing the bandwidth of two modules (or more) combined. By combining DDR400 (PC3200) with dual memory controllers, the nForce2 could offer up to 6.4 GB/sec of bandwidth in theory.

However, this extra bandwidth produced by dual channel cannot be fully utilitized by the Athlon XP and Duron family (K7) of processors. Data(bandwidth) will reach these processors no sooner than the system bus (FSB) allows them, and the processor therefore cannot derive an advantage from memory operating faster than DDR266 when operating on a 133/266Mhz FSB, DDR333 with a 166/333Mhz FSB or DDR400 at 200/400Mhz FSB even in single channel mode. Visualize a four lane highway, symbolizing your Dual Channel configuration. As you go along the highway you come up to a bridge that is only 2 lanes wide. That bridge is the restriction posed by the dual-pumped AMD FSB. Only two lanes of traffic may pass through the bridge at any one time. That’s the way it is, with the K7 processors and Dual Channel chipsets.

In case you’re wondering, the K in K7 stands for Kryptonite later changed to Krypton to avoid copyright infringement. Yes, that very same fictional element from comic books that could bring the otherwise all-powerful Superman (Intel ) to his knees. Intel’s P4 architecture, in contrast, is designed to exploit the increased bandwidth afforded by dual channel memory architectures. The 64-bit Quad Pumped Bus of the modern Pentium 4 CPU working at 800MHz, in theory, requires 6.4GB/s of bandwidth. This is the exact match of the bandwidth produced by the Intel i875 (Canterwood) and i865 (Springdale) chipset families. The quad pumped P4 FSB seemed like drastic overkill in the days of single channel SDR memory, but is paying handsome dividends in today’s climate of dual channel DDR memory subsystems. This is one lasting and productive legacy of Intel’s RDRAM efforts. As implemented on the P4 RDRAM was also dual channel architecture, and mandated the quad-pumped FSB for its extra bandwidth to be exploited. This factor continues to serve the P4 well in the dual channel DDR era we are currently in, and allows P4’s greater memory performance than all other PC platforms, save the new AMD Athlon64 FX with all its new bells and whistles.

The Athlon 64 FX processor has a fully integrated DDR Dual Channel memory controller providing a 128-bit wide path to memory and therefore eliminating the need for a Dual Channel interface on the motherboard which traditionally was always located in the Northbridge. The old term front-side bus has always represented the speed at which the processor moves memory traffic and other data traffic to and from the chipset. Since the AMD64 processors has the memory controller located on the processor die, that memory subsystem traffic no longer has to go through the chipset for CPU-to-memory transfer. Therefore, the old term “front-side bus” does no good as it is not applicable anymore. With AMD64 processors, the CPU and memory controller interface with each other at full CPU core frequency. The speed at which the processor and chipset communicate is now dependent on the chipset’s HyperTransport spec, running at speeds of up to 1600 MHz. Although the P4 (800fsb variety) and the A64 FX 940 pins, both share the same theoretical peak memory bandwidth of 6.4GB/sec, the Athlon FX realizes significantly more throughput due mainly to it’s integrated memory controller which drastically reduces latency. Even so, it still suffers from the required use of registered modules which are slower than regular modules. The upcoming Athlon 64 / A64 FX processors designed for Socket 939 will be free from this major drawback and will also feature Dual Channel memory controllers. One negative, though, of having the memory controller integrated into the processor is that to support emerging memory technologies, like DDR-2 for example, the controller has to be redesigned and the processor needs to be replaced.

The Slot

• If you’re using a single module, it’s best practice to use the first slot. If using two or more modules in a non-dual channel motherboard, populate the first slot and use any other slots you wish. Q: I’ve had my single module installed in slot 2 for the last few months now, should I change it? No, it’s also best practice to keep on using the slot(s) you’re been using before. If you replace RAM, then insert the new modules, in the same slots the older ones were in before.You may find the system overclocks better with the ram in a different slot. It is very hard to predict when this effect occurs, as well as which one might work best. In the overclocking game he who tries the most things wins, and if you are running an overclocked configuration that is asking a lot of the ram it is a good idea to try all available slots to make sure the one you are using yields the best results.
• If you’re using two or more modules of unequal size, you will get the best performance if you put the largest module(s) (in megabytes) in the lowest-numbered slot(s). For example, if your system currently has 256MB of memory and you want to add 512MB, it would be best to put the 512MB module into slot 0 and the 256MB module into slot 1.

Using Dual Channel

Dual Channel requires at least two modules for operation. It is recommended that the modules you use be of the same size, speed, arrangement etc. Dual Channel is optional on the original nforce2 motherboards and nforce2 ultra400. You can choose to run in single channel mode on these motherboards. Nforce2 400 boards are singe-channel only. Most dual channel capable nforce2 motherboards come with three slots. On these motherboards the first memory controller controls only the first slot (or the slot by itself), while the second memory controller controls the last two slots (which are usually closer together). Name them slots 1, 2 & 3 respectively. To implement Dual Channel, it is necessary to occupy the slot 1 (channel 0) and either one of the two slots that are closer together, slots 2 or 3 (channel 1). The entire config would be running in 128 bit mode.

You can use three modules in Dual Channel Mode, by filling the third unoccupied slot. With three sticks, slots 1 remains as channel 0 while slot 2&3 become channel 1. To maintain 128-bit mode, with all three slots filled, each channel must have an equal amount of memory. For example, slots 1 should be filled with a 512 Mb module, while slots 2 & 3 are populated 256 Mb modules. If you were to use three modules of the same size, then only the first two modules would be running in 128 bit Dual Channel Mode. Example, using 3x 256 Mb modules will have the first 512 Mb running in 128 bit Dual Channel mode, while the remaining 256 Mb will be in 64-bit Single Channel mode.

Intel dual-channel systems are different. The have either two or four slots, and to run dual channel mode must have either one or two pairs of (hopefully) matching modules. Running three modules on a P4 system will force it to run in single channel mode, and is therefore to be avoided.

Consult your motherboard manual for instruction on exactly which slots exactly to use.