How Much Memory Do You _Really_ Have?

Discussion in 'Mac Pro' started by Infrared, May 27, 2008.

  1. Infrared macrumors 68000


    Mar 28, 2007
    On a PC you don't generally have access to all your RAM.
    You might have 4GB in your machine, but only be able to
    access and use 3.5GB.

    This is not a Windows problem specifically. Rather, it has
    more to do with motherboards and BIOS implementations.

    So that's Windows machines. But how about your Mac Pro?
    You probably have a pretty good idea how much RAM it has
    inside, but how many of those installed gigabytes can you
    actually use?

  2. firestarter macrumors 603


    Dec 31, 2002
    Green and pleasant land
    Are you asking us or telling us?

    There have been some chipset/addressability issues in some of the early Core chipsets limiting to 3GB... and certainly on ancient 8086 variants there was difficulty addressing more than 640k. I don't recognise your assertation that .5GB chunks of memory mysteriously disappears in any current 4G systems though... do you have any references?

    I have 6GB in my Mac Pro, and I'm pretty certain that it's all addressable and usable.
  3. Infrared thread starter macrumors 68000


    Mar 28, 2007
    Several question marks, so I guess I would be asking :)


    The bit that is critical for Mac Pro/OS X users is this:

    "The BIOS must support the memory remapping feature."

    In which substitute EFI for BIOS.
  4. TheSpaz macrumors 604


    Jun 20, 2005
  5. zmttoxics macrumors 65816


    May 20, 2008
    The system only seeing 3.5 gigs is a limitation to the OS. 32 bit operating systems can only address the 3.5. Leopard being a 64 bit OS it can address larger amounts in full.
  6. Infrared thread starter macrumors 68000


    Mar 28, 2007
    I hope you are right. That's what I used to think,
    too. However, increasingly I'm tending toward the
    view that it's a motherboard and BIOS (or in our
    case EFI) thing.
  7. firestarter macrumors 603


    Dec 31, 2002
    Green and pleasant land
    Why are you tending towards that opinion? Please post some evidence that this is a Mac issue.

    So far this is just scaremongering based on a 32bit Vista issue.
  8. TheSpaz macrumors 604


    Jun 20, 2005
    Hehe... people sometimes assume that since the all mighty PC can't do it, the Mac also can't do it... it makes me laugh. I know for a fact that my Mac Pro uses ALL 5GB of RAM, I can watch it be used in Activity Monitor.
  9. Cryptic1911 macrumors member

    Mar 11, 2008
    It is a limitation of 32bit memory addressing. It can only address 4gb total at any given time. There are motherboards where a setting has to be flipped in the bios for it to address more than ~3gb, but its still a 32bit issue for the most part.

    Windows can address 4gb total, but the reason you only see ~3.2-3.5gb is because that 4gb also includes memory reserved for pci-e devices and also video memory.. meaning if you have 4gb ram and a 512mb video card, you're going to lose half a gig of ram because of the video card.

    In windows you need to set the /PAE flag in your boot.ini file so it uses physical address extensions, otherwise it wont see over 2gb or 3gb? (cant remember exactly). What this does is use a memory page table in active ram so that when it needs to use ram above what it has in its view at the moment, it pages out the addresses to the list so that it can reference them again... so the more ram you have (would only make a difference in a server), the more of a performance hit its going to take.. it works, but its a hack. Thats why 64bit is so much better. It can address a hell of alot more ram than 32bit. I cant remember the number, but its more ram than anyone is going to have in a computer for quite a long time
  10. gnasher729 macrumors P6


    Nov 25, 2005
    32 bit applications can see 4 GB of virtual address space, whether they are running on a 32 or 64 bit operating system. 64 bit applications can see basically unlimited virtual address space.

    The motherboard and BIOS determine how much real address space the operating system can see, and how much memory can be plugged in. Mac Pro's are limited to 64 GB RAM. New Core2 Duo Macs are limited to 4 GB RAM, but real address space is bigger, so that the OS can see the video card etc. without losing address space for RAM. Older Core2 Duo Macs are limited to 4 GB RAM _and_ 4 GB real address space, so less than the 4 GB are visible to the OS because some address space is needed for other hardware than RAM. Core Duo machines have lower limits (3 GB RAM or 2 GB RAM) and can only run a 32 bit operating system and 32 bit applications.

    So all MacPro's can have much more than 4 GB RAM, all new Macs can have 4 GB RAM installed and can see and use all of that RAM.
  11. TheSpaz macrumors 604


    Jun 20, 2005
    Most of Leopard is 64bit so that's good. Also, I don't run 1 32bit App that uses all of my RAM... I usually run like 6 apps that each use their own slice of my RAM and that gets used up sometimes.
  12. gnasher729 macrumors P6


    Nov 25, 2005
    Tiger was already able to use more than 4 GB in a system, but it could only run 32 bit applications. You can have a MacPro with 16 GB of RAM and Tiger; if you run four RAM eating application monsters that each use the full 4 GB they can use, this machine will run all four applications without any pageins or pageouts.
  13. Mackilroy macrumors 68040


    Jun 29, 2006
    I've got four gigabytes of RAM, and the computer sees (and uses) all four. :)
  14. echoout macrumors 6502a

    Aug 15, 2007
    Austin, Texas
    I have 16GB of RAM. After Effects uses it all which is all I care about, as that's where my paychecks are coming from. That's still only 2GB per core though.
  15. Cromulent macrumors 603


    Oct 2, 2006
    The Land of Hope and Glory
    Actually that is not true. It can run 64bit processes as long as they do no have a GUI. Therefore you can bolt a 32bit GUI on top of a 64bit base application and effectively have a 64bit application running on Tiger.
  16. squeeks macrumors 68040


    Jun 19, 2007
    my PC REALLY has 8gb and Xp-64 REALLY uses all of it
  17. JNB macrumors 604


    Oct 7, 2004
    In a Hell predominately of my own making
    Well, c'mon, who really needs more than 640K anyway?
  18. QuantumLo0p macrumors 6502a


    Apr 28, 2006
    32 bit XP can use only 2GB unless you use the /3GB switch

    64 bit XP, and I presume Vista, can use a lot more memory than that.

