Mixing Ram?

Discussion in 'iMac' started by alecmcmahon, Jan 8, 2013.

  1. macrumors regular

    Joined:
    May 18, 2007
    Location:
    Woodbridge, NJ
    #1
    I've got a 27" imac on order w/ standard 8 gig of ram ( 2 x 4gb )


    If I purchase a 16gig kit from owc ram ( 2 x8gb ) and the iMac 27" has 4 total ram slots, can I still utilize all ram sticks:

    8gb owc
    8gb owc
    4gb apple
    4gb apple

    total = 24gb ram


    any issues? or do all the sticks have to be " matching "

    Thanks
     
  2. macrumors Westmere

    GGJstudios

    Joined:
    May 16, 2008
    #2
    No, they don't all have to be the same brand or capacity. There is a slight performance benefit in using matched pairs, which you have, but as long as the specs are appropriate, you're fine.

    MR Guide to Matched RAM on Intel Macs
     
  3. macrumors regular

    WrrN

    Joined:
    Oct 11, 2012
    #3
    I did the same with ram, ordered 8 and added 16. I left the original 4's in the slots they were in and added the new 8's to the vacant slots.

    Have kept an eye on the memory in iStat Menus and all seems good. I ordered Crucial btw.
     
  4. macrumors member

    Joined:
    Mar 29, 2012
    Location:
    The Netherlands
    #4
    To get full dual channel performance you have to rearrange the dimms:
    Put the two 4 gig dimms your imac came with in one bank and the new 8 gigs together in the other bank

    System report must look like this:
    Bank 0 4 gig dimm
    Bank 0 4 gig dimm
    Bank 1 8 gig dimm
    Bank 1 8 gig dimm

    Read this thread for more info:
    http://forums.macrumors.com/showthread.php?t=1499124
     
  5. macrumors G3

    Joined:
    Jul 23, 2002
    Location:
    Sol III - Terra
    #5
    I believe this is incorrect. In a dual channel system, one bank goes to each channel. I believe the proper setup would be:
    Bank 0/DIMM 0: 4 GB
    Bank 1/DIMM 0: 4 GB
    Bank 0/DIMM 1: 8 GB
    Bank 1/DIMM 1: 8 GB
     
  6. macrumors member

    Joined:
    Mar 29, 2012
    Location:
    The Netherlands
    #6
    You are wrong, read the other thread!
     
  7. macrumors G3

    Joined:
    Jul 23, 2002
    Location:
    Sol III - Terra
    #7
    Of the various memory related threads, some clearly show what I posted as the way to do it. One of the others had people disagreeing on which was correct.

    There was nothing on Apple's support site I could find to document this one way or the other.
     
  8. roverma, Jan 9, 2013
    Last edited: Jan 9, 2013

    macrumors member

    Joined:
    Mar 29, 2012
    Location:
    The Netherlands
    #8
    No, what you ( and the others you quote for that matter) posted is wrong!!

    Google Intel's dual channel specification or read the Wikipedia article mentioned in the other thread!!

    (It's Intel cpu technology, so searching Apple support site won't give you the answer obviously......)
     
  9. macrumors G3

    Joined:
    Jul 23, 2002
    Location:
    Sol III - Terra
    #9
    What I wish we had was something official from Apple.
     
  10. macrumors regular

    Joined:
    Jul 20, 2011
    #10
    I think Apple would be really dumb sending the stock 8GB not arranged in dual channel.

    So i just added crucial 8GB x 2 in the free slots. Didn't even bother checking the slot/bank thingy, just works great and I'm happy the way it is.

    Apple sends the stock 8GB in DIMM0 (Bank0 + Bank1). Trusting them, i think that's the right way to achieve dual channel.
     
  11. macrumors member

    Joined:
    Mar 29, 2012
    Location:
    The Netherlands
    #11
    Not needed, just check the FACTS, the technology is perfectly explained by Intel
    In various papers , just google it.

