Resolved Is Apple installing the stock RAM incorrectly? Answer NO

Discussion in 'iMac' started by seanm9, Feb 23, 2013.

  1. seanm9, Feb 23, 2013
    Last edited: Feb 23, 2013

    seanm9 macrumors regular

    Joined:
    Dec 29, 2007
    Location:
    Cape Cod, MA
    #1
    The stock RAM in my iMac is in slots 1 and 3 and the open slots are 2 and 4...

    When you look at it in About This Mac More Info it shows up side by side.
    When you look at in System Information it has one listed as BANK 0/DIMM0 and the other as BANK 1/DIMM0.

    All of the screen shots people have posted here show the same step up... and when we install a pair of 8 GB SODIMM's in the open slots you end up with a 4GB stick and an 8 GB stick in BANK 0, and a 4GB stick and an 8 GB stick in BANK 1...

    everything I read says that RAM should be installed in matched pairs in the SAME BANK... SO SLOTS 1 and 2 are BANK 0 and 3 and 4 are BANK 1...

    What am I missing? Is interleaving that important? This is a new RAM expansion design for Apple and possibly for Foxconn, is it possible that they need to color code the slots like PC motherboards do?
     
  2. WilliamG macrumors 604

    Joined:
    Mar 29, 2008
    Location:
    Seattle
    #2
    1,3, and 2,4 are the correct pairings, from what I gather. That's how it comes from the factory in all new iMacs, so I'd keep it that way.
     
  3. Nermal Moderator

    Nermal

    Staff Member

    Joined:
    Dec 7, 2002
    Location:
    New Zealand
    #3
    Hmm, Apple's instructions and specs don't mention banks at all. Unfortunately I don't have a "known good" system to check against so I'm not sure whether it's important or not :(
     
  4. hfg macrumors 68040

    hfg

    Joined:
    Dec 1, 2006
    Location:
    Cedar Rapids, IA. USA
    #4
    Isn't there an installation diagram on the inside surface of the removable RAM cover?
     
  5. Nuke61 macrumors 6502

    Joined:
    Jan 18, 2013
    Location:
    Columbia, SC
    #5
    Two things to note:
    1) This matches the pairing designated by Intel and the last PC that I assembled for dual-channel mode. The Intel description is here.
    2) The difference in speed between single a dual channel isn't worth worrying about anymore because of DDR3 memory speeds. Will you see a difference in a benchmark? Yes. Will you see a difference in everyday use? Not unless you're using a stopwatch, which effectively makes it a test and not normal useage. http://www.behardware.com/art/imprimer/814/
     
  6. seanm9 thread starter macrumors regular

    Joined:
    Dec 29, 2007
    Location:
    Cape Cod, MA
    #6
    Well I just moved my 8GB sticks to Bank 0 and the 4 GB sticks to Bank 1 (or that is what System information says anyhow)
    it works... this probably also is only an issue for people with 2 different sized sets of RAM sticks (and even then the speed difference is probably minimal)

    there is but i think it was more how to move the tray out and not what slots are what bank but I didn't study it that much...

    Now if I am correct then Stock 8GB machines can be faster (slightly) by putting the ram in the top 2 or bottom 2 slots...

    ----------

    Wish I has seen this before:eek:... the intel link is what i have been looking for... looks like i'll be in flex mode for the night (or until I go and by 2 more 8GB sticks)
     
  7. drambuie macrumors 6502a

    Joined:
    Feb 16, 2010
    #7
    If you pair them as 4 + 8 and 4 + 8 in each channel, the worst that happens is that the first 8GB will run in dual channel mode and the higher 16GB will run in single channel mode. The result is that you will be running in dual channel mode, except for the situations where you need more than 8GB, and then, only the upper 16GB will be in single channel mode.

    In the MBP which has a single dual channel pair, the DIMMs are labelled as BANK0/DIMM0 and BANK1/DIMM0, which verifies the iMac's default pairings.
     
  8. Giuly macrumors 68040

    Giuly

    #8
    Obviously, 1/2 the bandwidth is nothing you need a stopwatch for. It takes twice as long. Also, why would you even install single DIMMs when they're more expensive than the dual-channel kits for the same size?
     
  9. drambuie macrumors 6502a

    Joined:
    Feb 16, 2010
    #9
    The bandwidth of single channel memory is about 10% - 15% less than that of dual channel, as measured in a bandwith benchmark test where the memory is addressed contiguously. In actual use the advantage of dual channel is less because there's a lot of random addressing.

    In single channel memory a RAS, (Row Address Select), action has to be performed for every address access. In dual channel, two addresses can be accessed by a single RAS, provided they are accessed in sequence. That's where the dual channel performance gain comes from.
     
  10. Giuly macrumors 68040

    Giuly

    #10
    Single channel RAM is accessed via a 64Bit address bus, an unganged dual channel implementation adds another 64Bit address bus, that is accessed simultaneously.

    It's a RAID0 of memory.
     
  11. Nuke61 macrumors 6502

    Joined:
    Jan 18, 2013
    Location:
    Columbia, SC
    #11
    It's not as simple as saying single channel is 1/2 the bandwidth of dual channel. If you read the test I referenced, there was roughly a 10% gain by going from single to dual channel when doing actual work and not just measuring bandwidth. You simply aren't going to notice a 10% difference unless you're timing it.
     

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