This is a brief guide on how to enable a PAE kernel on Ubuntu.
Ubuntu 12.10 (Quantal) and onwards
The generic default kernel already has PAE enabled.
For more on this, please see the Quantal technical overview, and the announcement by the Ubuntu Technical Board.
Ubuntu 10.04 LTS (Lucid Lynx) to Ubuntu 12.04 LTS (Precise)
Both the CD and DVD installer of Ubuntu automatically installs the PAE enabled kernel if it detects more than 3 Gb of available memory. In the case of the liveCD, a working network connection is required, since the PAE enabled kernel packages are not present on the CD.
If you need to enable PAE manually, follow the instructions below.
Enabling PAE manually
The PAE enabled kernel can be installed using the Synaptic Package Manager (accessible from the System menu under Administration -> Synaptic Package Manager): The relevant packages are called "linux-generic-pae" and "linux-headers-generic-pae" and should be easily found with a search for "pae".
Alternatively they can be installed using either apt-get or aptitude through the terminal:
sudo apt-get install linux-generic-pae linux-headers-generic-pae
After a reboot the PAE kernel should be booted as the default option in the GRUB boot menu.
To confirm that PAE was enabled correctly you can use the system monitor form the System menu under Administration -> System Monitor. The first tab shows usable memory.
Removing non-PAE kernels
If PAE was successfully enabled you may want to remove the meta-packages for the normal kernel, in order to disable unnecessary updates to the non-PAE kernels:
sudo apt-get remove linux-generic linux-image-generic linux-headers-generic
If you want to remove all of the old kernels from your boot menu, it seems like you have to do it all manually:
sudo apt-get remove linux-image-<version number>-generic linux-headers-<version number>-generic
Something went wrong
If something went wrong, just remove the PAE kernel packages using either synaptic or apt-get/aptitude. If you did not specifically remove the non-PAE kernel packages, they should still be present on the system and automatically selected as the default when the PAE kernel is removed.
The relevant PAE kernel packages are:
linux-image-generic-pae linux-image-<version number>-generic-pae linux-headers-generic-pae linux-headers-<version number>-generic-pae