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.

Nick Josevski

Posted 2009-07-15T09:10:17.457

Reputation: 6 747

3To be more accurate, any multiple of three works well, not just three. – Mehrdad Afshari – 2009-07-15T09:20:22.957

Answers

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.

Brian Knoblauch

Posted 2009-07-15T09:10:17.457

Reputation: 4 313

Ballpark, what are we talking for a performance improvement? – tbone – 2016-05-02T04:32:47.970

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/)

Richard

Posted 2009-07-15T09:10:17.457

Reputation: 8 152

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 is Fiddling by Firelight – 2010-07-14T17:57:16.020

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.

nik

Posted 2009-07-15T09:10:17.457

Reputation: 50 788

Many Dell and other OEM systems list Core i7 9xx systems with 8GB of RAM, and this is only possible in Dual Channel. – Stephen – 2010-01-17T18:47:33.197

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.

kastermester

Posted 2009-07-15T09:10:17.457

Reputation: 494

1@MaciekSawicki: We need to be careful with terminology. Percentages usually make things overly complex, especially relative comparisons. We start at 100%. Doubling something is a 100% boost. Tripling is 200% up. – underscore_d – 2015-09-27T16:09:40.380

1Only 100% boost of dual channel. – Richard – 2009-07-16T12:43:19.043

@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 – 2010-01-17T18:52:32.653

1@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 – 2010-01-18T08:58:20.540

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.

kpoehls

Posted 2009-07-15T09:10:17.457

Reputation: 766

-1

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.

Jim

Posted 2009-07-15T09:10:17.457

Reputation: 1