First - what version of Ubuntu do you have? The newest one is13.04, the latest LTS is12.04 (long term support). You have probably32bit kernel with ability to manage only3.2GB of RAM.
And therefore you have two options - stick to the64bit version of Ubuntu, or install kernel with PAE (kernel-generic-pae for Ubuntu32bit).
Good luck :)
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Fady naffa , Software Engineer , Boundless Drop
1) there is amount of reserved memory The limit for32 Bit is4GB from which it reserves between200MB and1GB of RAM2) Update your bios3) bios settings for reserved memory for integrated graphics it might take up to1.5 gb
make sure you are using64 bit version it's better if you have more than3.8 gb of ram
