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How can you tell if RAM is defective?

677 Views 22 Replies Latest reply: Dec 14, 2012 5:35 PM by Saxman RSS
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Saxman Level 2 Level 2 (175 points)
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Nov 13, 2012 3:03 PM

I recently upped my RAM from 8 GB to 12 GB. System Profiler shows I have 12, and I ran the Hardware Test, and it didn't say anything was wrong. However, since installing it, it seems if anything, my computer is running more slowly, especially my Safari. Would I not be able to tell the difference in speed of windows opening, functioning of programs, etc?

iMac, Mac OS X (10.6.8), 21.5" ,3.2 GHz i3, 12GB Ram
  • Kappy Level 10 Level 10 (221,005 points)
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    Nov 13, 2012 3:06 PM (in response to Saxman)

    Your problem isn't due to the RAM which is apparently working fine.

  • BobHarris Level 6 Level 6 (12,505 points)
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    Nov 14, 2012 6:25 AM (in response to Saxman)

    Do you have matched pairs of DIMM cards installed?

     

    Accessing RAM has 2 phases.  The first is draining the memroy cell to see if it had anything in it, and the second is refilling the memory cell now that it has been drained.

     

    If RAM DIMMs are installed as matched pairs (where matched means same size and technical specs), then the hardware will arrange for the RAM to be accessed first from one DIMM, then while it is restoring the drained values, it will access the paired DIMM.  This alternating between DIMMs allows for more data to be transferred from RAM to the CPU at a faster rate.

     

    If the DIMMs are not matched, the hardware will access RAM serially, waiting between each read for the previous read to refill the drained memory cells.

     

    Matched pairs does not double speed, as the CPU is not always accessing RAM, as the CPU has its own on-chip cache to defer going to RAM, but it does improve performance.

     

    If you did not properly pair your DIMMs when you installed them, that might explain some of your issues.

     

    Or I could be totally wrong, and you are experiencing some other problem.

  • BobHarris Level 6 Level 6 (12,505 points)
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    Nov 14, 2012 11:04 AM (in response to Saxman)

    What do you mean by "matched pairs"? The same sizes, same brand, position in the computer?

    A match pair are both the same size and have the same specs speed, DDRn etc...).  They do not need to be the same manufacture.

     

    I have one pair of 2GB, and one pair of 4GB, with one of each loaded into the two sides of the RAM storage space in the computer.

    Applications -> Utilities -> System Profiler -> Memory

     

    You will see

     

    BANK 0/DIMM0:  nn GB

    BANK 1/DIMM0:  nn GB

    BANK 0/DIMM1:  nn GB

    BANK 1/DIMM1:  nn GB

     

    Bank 0 should have a matched pair.

    Bank 1 should have the other matched pair.

     

    If they are not matched, then you will not get the benefit of memory interleaving.

  • Kappy Level 10 Level 10 (221,005 points)
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    Nov 14, 2012 11:43 AM (in response to Saxman)

    The top two slots should have a matched pair - same size. The bottom slot another matched pair - same size.

     

    upper slots: 2+2

    lower slots: 4+4

  • Kappy Level 10 Level 10 (221,005 points)
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    Nov 14, 2012 12:13 PM (in response to Saxman)

    Look in your computer's user manual. It's shown there as I've stated. Slot labeling is confusing.

  • BobHarris Level 6 Level 6 (12,505 points)
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    Nov 14, 2012 12:15 PM (in response to Saxman)

    Well I could be wrong.  Wouldn't be the first time.

     

    You could google "iMac memory banks" and you will most likely find some installation videos and maybe some guides on how to install non-equal sized RAM DIMMs

  • Kappy Level 10 Level 10 (221,005 points)
    Currently Being Moderated
    Nov 14, 2012 12:31 PM (in response to BobHarris)

    You are right, but the table is a little confusing for the less technically minded.

     

    I think it's because there are four banks. Bank 0 and DIMMA and Bank 0 and DIMMB are the upper slots. Bank 1 and DIMMC and Bank 1 and DIMMD are the lower slots. For memory interleaving DIMM A and DIMM B should be a matched pair as should DIMM C and DIMM D. However, the upper pair do not have to match the lower pair.

     

    So, as read your table it's correct.

  • Kappy Level 10 Level 10 (221,005 points)
    Currently Being Moderated
    Nov 14, 2012 1:22 PM (in response to Saxman)

    Farthest away from the screen will be the two lower slots since you are looking at them with the computer lying flat on the table, screen facing up.

     

    The computer comes, stock, with only two installed modules, and they are installed in the upper two slots.

     

    If you had them installed as:

     

    upper: 4-2

    lower: 4-2

     

    or

     

    upper: 2-4

    lower: 2-4

     

    Then the memory is not interleaved. Memory performance is degradated about 5-15%.

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