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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.

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To be more accurate, any multiple of three works well, not just three. –  LeakyCode Jul 15 '09 at 9:20

6 Answers 6

up vote 9 down vote accepted

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.

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Neelham (both Core i7 and Xeon varieties) will run in dual channel memory mode or three channel depends 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/)

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LGA1366 was designed to support the 6 core chips when they came out so it needed a wider memory bus. When the cheaper LGA1156 chips came out Intel stated (IIRC to Anandtech) that with DDR3-1333 two channels were enough to keep the CPU from bottle-necking in normal usage (vs memory benchmarks) but that the 3rd channel was needed to keep the 6 core chips running at full speed. –  Dan Neely Jul 14 '10 at 17:57

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.

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Many Dell and other OEM systems list Core i7 9xx systems with 8GB of RAM, and this is only possible in Dual Channel. –  Stephen Jan 17 '10 at 18:47

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 tripple channel ddr - theoretical (emphasis on theoretical!) 300% boost in speed on the memory Run with any other configuration and the least you will get is single channel DDR - but it should still work, just "may" be slower.

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Only 100% boost of dual channel. –  Richard Jul 16 '09 at 12:43
    
@Richard: if DDR dual channel gives (theoretical) 200% boost compared to single channel and DDR3 gives (theoretical) 300% boost compared to DDR1 then DDR3 gives "only" 50% boost compared to DDR2 –  Maciek Sawicki Jan 17 '10 at 18:52
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@Maciek: DDR1/2/3 is about the generation RAM technology not the number of banks (DIMMs working in parallel). (That said my earlier comment was unclear: 3 banks might, under ideal conditions -- which are not achieved even in performance benchmarks, give 50% better than two banks.) –  Richard Jan 18 '10 at 8:58

Yes, you are correct - you have to have 3. I'm currently using the new core i7 processor with 9G RAM.

You can use less than 3 sticks, however you will not get the tri-channel benefits.

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I received a Dell T5500 with three 2GB sticks in it, from Dell, but my company can only run Windows XP for now, as Windows 7 is not certified for our software.

The system, of course, just shows it has 3.37 GB of RAM, but it doesn't cause any issues.

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