The Pentium M CPU that comes on IBM Thinkpad X31 laptops (circa 2003…) claims to not support PAE (Physical Address Extension ). Luckily however it DOES support PAE if your kernel forces it, which you can do by following the instructions here:

https://help.ubuntu.com/community/PAE

They explain: “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.”

I can just imagine an IBM or Intel engineering thinking…it’ll be 10 years before operating systems require a PAE extension…none of these chips will still be running then…lets call it a night and not bother displaying the PAE flag…