I have total 6 GB of RAM installed.
Ubuntu 12.04 64-bit flavours refuse to install with a kernel panic indicating hardware failure if I enable memory remap in BIOS (which led me to change the memory module, luckily I happened to have a spare one lying around). If I disable the memory map in BIOS, installation proceeds smoothly but the operating system is only able to use 3 GB (as indicated by free -g).
The motivation behind installing 64-bit was to be able to use 6GB - I already had 32-bit flavours of Ubuntu and of Windows 7 running without any problem (and they continue to run without problem irrespective of whether memory remap is enabled in BIOS).
Please help me in using the whole 6GB in 64-bit install...