Pentium M and PAE

Last modified on 2014-10-17

I have a Thinkpad X31 with a Pentium M processor. A standard Ubuntu 14.04 install will bail thinking the machine has a non-PAE processor when in fact the CPU does support PAE extensions but omits reporting the necessary details.

From the Ubuntu Community Wiki:

A number of older Pentium M processors produced around 2003-4 (the Banias family) do not display the PAE flag, and hence a normal installation fails. However, these processors are in fact able to run the latest (and PAE-demanding) kernels if only the installation process is modified a little. The problem is not missing PAE, it’s about the processor not displaying its full capabilities.

Lubuntu is a Ubuntu-based distro using the lightweight LXDE desktop (good match for older machines like the X31) and provides instructions for adding the forcepae kernel option to the Lubuntu installer. This allows the installer to correctly detect a PAE-capable processor and the system to install as desired.

My own alternative is to prepare a GRUB-capable USB stick capable of booting the Lubuntu installer + other Linux images from the same device. In this scenario I create an entry in grub.cfg for Lubuntu to boot with forcepae pre-configured …

menuentry "Lubuntu 14.04 LTS - 32bit Installer ('forcepae' for Pentium M)" {
set iso="/iso/lubuntu-14.04.1-desktop-i386.iso"
loopback loop $iso
linux (loop)/casper/vmlinuz linux boot=casper iso-scan/filename=$iso noprompt noeject forcepae
initrd (loop)/casper/initrd.lz

Link: Transform a USB stick into a boot device packing multiple Linux distros for details.

Post-install - and before running any upgrades - the PAE flag needs to be set for the CPU, otherwise any attempt at upgrading the kernel will bail (package manager thinks its dealing with a non-PAE processor).

Add forcepae option to GRUB_CMDLINE_LINUX in /etc/default/grub


Run …

sudo update-grub

… and reboot.

Confirm that /proc/cpuinfo displays the pae flag (0=false, 1=true) …

cat /proc/cpuinfo | grep -cim1 pae

Happy hacking!