Shared Knowledge

"Windows Reports Less Memory Than Installed"

10 June 2011

Summary: of total physical memory installed in a personal computer (PC), some portion is consumed by operating system (OS) device drivers such as for the hard drive, a CD-DVD reader or burner, Ethernet, etc. These hunks of program must reside in memory for the OS to timely respond to various events.

Background:

Somewhere inside a Microsoft-based OS, you can see the amount of memory available  for applications to use.

In Windows XP, the amount of memory for application use is shown by left mouse clicking on the "System" icon in the control panel

The amount of memory shown is always less than the amount of physical memory installed in the PC.

Memory consumption by the OS:

Every operating system (Windows XP, Vista, Windows 7) must deal with all sorts of devices: hard drive, modem, audio, Ethernet, keyboard, video, etc. etc.

The programs that deal directly with a device is known as a device driver.

For an OS to respond in a timely manner to a device event, such as writing or reading from a hard drive, the device driver must be resident in system memory.

In Windows XP, if you go to control panel and then system and then hardware manager, if you click on properties of any device shown in the device manager list, you will have the option of seeing resources used. If you click on resources, you will see the starting and ending address of memory of the applicable device driver.

In most cases, a device driver is located in a specific section of memory such that the hardware automatically goes to the driver memory area when a device demands service.

All device drivers consume memory and thus, the amount of physical memory available for application use is reduced by the amount of memory consumed by all device drivers installed.

More devices mean more memory consumed.