This article is from the the Brief Linux FAQ (BLFAQ) Maintained by Neil Zanella firstname.lastname@example.org
Linux uses the BIOS to detect the amount of available RAM. At the present time there are two separate function calls available to do the job. Any version of Linux prior to 2.1.x uses the old call which is limited to 64M. The newer call (used by 95/NT and Linux 2.1.x) can return more than 64M. This problem should be fixed in the next stable release of the Linux kernel (release 2.2.0). For now it is possible to have Linux detect all of your RAM by adding the line:
append = "mem=128M"
to the file /etc/lilo.conf if you use LILO or to the end of the respective LOADLIN file if you use LOADLIN. This will enable 128MB of RAM, for example. You will need to rerun LILO for the changes to take effect. The next time you boot up, Linux will see all of your memory.