Pairing iMac RAM?

Discussion in 'iMac' started by Mightee, Jul 16, 2011.

  1. Mightee, Jul 16, 2011
    Last edited: Jul 16, 2011

    Mightee macrumors member

    Feb 25, 2004
    All right, this might be a silly question. I just bought myself a new i5 2.5" iMac, and I swapped out the two 2GB RAM modules for a pair of 4GB chips. The 2GB are currently sitting in my desk drawer.

    But since the iMac has four slots, could I just pop those two in as well for a total of 12GB? I seem to remember pairing being a big issue once upon a time, but I'm not sure that's still the case. Are there issues with this? Should I just wait a little bit until I can buy two more 4GB DIMMs? Or can I just slide those two old ones into the bottom slots and call it a day?

    Thanks folks.
  2. iLidz macrumors member


    May 19, 2011

    And congrats on the new machine!
  3. Mightee thread starter macrumors member

    Feb 25, 2004

    Thanks! So when did pairing become unimportant? There's no particular order in which I should arrange the DIMMs?
  4. GraceMolloy macrumors regular


    Oct 28, 2006
    I'm wondering the same thing. I remember older PCs going bananas
  5. rusty2192 macrumors 6502a

    Oct 15, 2008
    My Late 2009 iMac came with 2 x 2GB memory. I picked up a pair of 1GB sticks for $20 locally off craigslist and threw them in there with the stock to take it to 6GB. It works flawlessly and all 6 register just fine. Once Lion comes out I'll probably have to upgrade my MacBook from its original 2GB, so I'll probably buy 2 x 4GB, put that and the 2 x 1GB in the iMac, and move the 2 x 2GB into the MacBook. It works out great that they use the same type :D
  6. meli macrumors member

    Jun 3, 2004
    I'm not an expert on this, but I believe if you go with the 12GB you will not be running Dual-Channel mode. That's probably not a big deal, from what I've read dual-channel memory boosts speeds by only 3%-6%. Usually the benefit of having more RAM trumps running in dual-channel mode. So go for the 12GB.

    I've also read that you should install the larger DIMMs in the first slots because those slots are addressed first by the CPU.

    slot 1: 4GB
    slot 2: 4GB
    slot 3: 2GB
    slot 4: 2GB
  7. Vantage Point macrumors 65816

    Mar 1, 2010
    New Jersey
  8. meli, Jul 19, 2011
    Last edited: Jul 19, 2011

    meli macrumors member

    Jun 3, 2004
    I've been trying to figure this out for a couple days. It's confusing, and there's plenty of contradictory information out there. Here's something to contradict my post from ten minutes ago. The way I read it, it may be possible to achieve dual-channel with this configuration.

    slot 1 (ch. A, DIMM 0): 4GB
    slot 2 (ch. A, DIMM 1): 2GB
    slot 3 (ch. B, DIMM 0): 4GB
    slot 4 (ch. B, DIMM 1): 2GB


    Unfortunately, there does not seem to be any easy way to confirm if an iMac is running dual-channel mode. You can run Xbench tests with various RAM configurations and the results may lead you to the "best" configuration. Or you can install Windows, and using Bootcamp, run an application called Everest. That application will tell you if the computer is running dual-channel.

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