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Where does 4GB minus 3.25GB of memory go?

Posted on 2011-09-21
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Last Modified: 2012-06-27
Hi Experts,

I have just installed 4 GB of memory into my 32 bit XP computer and the OS is reporting 3.25 GB. I understand it is a limitation of the 32 bit OS not make the full 4 GB available. But what happens to the remaining memory. I have read that it is allocated to other parts of the computer such as the video etc. Is this true?
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Question by:DColin
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by:jmdl1983
jmdl1983 earned 100 total points
ID: 36574314
It is recognized simply not utilized however.  Technically you can install windows on a board with 48GB of RAM but you will still face the same limitations.  It simply will not use it, and will show the same amount.
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by:jrobillard
jrobillard earned 100 total points
ID: 36574317
Hi
Cause you probably have a Graphic card and other piece that use memory.
You can have your full memory when insalling 64 bit system.
Here the correct link to explain how it work:
http://www.experts-exchange.com/Software/System_Utilities/Remote_Access/Q_24651379.html
Text:
How graphics cards and other devices affect memory limits
 
Devices have to map their memory below 4 GB for compatibility with non-PAE-aware Windows releases. Therefore, if the system has 4GB of RAM, some of it is either disabled or is remapped above 4GB by the BIOS. If the memory is remapped, X64 Windows can use this memory. X86 client versions of Windows don’t support physical memory above the 4GB mark, so they can’t access these remapped regions. Any X64 Windows or X86 Server release can.
 
X86 client versions with PAE enabled do have a usable 37-bit (128 GB) physical address space. The limit that these versions impose is the highest permitted physical RAM address, not the size of the IO space. That means PAE-aware drivers can actually use physical space above 4 GB if they want. For example, drivers could map the “lost” memory regions located above 4 GB and expose this memory as a RAM disk.
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garycase earned 300 total points
ID: 36574411
The other memory is simply not used.    The issue is simply that a 32-bit OS only has 4GB of addresses -- and some of these addresses have to be used for other things.     BIOS shadowing; video aperture; PCI addressing; etc.

r.e. "... I have read that it is allocated to other parts of the computer such as the video etc. Is this true? " ==>  No, it's not true.    NONE of the unaddressable memory is used for anything.    If your system has onboard video that uses some of your memory, that memory would be part of what the OS reports -- it would simply also report that less than that was useable (depending on how much your video adapter used).     If you have a dedicated video card, then it has its own memory, and doesn't use any of the system's RAM (your 4GB) ... although it IS using some of the 4GB of address space (part of the reason you can't address a full 4GB of RAM)

But the simple answer to your question is No, none of the unaddressable RAM is being used.
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Expert Comment

by:jrobillard
ID: 36574419
Sorry i put the wrong link in my explanation:
Here the good one
http://msdn.microsoft.com/en-us/library/windows/desktop/aa366778(v=vs.85).aspx
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