Ubuntu 4GB Ram Limitation and Solution

Q. I've total 8 GB RAM installed in my dual boot Ubuntu Linux 8.10 (32 bit) version HP workstation. But free -m command only shows 3291 (3G) memory. How do I use 8GB RAM under Ubuntu Linux?

A. 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.
Without PAE kernel, you should see something as follows:
$ free -m
Sample output:

             total       used       free     shared    buffers     cached
Mem:          3291        801       2489          0         95        342
-/+ buffers/cache:        363       2927
Swap:         1906          0       1906

You have two options here as follows:

Option # 1: Use 64 bit Ubuntu Linux

64 bit Linux kernel will take care of 4G or more memory. Just grab latest 64 bit version and install it.

Option #2: Install PAE enabled kernel

Open terminal and type the following command if you are using Ubuntu version Ubuntu v9.04 and earlier:
$ sudo apt-get update
$ sudo sudo apt-get install linux-headers-server linux-image-server linux-server
If you are using Ubuntu v9.10 (Karmic Koala) and above, enter:
$ sudo apt-get install linux-generic-pae linux-headers-generic-pae
Once kernel images installed, just reboot your workstation, type:
$ sudo reboot
After reboot, login into your system and type the following command to verify memory usage:
$ free -m
Sample output:

             total       used       free     shared    buffers     cached
Mem:          8105       1292       6812          0         38        483
-/+ buffers/cache:        770       7334
Swap:         1906          0       1906