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I understand that DDR ram is basically sending the data on the falling and rising ends of the clock wave, but what is Dual Channel Memory. Google is just saying that it doubles the speed.

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up vote 4 down vote accepted

Dual channel memory doubles the throughput by talking to two memory modules at the same time. Instead of having to read 128 bytes from one module, you read 64 bytes from each of two modules at the same time and accomplish the same amount of data being read in less time.

Note that, unlike DDR memory, dual channel memory is not a type of memory. You can't go down to the store and buy some dual channel memory. It's a system capability that requires a memory controller with two channels each of which must be connected to at least one memory module. Many motherboards have other requirements -- for example, usually each channel must be populated with the same amount of memory.

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Correct me if im wrong, but basically the memory controller has 2 channels and each channel is connected to one dimm? vs if there was only one channel, the memory controller would have to alternate between the two>? – agz Jan 29 '13 at 23:53
Essentially, yes. If you have more than one DIMM connected to a single channel, then the memory controller can only operate on one DIMM at a time. (And often it needs an extra command cycle to select which DIMM it's accessing.) – David Schwartz Jan 29 '13 at 23:57
One more come do they have to have the same size and timing? Is it because they are accessed at the same time? – agz Jan 30 '13 at 2:48
They have to have the same size and timing because the memory controller actually interleaves the memory across the two channels (first chunk on channel one, second on channel two, third on one, fourth on two, and so on). If they were different sizes, interleaving would be impossible. They must be the same timing because the memory controller issues operations to the channels simultaneously. To read the first, say, four chunks of memory, the controller issues "read your first chunk" to both channels at once. Then "read your next chunk" to both at once. Now it has read four chunks. – David Schwartz Jan 30 '13 at 2:51
Ok great, thanks for your responses!, they were really informative. – agz Jan 30 '13 at 3:00

Then google is wrong. (or you misinterpreted).

Dual channel is a term used when a memory controller (read: North bridge of the chipset or the CPU) can use two independent memory channels. That theoretically doubles the memory bandwidth.

Not because it is twice as fast, but because two can be used simultaneously.

Analogy: You can transport twice as much good per time with two cars, but the cars do not get twice the top speed.

In praxis this means that your computer runs about 5% faster because it spends less time waiting for memory accesses.

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