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Basically, they don't make triple-channel kits of memory for my LGA1366 i7-980x processor anymore. But they do make quad-channel kits for the new 2nd generation i7-3930K processor. Is this memory compatible with my older processor? I plan on buying a quad-channel kit and using only 3 sticks. Is there any actual difference between memory based on generation of processor?

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There is no real notion of "memory based on generation of processor". The memory specs are the memory specs, and they're really independent of the processor generation. Some processor generations require new RAM specifications; some don't. For example, the Ivy Bridge mainstream processors -- e.g. i7-3770K, which is not to be confused with Sandy Bridge-E, the 3930K you referred to -- use the same dual channel DDR3 memory that has been in use since at least the Core 2 era (older than your LGA1366).

The real question you should be asking is whether quad-channel memory can run in compatibility mode in a triple-channel configuration.

My understanding is that, overall, memory sticks are not specifically manufactured to run in a specific channel configuration, except, I think, in the case of dual channel. So the general rule is, if you have dual, triple or quad channel memory, it can probably be made to work with any dual, triple, or quad channel motherboard that otherwise supports the timings, clock rate, capacity (amount) and generation (e.g. DDR3) that the particular RAM module has.

However, there are some memory modules that are manufactured with less rigorous specifications / procedures to meet a lower price point. These modules can sometimes fail to work in channel configurations of more channels than they are advertised for. So for example, certain models of Kingston RAM that are specifically "dual channel" will not work on triple or quad channel configurations. However, quad channel modules should always be able to go "down" to triple channel.

From Wikipedia:

When operating in triple-channel mode, memory latency is reduced due to interleaving, meaning that each module is accessed sequentially for smaller bits of data rather than completely filling up one module before accessing the next one. Data is spread amongst the modules in an alternating pattern, potentially tripling available memory bandwidth for the same amount of data, as opposed to storing it all on one module.

(and further down in the quad-channel section):

The architecture can only be used when all four, or a multiple of four, memory modules are identical in capacity and speed, and are placed in quad-channel slots. When two memory modules are installed, the architecture will operate in dual-channel architecture mode. When three memory modules are installed, the architecture will operate in triple-channel architecture mode.

The first paragraph, as I interpret it, tells me that the system will tax the memory modules "harder" in higher channel configurations, which might explain why certain cheap dual channel RAM will not work in triple or quad configurations, yet quad configurations tend to work very well as triple channel. The second paragraph says that, on a quad-channel motherboard, if you plug in four quad-channel memory modules and then remove one (or insert any multiple of 3 sticks that is not divisible by 4), the system operates in triple channel mode. So that tells me that for certain, quad-channel RAM can operate in triple-channel mode.

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The memory is the same, just sold in two- or three- or four-packs. They operate at what the motherboard specifies the should run at. The K2 or K3 at the end of a Kingston KVR part number just says how many are in that kit. Aside from that they are the exact same. You can test this by getting a triple channel system, and a dual channel system, and you can interchange the RAM between them and run a MemTest86. I agree triple channel will be faster, but the memory is the same spec-wise –  Canadian Luke Sep 13 '12 at 14:48
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A quad channel kit is just 4 identical sticks of memory, from the same batch, tested to work together so ya, it should be fine. Just make sure its the right generation of DDR (DDR2/DDR3) and it'll be fine. –  Journeyman Geek Sep 13 '12 at 14:48
    
Amazing answer. I guess the biggest thing was that I was concerned about the XMP built into each one, and if that would cause problems. I don't plan on using XMP, because I want to overclock them manually. If I go this route, I hope I don't have any problems. –  eek142 Sep 13 '12 at 15:13
    
The answer should actually be updated to reflect what actually is true about double, triple, and quad channel memory. Currently that part of the answer is WRONG. –  Ramhound Sep 13 '12 at 16:37
    
@Ramhound, Edit proposals will be reviewed and probably accepted. –  allquixotic Sep 13 '12 at 17:05

There will be no difference between the memory modules in a "quad channel" kit and using only three memory sticks (modules) compared to a "triple channel" kit.

The kit is basically just a group of memory modules that came off the production line at the same time and so should have exactly the same timings and other features as each other module in the kit. It is entirely possible that you could go into a shop, buy three of one type of memory module and be given three sticks that came from the same batch but there's a chance you'd get one or more from different batches.

Having the modules from different batches is not necessarily a problem in itself, the only problem comes if they came from batches before and after a specification change whereby one module might be working to slightly faster timings compared to other modules.

Even then having these difference may not affect the system in a way that you would even notice, it might be down to loosing a couple of MB/s on a benchmark.

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