Optimal Memory Layout for Late 2015 iMac I purchased an additional 2 8GB ram sticks for my iMac.  My intent was to upgrade the original 8GB (from Apple) to a total of 24GB.  

When I opened the back, I noticed Apple had installed a 4GB slot in the 1st &amp; 3rd slot.  I thought that odd and would have expected it in the 1st &amp; 2nd.  I googled around to see where I should add my additional 8GB cards, but didn't come up with anything.  So, I installed in the 2nd &amp; 4th slot.  

Here's the current configuration:



Just to confirm, this is a poor setup?  

I should have the 4s in the same bank together &amp; the 8s in the same bank together?  I.e., 4GB in B0/D0, 4GB in B1/D1, 8GB in B1/D0, and 8GB in B1/D1?
I recently bought RAM from a thirdparty vendor and faced the same issue/question. Well, initially not (for a day or so) but after I wanted to play some games in Windoes 10 on bootcamp (Battlefield1 and Dragon Age Inquisition, both using the Frostbite Engine) I faced some issues. Both games crashed to the desktop after 5-20 minutes of gameplay. This did not happen before I added the two new RAM modules and I changed nothing else.

I had 2x8GB original RAM installed in slot 1&amp;3 and I placed the new modules into the slots 2&amp;4. After I noticed the crashes I searched the internet and also found the apple instructions mentioned (https://support.apple.com/en-us/HT201191). But the guide doesn't mention anything about the correct slot usage.

Contacted the vendor and they told me that it would be best to use slot 2&amp;4 for the larger RAM modules and 1&amp;3 for lower modules. But of course all my modules were 8 GB - so equal size.

Searching the internet I found some sites suggesting that memory modules of the same type should be in the same bank.

I thought to give it a try and placed the original modules in slots 1&amp;2 (bank 0) and the new modules in slots 3&amp;4 (bank 1). After that change the issues with the game crashes were gone.

I know, its not a scientific answer with hard facts but I would still suggest this layout. Cannot tell if this is a good layout for dual channel mode though.