Windows 7: Video Memory Virtualization - A Brief Guide

The new Microsoft Windows Vista Display Driver Model (WDDM) brings fundamental changes to the management of graphics memory in a system.
Prior to Windows Vista, graphics memory was reported by the graphics driver as a single number through the Display applet in the control panel. Some legacy APIs exposed two types of memory: local and non-local. However, these numbers were selected by the driver and were mostly inaccurate.
With the introduction of Windows Vista, the operating system is putting a much heavier load on the GPU than ever before. Overall system performance is now closely associated with the graphics subsystem performance and is directly affected by the amount of graphics memory available. Microsoft is enhancing how graphics memory is reported, so that end users can better understand the factors that directly impact the system performance. With WDDM, Windows Vista can accurately account for each of the graphics memory contributors and report available memory precisely through new APIs.
It's important for software running on Windows Vista to be able to accurately determine the amount of available graphics memory. WDDM manages the virtualization of graphics memory in itself and also ensures accurate reporting of various aspects of graphics memory. Application developers and software vendors are encouraged to take advantage of the DirectX 10 API for retrieving the accurate set of graphics memory values on systems that have WDDM drivers.