I have a Toshiba A100, which I upgraded to 4 GB of RAM. The hardware startup indeed shows 4 GB of RAM, and I recently installed Windows 7, just to see how it behaves on it. So far so good, it displays 4 GB of RAM. Not that I tried to use it all, but it displays it. Previously under XP, I also would see 4 GB of RAM.
But under Ubuntu 9.10 (32 or 64 bits), it only displays 2.9 GB of RAM. And my kernel is the "pae" compiled one, which is supposed to do the trick to work around the 32-bit CPU limitation.
How can I get Ubuntu to fully use my 4 GB of RAM?