If you install Ubuntu 32bit , and you have more then 4 GB of RAM, you won’t be able to use it, The easiest solution is to grab latest 64 bit version and install it (Only if your hardware supports 64bit architecture) otherwise you have to use PAE enabled kernel.
For example if you have 8GB of ram, and you issue the command,
you will see only 3-4 GB of total ram. You need to install Physical Address Extension (PAE) aware kernel under 32 bit Ubuntu Linux. It is a feature of x86 and x86-64 processors that allows more than 4 Gigabytes of physical memory to be used in 32-bit systems.
HOWTO Install PAE enabled kernel in UBUNTU
Open terminal and type the following command
If you are using Ubuntu v9.10 (Karmic Koala) and above, enter:
apt-get install linux-generic-pae linux-headers-generic-pae
Once kernel images installed, just reboot your workstation, type:
After reboot, login into your system and type the following command to verify memory usage:
if you are using Ubuntu version Ubuntu v9.04 and earlier:
apt-get update sudo apt-get install linux-headers-server linux-image-server linux-server
Now reboot and check memory by free -m