iMac memory upgrade - which slots need to match?

[Gatnus]

I have a late 2012 27" iMac that came with 8GB of memory in 2 4GB cards. The existing memory was in the 1st and 3rd slot (from bottom). I picked up 2 8GB memory cards to install and put the existing two 4GB cards in slot 1-2 and the new 8GB cards in slots 3-4. Is this correct or should the 4GB cards have remained in slots 1,3 with the 8GB cards going in slots 2, 4 to have them matched?

Here's what the memory looks like after my install:

[/URL][/IMG]

 

[SaSaSushi]

Matching DIMMs should be in alternating (1st/3rd and 2nd/4th) slots for full dual channel. You can confirm this in the System Report > Memory.

 

[drambuie]

Swap one of the 4GB modules with one of the 8GB modules so that both DIMMs in each row are the same so that the sizes are interleaved per SaSaSushi's post. This will provide slightly faster memory bandwidth for the full 24GB, but will not be noticed in use unless you use large memory intensive applications.

With your present configuration addresses from 0GB to 8GB will operate in dual channel mode, then from 8GB to 12GB will be single channel mode. This pattern will repeat in addresses from 12GB to 24GB, so most of your RAM will still operate in dual channel mode.

 

[Gatnus]

Thanks all - I swapped them so the 4GBs are in 0/2 and the 8GBs are in 1/3. It appears that 24GB memory is more than I need so I won't be upgrading to 32GB any time soon.

 

[iF34R]

Is this correct: