Easy.
Hardware and software!
Did you ever try to use a computer with only mouse and monitor?
Windows 7 reports how much physical memory is currently installed on your computer along with how much memory is available to the operating system and the hardware reserved memory.
Windows 7 may show that the usable memory may be less than the installed memory.
The indicative Usable memory is a calculated amount of the total physical memory minus “hardware reserved” memory.
