Can I use 1066 ddr3 in a 2010 iMac?

Discussion in 'iMac' started by newdeal, Aug 15, 2010.

  1. newdeal macrumors 68020

    Oct 21, 2009
    Ok here is the deal. I ha e 2x2gb of 1066 ddr3 ram sitting here. I am going to buy an iMac. I can't decide between the refurb 27 core2duo, the refurb 27 core i5 or the new 21.5" i3. Of course I am torn between cost and performance. I would prefer the bigger display but it's not necessary. If I could use my existing ram in the new 21.5 that may put it as the winner but I am unsure if I can mix 1066 and 1333 ram or not. If not then I will probably get the refurb 27 and then it's just to decide between i5 or core2duo
  2. TMRaven macrumors 68020


    Nov 5, 2009
    You can mix them, yes. The 1333mhz will just be downclocked to 1066mhz.
  3. newdeal thread starter macrumors 68020

    Oct 21, 2009

    Alright thanks for the info. I was looking at some benchmarks and looks like the 1333 offers a extremely small improvement in performance anyway, defiantly 8gb of 1066 will be better for me than 4gb of 1333. It also looks as though the deal on the 21.5 i3 with the student promo is hard to beat even when looking at the 27" i5
  4. drambuie macrumors 6502a

    Feb 16, 2010
    The iMac has two banks of dual channel DDR3 RAM. Within each bank, the two channels should have identical DIMMs as each channel is 64 bits, but both work together as a 128 bit unit. It is recommended that the upper and lower banks have the same timings, but is not a necessity. With current memory controllers, especially the on-chip controllers in Core i CPUs, the timings for the two banks are independent. In your case the lower bank will run at 1333MHz and the upper bank at 1066MHz. Since the OS and apps will run in the lower 4GB, (unless you're concurrently running a number of large apps), the lower speed of the upper bank will have minimal impact.

    Another consideration is the memory latency timings. The 1333MHz RAM will probably be running at 9-9-9-24, (CL9), and the 1066MHZ at 7-7-7-20, (CL7). When the Core i CPUs were first introduced, a lot of RAM frequency vs latency timing benchmarks were done, and it was discovered that with the Core i's QPI bus, memory latency timing is almost as significant to memory bandwidth as memory clock speed. That's why 1333MHz at CL9 is not that much faster than 1066 at CL7 in bandwidth benchmarks. If the 1333MHz RAM could be overclocked to CL7 timings, then you would see the expected bandwidth difference. That's one advantage that PC desktops with fully accessible BIOSs have.

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