iMac Memory and Bus Speed

Discussion in 'iMac' started by jmeggers, Sep 15, 2008.

  1. jmeggers macrumors newbie

    Joined:
    Dec 10, 2005
    #1
    What is the relationship between bus speed and memory speed? I have an iMac 2.4GHz that I purchased off the Apple Refurb Program earlier this year, but I can't tell for sure what generation it is. Apple's web site seems to indicate that Gen 1 uses PC2-5300 667MHz (which is what's in my iMac) while the Gen 2 model uses PC2-6400 800MHz DIMMs. System Profiler is saying the bus speed on mine is 800 MHz, but the memory is 667 MHz. Will either DIMM work in mine? If so, is it necessary for both DIMMs to match, and is it worth replacing my 1GB DIMM with two 1GB 800 MHz DIMMs? Should I call Apple and complain that this should have been shipped with an 800 MHz DIMM?

    Thanks for any information.

    John
     
  2. Berlepsch macrumors 6502

    Berlepsch

    Joined:
    Oct 22, 2007
    #2
    Whoa, lots of questions. Let's break it down one by one.


    Intel-based computers run with different speeds for the connections CPU <-> memory controller (front side bus speed) and memory <-> DIMMs (memory bus speed). The FSB speed is usually higher than the MB speed. The 1st generation alu-iMac clocked the FSB at 800 MHz, and memory at 667 MHz. The current model uses 1066 MHz and 800 MHz, respectively.

    That looks like the 1st gen. alu to me.

    You can install both 800 and 667 MHz DIMMs in your iMac, but they will always run at 667 MHz. You can mix DIMMs with both speeds, and also combine different capacities, e.g 1 GB and 2 GB. In that case, 2 GB are accessed in dual channel mode, 1 GB in single channel mode, which is a bit slower.

    With the current prices for memory, it is probably the easiest to buy 2 x 2 GB 5300-667 MHz DIMMs, which is the maximum you can install.
     
  3. Cheffy Dave macrumors 68020

    Cheffy Dave

    Joined:
    Feb 5, 2007
    Location:
    Sunny Florida, on the Gulf Coast in Homosassa Fl
    #3
    When I need to know what ram to install in my systems, I just run the app from Crucial. Scans my system, finds the Ram and price, I order. Works every time
     
  4. rick.marshall macrumors newbie

    Joined:
    Sep 27, 2009
    #4
    800MHz Crucials Didn't Work in iMac (Mid-2007)

    Hi Berlepsch, thanks VERY much for the detailed explanation. I had exactly the same problem as jmeggers -- thought an 800MHz FSB equated to PC2-6400 (800MHz) DDR2 -- nope!

    Just FYI, I actually got so far as buying a 4GB upgrade from Crucial. When I installed the 800MHz DIMM's into my Aluminum iMac 24" Core2 Duo 2.8 (mid-2007) it refused to boot.

    I'm swapping them out for the correct set of 667MHz modules, but was wondering if you it would surprise you that they didn't work?

    The failures was pretty dramatic -- the system would act like it was booting (the drive would spin up and the fans would come on,) but there were no tones, no display, nothing. Talk about scary!

    Thanks again for the info.
     

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