Dual-channel memory

by Parm Mann on 3 August 2006, 00:00

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Dual-channel RAM

The amount of RAM (Random Access Memory) in a system is an important factor in its overall performance. So to is that RAM’s speed and latency. Whether the RAM is operating in single- or dual-channel mode is also important. So what is dual-channel?

Dual-channel technology depends on several factors, so in this HEXUS.help guide we’ll explore what those are and how they work together.

The Technology

The RAM in most modern PC systems is either DDR or DDR2. As you might expect, DDR2 supersedes DDR and the two technologies are in some ways similar; something we’ve explored in another HEXUS.help guide.

Irrespective of whether the RAM is DDR or DDR2, it connects to the system’s memory controller by a 64-bit wide bus or channel. In current Intel-based systems, this memory controller resides on the ‘northbridge’ of the chipset on the motherboard. In AMD-based systems, the processor contains the memory controller.

With just one memory channel, it doesn’t matter how much memory you put on it, there’s still a limit to the rate at which data can be transferred.

However, what if there were two memory channels? The RAM itself is still only 64-bits wide and can only interface with one channel, but there’s now two 64-bit memory channels… a dual channel configuration. To make use of this feature, two RAM modules will be required.

Two RAM modules, each with their own channel… dual-channel RAM. It’s like RAID-0 for memory. Simple! Well, there are still a few things to consider.

You’ve probably seen ‘dual-channel kits’ of memory available. These consist of two modules of the same RAM. In many cases, the system’s memory controller can only operate in dual-channel mode when both channels have the same RAM in them. Sometimes you can get away with same-sized modules running at the same speed, but to play it safe the same modules should be used.

If a motherboard has more than two DIMM slots (say, four) then where does the RAM go? The motherboard’s manual, or colour coding, should indicate which slots are used for which channel and how to install the RAM for a dual-channel configuration. Dual-channel with four DIMMs is often possible too, but overall operating speeds can drop as a result.

Just as motherboards have varying support for certain memory modules, dual-channel support varies too. It’s often useful to check a motherboard manufacturer’s website to see what compatibility is like before making a RAM purchase.

So, once the dual-channel configuration is up and running, what are the benefits? In theory, double the bandwidth that was available before. Parallel solutions like dual-channel doesn’t always translate directly to double the performance, however, so the benefits will vary depending on the circumstances.

The Market

All modern enthusiast motherboards and even mid-range boards should support dual-channel memory. AMD’s Socket 754 processors (for their budget CPUs) are not dual-channel capable. Socket 939 processors are.

Dual-channel memory kits are readily available, although buying two of the same RAM module will often be as good, even though the modules won’t have been verified as dual-channel capable together.

Dual-channel technology exists for both DDR and DDR2 RAM and there’s little sign of (and no point in) reverting back to a single-channel architecture. It’s commonplace now, compared to just three or so years ago.

The Players

AMD is responsible for dual channel support on the memory controllers of its CPUs. Chipset manufacturers have a role to play too, particularly for Intel systems, so that includes ATI, NVIDIA, SiS and VIA. Then of course, there are the motherboard manufacturers, of which there are too many to list.

As for the RAM itself, some companies provide special kits, as we’ve already mentioned. These include Corsair with their TwinX range, Mushkin, OCZ, Kingston and many more. A lot of the modules will be derived from the same memory chips, but they may be rated to run at different speeds, configured or tested differently.

As with any purchase, making the right decision is important if you want the best performance, so seek the advice of those on hardware forums if you have any doubts.

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