Many lower-end PCs ship with an integrated graphics controller – a video system built into the motherboard rather...
than installed as an add-on card. In fact, a fair number of PCs that come with integrated video don't even have a dedicated slot for an outboard video card. This is presumably a cost-saving measure, but it's also a way to reduce the size of the motherboard and allow it to fit into a smaller chassis.
Motherboards with integrated video used to have their own memory modules, but the newest integrated video controllers have a different approach. They share out a section of RAM from the machine as a whole, which is used to render the display graphics (anywhere from 8MB to 128MB).
The actual amount of memory used depends mainly on the bit depth and dimensions of the display, but it varies depending on what's running. For instance, a conventional 2D desktop program won't use as much shared display memory as a 3D application.
Shared video memory saves the manufacturer some money, since the motherboard doesn't have to be outfitted with dedicated video memory. Of course, shared memory is slower than dedicated memory. Furthermore, shared memory creates a side effect that puzzles many Windows users when they first encounter it: It causes Windows to misreport the amount of physical memory available in the system. If you right-click My Computer and select Properties on such a system, the "General" tab will list a memory tally that's short of the actual amount of physical memory installed in the PC.
In the screenshot above, the machine in question actually has 512 MB of RAM, but is reporting as 504 MB. The integrated video controller in the system is pre-emptively using 8MB of RAM—staking it out before Windows itself can make use of it. Because this happens on the hardware level, the actual amount of available system memory as reported to Windows is going to be less the amount used by the video controller.
The most foolproof way to get an accurate tally of physical memory is to examine the machine's POST (power-on self-test) memory tally, or to enter the system BIOS and examine the memory count listed there. In BIOS, the total amount of physical memory installed will always be reported accurately. If you see less memory installed there than you expect, there may be a problem with one of the DIMMs themselves.
About the author: Serdar Yegulalp is editor of the Windows Insight, (formerly the Windows Power Users Newsletter), a blog site devoted to hints, tips, tricks and news for users and administrators of Windows NT, Windows 2000, Windows XP, Windows Server 2003 and Vista. He has more than 12 years of Windows experience under his belt, and contributes regularly to SearchWinComputing.com and SearchSQLServer.com.
More information on this topic: