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I was using a laptop with 2G RAM. The laptop has Ubuntu v10.04 running.

Today, I asked our IT-support guy to upgrade my laptop to have total 6G RAM, he did it but told me "Though you have 6G RAM in your laptop, but when you use your current Ubuntu, you have only 3G available, you have to install the latest version of 64bit Ubuntu to enable the 6G RAM."

He was in a hurry to leave without explain me more. Then, I turned on my laptop, use gnome-system-monitor to check, like what he said, it shows me 3G RAM only.

Could someone explain me why ? Why I have 3G available only and why by installing 64bit latest version Ubuntu I will have 6G available?

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Could you post what the uname -a command returns to you ? This will tell us what version you're using. – Maxime R. Jan 18 at 15:44
I saw "i686" which means 32bit. Anyone can tell me what I am asking, that's why I have 3G available only, is it because of the 32bit version I am using? If so, why is that? – user842225 Jan 18 at 15:51
Actually, 32bit systems have an hardware limitation of 4GB (see my comment to duffydack's answer) so you either need to install the pae version of the kernel to tweak this or to reinstall a 64bit version of ubuntu if you have a 64-bit cpu (which is more than likely) – Maxime R. Jan 18 at 16:02
What you're asking is a duplicate of this question: askubuntu.com/q/43422/7567 (or this one askubuntu.com/q/20049/7567) – Maxime R. Jan 18 at 16:07
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2 Answers

up vote 6 down vote accepted

If you have the 32bit version installed then you either need to install the 64bit version or you can just install a PAE kernel that can access all your ram. From terminal or ALT+F2 run sudo apt-get install linux-image-generic-pae linux-headers-generic-pae and reboot

You need a pae/64bit kernel to use more than 3.2gb as 32bit can not address more than that. It's a hardware limitation, not a linux one.

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How can I check my ubuntu bit version? – user842225 Jan 18 at 15:40
Will the installation harm anything on my current system? – user842225 Jan 18 at 15:41
And I still need explanation of why I have only 3G RAM available? – user842225 Jan 18 at 15:46
Hi, can you put your last comment to your answer, that's what I need and what I am asking for, though your existing answer provides excellent further help. – user842225 Jan 18 at 15:53
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In a 32-bit system only 2^32 adresses can be referenced, thus the 4GB ram limitation. More info on Wikipedia thought you'll have to wait tomorrow to read it thanks to the sopa mess... – Maxime R. Jan 18 at 15:56
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up vote 1 down vote

You will be able to use all 6G of RAM if you will choose the *-pae kernel when you boot in the Grub menu. PAE stands for Physical Address Extension and if you can't see it in the menu you can use Synaptic to add it to your current kernel. Be sure to install the same version though.

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How can I check my ubuntu bit version? – user842225 Jan 18 at 15:39
Can you be more detailed on how to use Synaptic to add the PAE? And will it harm anything after install it? – user842225 Jan 18 at 15:40
uname -p in a terminal should tell you if you are running 64 or 32-bit. Just search for "linux-image-generic-pae" in synaptic. – Zoke Jan 18 at 15:42
And I still need explanation of why I have only 3G RAM available? – user842225 Jan 18 at 15:44
@ Zoke, uname -p returns me 'unknown' – user842225 Jan 18 at 15:47
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