    ----------

    Man, read the other thread, you clearly have no clue what dual channel means....
    ( sorry getting a bit annoyed from explaining the obvious)
    Why do you people refuse to read the Intel paper in which dual channel is clearly explained (with samples)
     
  12. roverma, Jan 9, 2013
    Last edited: Jan 9, 2013

    macrumors member

    Joined:
    Mar 29, 2012
    Location:
    The Netherlands
    #12
    Last try

    According INTEL specification:
    In order to perform optimal each channel has to contain dimms with equal amounts of ram
    The stock iMac has a 4 gig dimm in each channel ( bank 0 and bank 1) correct, as long as you don't add dimms with different amount of ram.
    If you add an 8 gig dimm in each bank the dimms per bank are not the same amount (namely 4 and 8)
    In order to obtain optimum dual channel performance you have to put the 4 gig dimms in one channel (bank 0 ) and the two 8 gigs in the other bank/channel
    So you can add two 4 gig dimms in the free slots of an stock iMac, but if you are going to add two 8 gig dimms you have to rearrange: the two 4 gigs in bank 0 and the two 8 gigs in bank 1
     
  13. macrumors regular

    WrrN

    Joined:
    Oct 11, 2012
    #13
    Okay, I'm really trying to wrap my mind around this. I feel like you have contradicted yourself, but I may be lacking mind-power right now. Your last statement (quoted), says that the DIMMs must have equal ram. My SySInfo window shows this:
    [​IMG]
    which leads me to believe that my memory is physically configured correctly -- The DIMMs have equal amounts, not the BANKs having equal amounts.

    Please let me know if my brain is needing a boost.
     
  14. macrumors member

    Joined:
    Jul 15, 2009
    Location:
    Austria
    #14
    I give you a boost: The channels are the banks, so bank 1 needs the same amount of RAM in DIMM (=slot) 0 and 1. The RAM is configured wrong in your iMac because Bank 0 uses one 4GB and one 8GB.

    On a technical level think of the Banks as something like RAID set up. To have this working correctly Intel needs the same sizes per bank.
     
  15. macrumors regular

    WrrN

    Joined:
    Oct 11, 2012
    #15
    Got it, thanks.

    So this raises the question ... why would Apple do it this way at the factory?
     
  16. macrumors newbie

    Joined:
    Dec 28, 2012
    #16
    I have a quick question about 27" iMac Ram

    I ordered min with 16GB.

    Will it be 2x8 GB or 4x4 GB ?

    I want to up it to 32Gb but don't want to order 2x16GB and only end up having to get 2 more.

    Thanks
     
  17. macrumors member

    Joined:
    Mar 29, 2012
    Location:
    The Netherlands
    #17
    Can you read?........
    It clearly says in the apple store that the 16Gb is 2x8
     
  18. macrumors newbie

    Joined:
    Dec 28, 2012
    #18
    Tanks !!
     
  19. macrumors newbie

    Joined:
    Dec 2, 2012
    #19
    This is a very interesting question...
     
  20. macrumors member

    Joined:
    Jan 1, 2013
    Location:
    The Netherlands
    #20
    Ok, I now have:

    Bank 0/DIMM0 : 8GB
    Bank 1/DIMM0 : 4GB
    Bank 0/DIMM1 : 8GB
    Bank 1/DIMM1 : 4GB

    and XBench memory test shows 10% better performance.
    +1 for @roverma

    Still the question remains: why has someone put 2x4GB in different banks?
     
  21. macrumors regular

    WrrN

    Joined:
    Oct 11, 2012
    #21
    When you say "someone," are you referring to a factory assembler/tech?
    This is indeed an interesting question.
    I guess I need to power down and reassign my RAM to new slots.
     
  22. macrumors regular

    Joined:
    Jul 20, 2011
    #22
    that's strange... I tried it yesterday, both ways, xbench + geekbench, perfectly aligned results.
     
  23. roverma, Jan 10, 2013
    Last edited: Jan 11, 2013

    macrumors member

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    Mar 29, 2012
    Location:
    The Netherlands
    #23
    Okay, that 10% difference is to be expected according to Intel papers....
    Apple ships with only 2 modules, one in bank 0 and the other in bank 1
    With only 2 modules it doesn't matter where they put them as long as they are the same amount ram in both banks.....
     
  24. macrumors regular

    Joined:
    Jul 20, 2011
    #24
    this seems somewhat contradicting... one time you say it has to match the amount of ram in paired banks to achieve dual channel (8gb in both banks 1 + 4gb in both banks 0) but here you say that it does not matter, as long as there is the same amount in both different banks (4gb in bank 0 + bank 1, like apple ships them).
     
  25. macrumors member

    Joined:
    Jan 1, 2013
    Location:
    The Netherlands
    #25
    But you see what happens if the user uses the empty slots with different kind of memory. I knew about dual channel and I assumed Apple would put the standard RAM in the same bank. I did not check it!

    BTW: I have Kingston KTA 1600/8GB.
     

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