Windows Vista: Creating a 3-D Desktop

Windows Vista's desktop environment requires considerably more computer resources than previous versions of the OS. For this reason, and to make the OS more stable, Vista's graphics subsystem is different from its predecessors.

First, Windows Vista uses a new graphics driver model, known as the Windows Display Driver Model (WDDM). Previous Windows graphics drivers ran in kernel mode. They had direct access to the graphics hardware, and their performance could affect the operating system. This is why graphics errors could cause the entire system to stop responding. WDDM, however, runs primarily in user mode. It has little direct access to the graphics hardware or to critical parts of the operating system. Microsoft instituted a similar change to Vista's audio subsystem as well. These changes should help make the OS more stable.

The WDDM manages the workload of the graphics processing unit (GPU). It allocates the video memory required for different tasks, and it prioritizes applications that need access to the GPU. In other words, it helps budget the computer's video processing resources. This is particularly important, since the OS and applications that use lots of 3-D graphics have to share the computer's graphics resources.

A driver called the Desktop Window Manager (DWM) is part of the WDDM. This driver is responsible for updating what you see on the desktop. The DWM draws all of the objects you see on your screen and holds them in a buffer until you need them. By keeping different desktop views in a buffer, the DWM should help prevent the blank square of space that often appears when programs stop responding. The DWM creates the thumbnails used in Flip and Flip-3D, and it can scale on-screen images to fill high-resolution monitors.

Cap Bits
Previous versions of DirectX used capability bits, or cap bits, to describe different DirectX features. Hardware did not necessarily have to support all of the cap bits to be DirectX compliant. For this reason, video cards and other components didn't always work properly even if they were DirectX compliant. DirectX 10 does away with this system, designating only three features as optional.