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I have an 8GB RAM System but Ubuntu 12.04 only detect 4GB by system start. Every time I reboot the System Ubuntu will detect the full 8GB RAM!?!?!?

AMD Zacate E350 APU Radeon HD 6310 512MB shared memory 8GB GSKILL RAM 2TB Segate Barracuda

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Can you post the output of free -m by editing your Qn. – tijybba Aug 28 '12 at 6:32
How many modules is it? Try removing one module, boot the machine, see how much ram is detected, and then try it with the other. You may have a bad module. – Mitch Aug 28 '12 at 6:52
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Can you please post the output of uname -a and dpkg -l | grep linux-generic – Histo Aug 28 '12 at 7:05
regarding @Histo comment: BOTH situations (when on 4 and when on 8 Gb). Maybe 1 mem stick is too slow(?) – Rinzwind Aug 28 '12 at 7:46
did u enable / install PAE? – penreturn Aug 28 '12 at 9:18

2 Answers

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You need to run 64bit Ubuntu to address more than 4GB of ram.

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Not exactly true. Ubuntu uses a PAE kernel for 4+Gb and 32bits. en.wikipedia.org/wiki/Physical_Address_Extension – Rinzwind Aug 28 '12 at 6:42
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pae kernel is only installed on 12.04 if the installer detects more than 3GB of free ram. I suspect this is the problem he is running a non PAE kernel. – Histo Aug 28 '12 at 7:04
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hmmm and the reboot does activate a pae kernel? :-\ you could be on something there – Rinzwind Aug 28 '12 at 7:46
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I think you installed the 32bit version with the pae kernel. Enter in the terminal: 

uname -a

If you see pae mentioned or i386, you are using the 32bit version. To use the full amount of RAM you need to install the 64bit version.

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