When I tried to install Ubuntu 12.04 on an old ThinkPad X40 I was surprised to discover this warning on my screen:

This kernel requires the following features not present on the CPU:
pae. Unable to boot - please use a kernel appropriate for your CPU.

In few words I wasn’t able to install Ubuntu 12.04 because the kernel on Ubuntu 12.04 was not supporting my CPU and I was really upset because I have always promoted Linux distributions as the most versatile and appropriate OS to keep using “old” hardware.

Sincerely, as described in an older post, I previously decided to install MATE desktop environment because the graphic cards on many laptops didn’t support Unity and Gnome 3 but this was the first time I had to renounce the installation of Ubuntu. I didn’t want to surrender to this difficulty (that appeared to me as a bug)!

First of all, I discovered that pae is “a feature to allow 32-bit x86 processors to access a physical address space (including random access memory and memory mapped devices) larger than 4 gigabytes“. After I clarified this point, I was not sure that my processor was totally inadequate to support Ubuntu 12.04.

After some web surfing I discovery that there are two easy solutions to avoid this problem.

1 – As first solution you can install Lubuntu (my choice) or Xubuntu on your computer then you can install the ubuntu-desktop using the Package Manager.

2 – If you prefer not to use the Package Manager you can install Ubuntu 11.04 or 11.10 and then upgrade Ubuntu to 12.04.

More in general, it is also possible to assume that if you have a non PAE CPU, your hardware could not be able to support the Unity desktop. For  this reason you can keep your Lubuntu – Xubuntu (based on the light but effective LXDE desktop) or switch your desktop environment to Gnome (the Classic, of course) installing the gnome package instead of ubuntu-desktop.

In any case, if you want to know if your CPU supports pae you have to open a Terminal and type:

grep --color=always -i PAE /proc/cpuinfo

If something like “flags: fpu vme de pse tsc msr pae mce cx8…” outputs on your Terminal, you have a pae CPU otherwise your CPU doesn’t support pae.