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I have a pae kernel but free -m only shows 3.2 Gigs of the 4 GB that is physically installed. The BIOS shows 4 GB available. uname -a gives:

3.2.0-35-generic-pae #55-Ubuntu SMP Wed Dec 5 18:04:39 UTC 2012 i686 athlon i386 GNU/Linux

So it is 32-bit but as I say pae. Running Ubuntu 12.04.

Anybody any idea why it can't see the full 4GB?

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Do you have a dedicated video card? Some video cards reserve part of the system memory for graphics. –  To Do Jan 7 '13 at 14:08
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2 Answers

Motherboards reserve a chunk of address space ( usually 800M or so ) just below the 4GB line for use by other hardware instead of ram. On cheap or buggy motherboards this leaves the ram that would otherwise have those addresses inaccessible. Some motherboards have an option in the bios to enabled memory hole remapping or hoisting, so that ram will be assigned an alternate address above 4GB, allowing it to be accessed by a 64 bit or pae enabled kernel, so poke around in your bios for such an option.

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Thanks for the replies. Is there any way to tell for sure if the graphics card took some of the memory (dmidecode for example)? –  Kevin Sheahan Jan 7 '13 at 14:30
    
@KevinSheahan, you can check dmesg or /var/log/kern.log for the bios e820 memory map if you want to see exactly what ranges of memory your bios is reporting. lspci should show what memory each card is using, but whether it is used or not, that area of memory is still reserved so that it can be used. –  psusi Jan 7 '13 at 18:54
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There was a great anser here, where is it? it gone? he showed that the user has 3.72 GB (calaulating right), and that 3.72-3.2 are used by the graphic card.

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psusi pointed out the mistake. The calculation isn't working for my own system. :P So I removed it. –  Web-E Jan 7 '13 at 14:29
    
I think you should ask him if the calculation is right or not. he gave a good answer, and he gave his time for it. Why to delete his work? couldn't you just say "i'm sorry but you may be wrong" ?? –  yinon Jan 7 '13 at 15:39
    
no no, I gave the answer. I understood my mistake. –  Web-E Jan 7 '13 at 18:04
    
The calculation was incorrect because ram is always in powers of two, thus 1 GB of ram is 1024 x 1024 x 1024 bytes, not 1,000,000,000. It is just hard disk makers that use the power of 10 lie to make their drives look larger. –  psusi Jan 7 '13 at 19:01
    
oh. got it (: (2 more to go..) –  yinon Jan 15 '13 at 11:59
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