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In each generation of hardware, the rules change. I was under the impression that the last generation of intel processors were using two banks of memory, and that you would get significantly better performance therefore if you had both memory slots populated.

To be specific, in a Dell Vostro 3750 for example, there are two memory slots. Dell sells this laptop with 4Gb in one slot, or 6Gb with two slots, or 8Gb.

Is there significantly less memory bandwidth in the configuration with only one memory module? And in those with 6Gb, do the accesses beyond the 2Gb of one module happen slower?

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up vote 2 down vote accepted

Modern PCs use a multi-channel memory architecture to enable faster access to memory. For instance the Core i3-2xxxM processor used in your Dell has two memory channels.

However, all of the slots must have the same amount of RAM, otherwise only one memory channel can be used. If the second channel was used, it would attempt to access "empty space" where no RAM actually physically exists.

So you can install mismatched sizes of RAM and it will work, but the memory controller will switch to single channel.

In short, yes it will run slower.

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does it run slower for accesses where both banks are available? For a 4Gb and 2Gb module, half the accesses would have two banks. Also how significant is the loss of half the memory bandwidth? On my CPU-intensive code doing array references, I'm sure it would be a lot, but is it noticeable when running desktop applications? –  Dov Oct 22 '12 at 15:13
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