Do I need to have 3 RAM DIMMs to use DDR3, or even be able to run a 3 channel CPU?

The Core i7 CPUs from Intel (Bloomfield in the Nehalem line) are an example of a CPU that handle 3 channels of memory input, and on an appropriate motherboard, ideally making use of 3 DIMMs the best option.

But lets say I need to diagnose a problem, or I spent all my money on the CPU and can only afford two, 2Gb DIMMs, or I just want to run only 2 DIMMs (for another reason), will it work?

Or will the CPU or other DIMMs have an issue in such a configuration?

Note: I don't have this hardware yet, so "try it" isn't an option.

Solution 1:

No, 3 DIMMs are not required for the system to function (unless you've got some weird mobo that for some reason requires them). It's just preferred for performance reasons, so that the the system can run triple-channel mode.

Solution 2:

Nehalem (in both of its Core i7 and Xeon varieties) will run in dual-channel memory mode or triple-channel, depending on which DIMM slots are populated.

Checking the manual for my new motherboard, it will support 2, 3, 4, or 6 DIMMs installed. But they do have to be installed in specific slots for each different count of DIMMs.

(I've seen a review of memory for i7 which found little benefit from triple channel operation — but this is likely to be very dependent on the (benchmark) application in use: http://www.reghardware.co.uk/2009/07/01/review_memory_for_intel_core_i7_cpu/)

Solution 3:

Short answer, YES, you need 3 DDR3 'sticks' together.
However, you should check this Intel page describing Triple Channel mode.

If only two of the blue memory connectors are populated with matched DIMMs, dual channel memory is enabled.

Check if your board can handle that.

Solution 4:

As far as I have understood, this is just like the old dual channel DDR, meaning:

Run with any multiple of 3 (identical) memory blocks and they go triple channel ddr - theoretical (emphasis on theoretical!) 200% boost in speed on the memory, i.e. 3x the speed

Run with any other configuration and the least you will get is single channel DDR - but it should still work, just "may" be slower.