Unibody MacBook: real available ram

Discussion in 'MacBook' started by beneventi, Feb 9, 2009.

  1. beneventi macrumors member

    Joined:
    Jun 6, 2006
    Location:
    Italy
    #1
    Dear forum users, I've just purchased a new unibody MacBook with 2GB of ram to replace my black MacBook (with 2GB of ram too).

    I have Bootcamp installed on both of them and I've noticed in Windows XP that the real ram in the new unibody MacBook is less than the old one. To be precise: the old one gives me 1,96 GB of ram, the new unibody gives me 1,72. Is this a problem?

    Where are those 256 MB of Ram? Are they for the NVIDIA Chipset? I thought that the new graphics card had its own ram.

    Best regards

    Alessandro
     
  2. ayeying macrumors 601

    ayeying

    Joined:
    Dec 5, 2007
    Location:
    Yay Area, CA
    #2
    The 256MB is shared, not dedicated. The 9400M is still an integrated card, a better one, but still integrated.
     
  3. Eric S. macrumors 68040

    Eric S.

    Joined:
    Feb 1, 2008
    Location:
    Santa Cruz Mountains, California
    #3
    I have 4 GB in my aluminum MB and Windows XP says 2.72 GB. Will XP not use anything above 3 GB?
     
  4. Chundles macrumors G4

    Chundles

    Joined:
    Jul 4, 2005
    #4
    Don't think so.
     
  5. Stetrain macrumors 68040

    Joined:
    Feb 6, 2009
    #5
    I believe XP assigns 512mb to the NVidia chip. Windows 32-bit can only address 4GB of total memory space, so its probably seeing around 3.25gb of your system RAM and then taking 512mb of that for the video card. Vista x64 will see all 4GB and dedicate 256mb of that to the video chip, like OSX.
     
  6. jbrenn macrumors 6502a

    Joined:
    Aug 27, 2008
    #6
    32 bit is technically supports 4 but i have never seen it register more than 3 gig. vista sp1 will show 4 but it wont use more than 3.
     
  7. ayeying macrumors 601

    ayeying

    Joined:
    Dec 5, 2007
    Location:
    Yay Area, CA
    #7
    32Bit can use 4GB, however, remember the overhead (All the controllers that also require their own memory space). The overhead takes up ~1GB of space. Since 32bit cannot see above 4GB. 4GB - 1GB = 3GB of available memory space for the actual "Ram". Then minus 256MB for the 9400M video card and you get ~2.75GB of usable ram in Windows XP or any 32-bit OS.
     
  8. arthursiew macrumors 6502

    arthursiew

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    Sep 13, 2008
    Location:
    SF Bay Area
    #8
    It's normal. The graphics card is integrated so it will share RAM.
     
  9. Eric S. macrumors 68040

    Eric S.

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    Feb 1, 2008
    Location:
    Santa Cruz Mountains, California
    #9
    But then I wonder why the OP said he has 2 GB and the system reports 1.72, but I have 4 and the system reports 2.72. It almost seems as though it's starting from 3, not 4.
     
  10. ayeying macrumors 601

    ayeying

    Joined:
    Dec 5, 2007
    Location:
    Yay Area, CA
    #10
    He has 2GB. 2GB - 256MB (video card) = ~1.75GB.

    You have 4GB. 4GB - 1GB (overhead) = 3GB - 256MB (video card) = ~2.75GB
     
  11. Eric S. macrumors 68040

    Eric S.

    Joined:
    Feb 1, 2008
    Location:
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    #11
    I don't get it. Why would there be overhead on 4GB and not on 2GB?
     
  12. xkRoWx macrumors regular

    xkRoWx

    Joined:
    Oct 15, 2007
    Location:
    Canada
    #12
    One possible explanation is that Window XP only recognize as much as 3GB.

    Try taking off one of the 2GB modules and put the original 1GB stock RAM back, run XP and see if it still only recognize 3GB. If it does, then the max RAM recognized for your XP is only 3GB.
     
  13. ayeying macrumors 601

    ayeying

    Joined:
    Dec 5, 2007
    Location:
    Yay Area, CA
    #13
    Okay. For 32bit, the max it can see is 4GB. That includes both overhead/available ram, everything.

    Now, the overhead takes up ~1GB. Since 32bit is max of 4GB, the overhead moves into that space and steals 1GB. Therefore, you're left with 3GB possible of being accessed for the physical ram.

    The entire 32bit coding only allows 4GB.

    This is the maximum 32-bit can occupy:

    | ---- | ---- | ---- | ---- | = 4GB

    Now, if we include in the overhead which is required for the stuff such as USB controllers, input/output and such, etc.

    | OVHD | ---- | ---- | ---- | = 4GB

    Here, 1GB is taken for overhead. 3GB is left for everything else.

    When you have 2GB of physical ram installed, it is (Physical ram = PHRA):

    | OVHD | PHRA | PHRA | ---- | = 4GB

    Here, you see that we haven't reached the 4GB limit. Thats why all of the 2GB of physical ram can be used.

    Now, if we have 4GB installed:

    | OVHD | PHRA | PHRA | PHRA | = 4GB

    Uh oh, we only got 3GB of physical ram to fit in the room. What happened to the extra 1GB? It is wasted and not used at all. There's not enough room for the extra "1GB" to fill anywhere since overhead is required and cannot be avoided.

    Note: this is a crude explanation but simple explanation.
     
  14. Eric S. macrumors 68040

    Eric S.

    Joined:
    Feb 1, 2008
    Location:
    Santa Cruz Mountains, California
    #14
    That's what I've been saying. :)

    Well I'm curious about it but not enough to perform surgery on the MB. Anyone it doesn't have an "original" 1GB RAM; it came this way.

    OK, I see what you're saying now. The bottom line is, as I was asking, XP can only use 3GB.
     

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