Resolved iMac 27" (5K, Late 2015) correct RAM configuration

Discussion in 'iMac' started by Edgecliff, Oct 23, 2016.

  1. Edgecliff macrumors newbie

    Joined:
    Apr 7, 2013
    Location:
    The Netherlands
    #1
    This is discussed many times before, but without definitive answer.
    What is the correct way to arrange 16GB of additional memory?

    By default, the iMac has got 8GB of memory installed in a 4/0/4/0 way, or:
    BANK0/DIMM0 4GB
    BANK0/DIMM1 Empty slot
    BANK1/DIMM0 4GB
    BANK1/DIMM1 Empty slot​

    Now, some people advice to add 16GB in the empty slots, making it 4/8/4/8, or:
    BANK0/DIMM0 4GB
    BANK0/DIMM1 8GB
    BANK1/DIMM0 4GB
    BANK1/DIMM1 8GB​

    Others say the banks must have equal memory in their DIMMs, making it 4/4/8/8, or:
    BANK0/DIMM0 4GB
    BANK0/DIMM1 4GB
    BANK1/DIMM0 8GB
    BANK1/DIMM1 8GB​

    Does the memory have to be equal in the banks or in the DIMMs to run in dual channel mode?

    Also, some say it's better to have the higher capacities first, making it 4 possible combinations:
    4/8/4/8
    4/4/8/8
    8/4/8/4
    8/8/4/4​

    Which of these is the best configuration? And why?
     
  2. redheeler macrumors 603

    redheeler

    Joined:
    Oct 17, 2014
    #2
    Arrangement doesn't affect performance as long as the RAM is installed in matched pairs, which the factory RAM should be. Easiest to just use the empty slots.
     
  3. Edgecliff thread starter macrumors newbie

    Joined:
    Apr 7, 2013
    Location:
    The Netherlands
    #3
    So you're saying slot 1 matches 3 and slot 2 matches 4, making a 4/8/4/8 configuration preferable?

    Anyone who can confirm this?
     
  4. loekf macrumors 6502

    loekf

    Joined:
    Mar 23, 2015
    Location:
    Nijmegen, The Netherlands
    #4
    Should be:

    BANK0/DIMM0 4GB
    BANK0/DIMM1 8GB
    BANK1/DIMM0 4GB
    BANK1/DIMM1 8GB

    A DIMM in a slot in bank 0 ... should have the same DIMM in bank 1, same slot. Otherwise dual-channel
    can't work. In order words... dual channel means the memory controllers accesses data in two banks
    at the same time. So not to have timing issues, you should the same DIMMs.
     
  5. Edgecliff, Oct 24, 2016
    Last edited: Oct 24, 2016

    Edgecliff thread starter macrumors newbie

    Joined:
    Apr 7, 2013
    Location:
    The Netherlands
    #5
    This is exactly what I was looking for, thanks!

    From another thread:
    Combined with the input from loekf, I assume 8/4/8/4 is the way to go then. Correct me if I'm wrong.
     

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