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I have an Asus Maximus VIII Ranger motherboard, which has four RAM slots. The slot configuration is same as in this image:

https://www-ssl.intel.com/content/dam/support/us/en/images/motherboards/desktop/sb/img/singlewith1.jpg

Currently I have installed one stick of 8 GB capacity and 3000 MHz speed in the DIMM 1 slot and I'm looking to upgrade my RAM, and want to make use of dual channel configuration.

While researching about dual channel behaviour, I read this https://www-ssl.intel.com/content/www/us/en/support/boards-and-kits/000005657.html#dual and got a little confused when I saw the second image (Dual-channel with three DIMMs).

Is it that the DIMMs in slots 1 and 2 are operating in dual channel mode and the 4 GB DIMM in slot 3 is operating in single channel mode, or is it that DIMMs 1 and 2 are operating together with DIMM 3 in dual channel mode? In other words, is it [2 GB] matching with [2 GB] and the [4 GB] stick could have been of any other capacity, or is dual channel working here because it's [2 GB + 2GB] matching with [4 GB]?

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  • I don't know how much this applies anymore... RAM used to come as single-sided (1GB, 4GB, 16GB, 64GB modules) and dual sided (2GB, 8GB, 32GB modules), with a lot of luck you would run dual channel memory mode with 3 dimms, 1 dual-sided and 2 single-sided dimm, provided the BIOS would treat the two SS dimms as a single DS dimm, it worked on some boards with some RAM, therefore allowing dual channel mode to work with the one DS dimm module and the two SS dimms acting like a single DS dimm. It doesn't work on every board or with every RAM stick, it is hit and miss. Does that make sense?
    – acejavelin
    Jul 24, 2016 at 18:11
  • Aaah... So you're saying the configuration in the second image would run dual channel only if the motherboard treated the two (proably single-sided) 2 GB modules as one single (dual-sided) 4 GB module, right? Jul 25, 2016 at 5:53
  • Sort of... But I don't believe two 2-sided modules can be treated as a single unit... A single 8GB and two 4GB sticks might work, or a 2GB and two 1GB sticks, but not a 4GB stick and two 2GB dimms. 256M, 1GB, 4GB, 16GB, and 64GB modules are single sided or single density, 512M, 2GB, 8GB, and 32GB are all double sided or dual density. They will all work together for the most part, but only certain configurations will work in dual channel mode. It is highly recommended to only matching dimms be used for dual channel applications
    – acejavelin
    Jul 25, 2016 at 22:00
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    Okay, got it. Thank you! As for my upgrade, I guess I'll be better off buying a fresh kit of 2*8GB RAM. If my existing stick works with these, great. Else, I'll just see if I can sell it. Jul 27, 2016 at 16:47
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    BTW, the chances the performance gains will be noticeable to the user from dual-channel vs single-channel memory configuration are slim... benchmarks will show a slight increase in performance but the real world differences are difficult to be seen. I suggest reading this article and looking at the benchmarks, depending on what your doing single-channel memory can even be faster. gamersnexus.net/guides/…
    – acejavelin
    Jul 27, 2016 at 21:23

2 Answers 2

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DIMM slots of the same colour indicate different channels. In the figure, DIMM1 and DIMM3 are on channel A, whereas DIMM2 and DIMM4 are on channel B. In order to make use of dual-channel operation, both channels should have a RAM module with matching capacity, hence the manual suggests installing two matching-capacity modules into DIMM1 and DIMM2. (DIMM1 and DIMM2, as well as DIMM3 and DIMM4, seem to be paired, so you cannot install matching size modules into DIMM1 and DIMM4, for instance, despite them technically being on different channels as well.) Extra, unpaired RAM modules will run in single-channel mode, no matter their capacity. In the example, that means 4 GiB dual channel RAM from the 2+2 configuration plus 4 GiB of single channel RAM from the single 4 GiB RAM module.

In other words, the 4 GiB size of the other module is just for illustration that the sizes need not match in that case. That 2 GiB + 2 GiB = 4 GiB is a coincidence. The following figure also shows a 2+2 & 4+4 configuration in dual channel, illustrating that the paired slots will continue to run in dual channel mode as long as the capacities across channels match (thus it's 6 GiB per channel in dual-channel configuration).

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Slots 1 and 2 which both have 2gb will be dual-channel and the 4gb one in slot 3 will be single-channel. I suggest getting another 4gb matching DIMM for slot 4 so you are completely dual channel.

It's all about the lines between memory and CPU. Each slot pairs with 32 pins out of 64 register. If you don't fill both slots in the pair you are using only half the lines, thus not utilizing the full throughput between CPU and memory. You literally cut throughput in half with single-channel.

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