My machine has 4GB RAM, but when I run the free command or view or /proc/meminfo, it shows 3.7 GB of RAM being used. Why?

Release Found: Red Hat Enterprise Linux

Symptom: The computer correctly sees all the physical memory during POST, but the operating system does not recognize all the installed memory. The memory has been verified as good. Passing value of the physical RAM to the operating system during bootup (either via
GRUB
...