    From what I've read on the subject from M$ support articles, a standard configuration of 32 bit XP can only address 2GB for any process. The total ram it will recognize may be only 4GB. I'm not sure about the latter.

    There is the /3GB switch you can use so 32 bit XP can use that much ram for a process.
  19. MacsRgr8 macrumors 604


    Sep 8, 2002
    The Netherlands
    4 GB installed in Mac Pro '08

    Mac OS X Leopard sees all 4 GB

    Win XP Pro 32 bits SP3 sees only 2 GB
    Vista Ultimate 32 bits sees only 2 GB
    Vista Ultimate 32 bits SP1 sees all 4 GB
    Vista Ultimate 64 bits sees all 4 GB
    Vista Ultimate 64 bits SP1 sees all 4 GB.

    Haven't tried Ubuntu yet... will be doing shortly.
  20. Infrared thread starter macrumors 68000


    Mar 28, 2007
    Well, it seems I was wrong about my Vista installation.
    It does now appear to allow the use of all 4GB of RAM:


    Sorry about that :)

    I think what confused me is that memory availability is
    reported in different ways in different places by the OS,
    and that reporting changed with SP1.
  21. netnothing macrumors 68040


    Mar 13, 2007
    Prior to SP1, Vista would only report what was available. So people would go to the System Info screen and scream because it only showed 3.5GB of the 4GB they just installed.

    SP1 for Vista changed that. Now the system information screen will show the total memory installed in the system. However, going into Task Manager, you will see the actual usage.

    Now, this is with a real PC, not running on a Mac. I thought there are still issues with running XP or Vista on a Mac....something to do with the BIOS emulator or something.

    Anyway, my Dad found out the hard way. He had a Vista 32-bit machine at home with 2GB. When my Mom got her MacBook, I put 4GB in it.....she uses iPhoto a lot. So, he went out and bought 4GB more RAM. Technically his motherboard supports it, as the BIOS states at bootup. And he looked at the System Info screen in Vista SP1 after installing it saw the 6GB being reported and was a happy camper. Wasn't until I told him to go the the task manager and sure enough, only about 3.58GB was being used.

    Plain an simple, 32-bit Vista or XP will only see this much.

    I think it's wrong how Microsoft changed the way the sys info screen shows the installed RAM in the system. They could at least qualify it with the fact that you an put in as much as your motherboard will take, but Vista 32-bit will only use 3.5GB.

    My MacBook is happily running and seeing 4GB. My Mac Pro is chugging away with 11GB.

  22. speakerwizard macrumors 68000


    Aug 8, 2006
    mac pro with 10GB baby, why close applications? :) haha
  23. Cryptic1911 macrumors member

    Mar 11, 2008
    on XP, open your c:\boot.ini file (its hidden i believe..its been a while) and add /PAE after the line that has all the boot info (disk, etc) and it will see over 2gb
  24. GoKyu macrumors 65816


    Feb 15, 2007
    New Orleans
    That was my thinking too....aside from the fact that I also use programs like Photoshop and Lightroom constantly :)

    I keep a lot of stuff open, but I still manage to have minimum of 2-3 gigs free at any time....

  25. Infrared thread starter macrumors 68000


    Mar 28, 2007
    The whole thing is pretty confusing!

    There's the "System Information" referred to here:

    But if that's "System Information", what's this then? :)


    Four of the places where I've found the memory stats.

    ([B]1[/B]) "System Information" as at the codinghorror site. 
      Memory (RAM): 4.00 GB
    ([B]2[/B]) "System Information" as in my screenshot.
      ([B]a[/B]) Installed Physical Memory (RAM) 4.00 GB
      ([B]b[/B]) Total Physical Memory 3.99 GB
      ([B]c[/B]) Available Physical Memory 2.93 GB
    ([B]3[/B]) "About Windows"
      Physical Memory Available to Windows: 4,183,244 KB
    ([B]4[/B]) Task Manager
      Physical Memory (MB) Total 4085
    Running through this:

    1 is the same as 2a. It's the amount of memory physically on the FB-DIMMs before
    Windows and the hardware has had a chance to mangle it. 3 sounds a bit like 2c
    because the word "available" is used for both. But that's not correct. Rather, 3
    is the same as 2b and 4, but that fact is obscured slightly by the use of
    different units. The description "Total Physical Memory" isn't helping anyone
    here. 3, 2b and 4 refer to the amount of memory available for use before Windows
    has booted. It's 4GB minus any "memory holes". Finally, as best as I can make
    out, 2c refers to the amount readily available now for processes that want new
    pages of memory. Insert a suitable definition of "readily available" :)

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