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