If the Linux Kernel finds Wrong Memory size at the boot time:

The Linux kernel will detect various settings from your computer configuration. This includes the size of memory you have.

In some cases, it will find the wrong size. For example, it could find only 256 MB of memory when in fact you have 512 MB.

The trick here is to specify the amount of RAM memory you have with the "mem=" parameter.

Here is what you would type when your system boots if you have 512 MB of memory:

•use the arrows to select the boot entry you want to modify on the GRUB splash screen at boot time.
•press e to edit the entry. You will find the following entries as shown below:

root (hd0,0)
kernel /vmlinuz-2.6.18-53.el5 ro root=LABEL=/ rhgb quiet
initrd /initrd-2.6.18-53.el5.img

•use the arrows to go to kernel line
•press e to edit this entry
•Type the entry as shown below at the end of Kernel line:

root (hd0,0)
kernel /vmlinuz-2.6.18-53.el5 ro root=LABEL=/ rhgb quiet linux mem=512M
initrd /initrd-2.6.18-53.el5.img

•press ESC to go back to the parent menu
•press b to boot this kernel

This will tell GRUB to load the linux kernel with 512 MB of memory.

The above steps will be persistent only for the first boot time. Incase you want to make it permanent i.e. at every time the computer boots then make the same change in the "/etc/grub.conf" file in the kernel line below the Linux Operating System Title.