I have ubuntu 11.10 64 bit:
Linux 3.0.0-12-generic #20-Ubuntu SMP Fri Oct 7 14:56:25 UTC 2011 x86_64 x86_64 x86_64 GNU/Linux
I have installed 4 Gb of RAM:
$ sudo dmidecode --type 17 | grep -E "Size|Form Factor|Memory Device|Type"
Memory Device
Size: 1024 MB
Form Factor: DIMM
Type: DDR2
Type Detail: Synchronous
Memory Device
Size: 1024 MB
Form Factor: DIMM
Type: DDR2
Type Detail: Synchronous
Memory Device
Size: 1024 MB
Form Factor: DIMM
Type: DDR2
Type Detail: Synchronous
Memory Device
Size: 1024 MB
Form Factor: DIMM
Type: DDR2
Type Detail: Synchronous
But my PC can see only 3.2 Gb of it.
$ free
total used free shared buffers cached
Mem: 3347240 1404628 1942612 0 47156 603992
-/+ buffers/cache: 753480 2593760
Swap: 2093052 0 2093052
I found this article: https://help.ubuntu.com/community/EnablingPAE. It says that PAE is enabled by default on 64-bit operating systems, so my system should see my 4Gb RAM. But it doesn't.
What could be the problem?
This duplicate question contains even more detail.