How can the new iMac have an 800 MHz system bus and only use 667 MHz RAM?

Discussion in 'iMac' started by rogersmj, Aug 7, 2007.

  1. rogersmj macrumors 68020

    rogersmj

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    Sep 10, 2006
    Location:
    Indianapolis, IN
    #1
    Was thinking about the new iMac, and noticed the above when looking at the specs. Now, it's been a long time since I got into the nitty-gritty of PC architecture, but wouldn't the slower memory be holding the system back? Does the 800 MHz bus spec of the SR platform imply it should actually take 800 MHz memory?
     
  2. Dont Hurt Me macrumors 603

    Dont Hurt Me

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    #2
    They cranked it down for the cheaper memory no doubt. Sure its holding it back.
     
  3. Eidorian macrumors Penryn

    Eidorian

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    #3
    We went through this when the MacBook Pro was updated. It's the memory controller.
     
  4. Vidd macrumors 6502a

    Vidd

    Joined:
    Mar 7, 2006
    #4
    Santa Rosa has a 800Mhz frontside bus but the hardware that handles RAM runs at 667Mh and this is a limitation in the hardware; it is not Apple's fault.
    EDIT: Whoops, Eidorian's reply wasn't there when I started to type!
     
  5. chewietobbacca macrumors 6502

    Joined:
    Jun 18, 2007
    #5
    This is going to be a very very crude explanation but: Intel rates their front side bus or FSB by quad pumped values.

    On PC's, it goes like this:

    A 2.4GHz E4500 core 2 duo allendale has an 800 MHz FSB quad pumped. It's 2.4GHz comes from the 12X multiplier so 12 x 200 mhz = 2.4GHz.

    RAM can be set to linked (which is a 1:1 ratio) to FSB or unliked (and arbitrary multipliers). Ram, such as the DDR2 ram here, is the base FSB * 2. So if the processor is running at 200FSB (or 800FSB quad pumped, as advertised here), and RAM is linked in a direct 1:1 ratio (or 2.0 multiplier in other words), the RAM is running at 2 * 200 mhz = 400Mhz, or DDR2 400.

    If multiplier is 3.33, then 3.33 * 200 = 667Mhz or DDR2 667.

    For example, my brother's PC has a Core 2 Duo E6550 2.33GHz running at 1333FSB (or 4 x 333). Thus 2.33GHz / 333 MHz = 7 x multiplier. When overclocked to 450 FSB, the computer is running at 7 x 450 = 3.15 GHz.

    The RAM came stock at DDR2 800 but when set to linked 1:1, at the overclocked settings, the RAM is running at 2 x 450 = 900 MHz or in other words, it is overclocked over its originally rated speed.

    I know this pertains more to PC's and their BIOSes than Macs but that's the gist of it. In a nutshell: Lower rated RAM doesn't mean lower speeds at all unless overclocking is involved, and that's not involved here at all.
     
  6. rogersmj thread starter macrumors 68020

    rogersmj

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    Sep 10, 2006
    Location:
    Indianapolis, IN
    #6
    My, aren't we full of flippant responses today. Sorry, I don't have time to keep up with EVERY SINGLE thread about every product line. I figured there was some explanation for it, but "It's the memory controller" isn't a very substantive response.

    Thank you! I feel rumblings in the back of my brain from when I used to build PCs...I think that makes sense. Appreciate the help!
     
  7. Eidorian macrumors Penryn

    Eidorian

    Joined:
    Mar 23, 2005
    Location:
    Indianapolis
    #7
    There's no need to have high speed RAM unless you plan to overclock?

    For best performance you're going to need DDR2-800 for single channel or DDR2-400 for dual channel.

    http://en.wikipedia.org/wiki/Core_2_duo#DDR2_memory_modules

    http://www.intel.com/products/chipsets/pm965/index.htm

    It has been 3 months since the MacBook Pro update. My answer is correct.
     

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