Windows Display Driver Model (WDDM, also WVDDM) is the graphic driver architecture for video card drivers running Microsoft Windows versions beginning with Windows Vista. WDDM provides the functionality required to render the desktop and applications using Desktop Window Manager, a compositing window.

Need for a new display driver model
A typical application that relies on the Windows Display Driver Model is the Desktop Window Manager. Since the desktop and application windows managed by DWM are Direct3D applications, the number of open windows directly affects the amount of video memory required. Because there is no limit on the number of open windows, the video memory available may prove insufficient, necessitating virtualization. As the window contents that DWM composes into the final desktop are generated by different processes, cross-process surface sharing is necessary. Also, because there can be other DirectX applications running alongside DWM on the DWM-managed desktop, they must be able to access the GPU in a shared manner, necessitating scheduling.

Though this is true for Microsoft's implementation of a composited desktop under Windows Vista, on the other hand, a composited desktop need not theoretically require a new display driver model to work as expected. Successful implementations of composited desktops were done before Windows Vista on other platforms such as Quartz, Compiz, WindowFX). The approach Microsoft attempted was to try to make sure WDDM was a unified experience across different GPUs by standardizing their features and performance. The software features missing from other driver models could be made immaterial by extensions or if a less restrictive or simply different driver model

Limitations
One of the current limitations of WDDM driver model version 1.0 is that it does not support multiple drivers in a multi-adapter, multi-monitor setup. If a multi-monitor system has more than one graphics adapter powering the monitors, both the adaptors must use the same WDDM driver. If more than one driver is used, Windows will disable one of them.[1].

WDDM does not allow some modes that were previously handled by the driver such as spanning mode (same desktop view across two monitors). The new driver model also currently puts a limit on what hardware can support it, it needs to have Shader Model 2.0 support at least (fixed function pipeline is now translated to 2.0 shaders) and some other hardware features that were not previously enforced (causing, for example, SM 2.0-supporting hardware such as Intel GMA 900 to fail the WDDM certification [2]).

WDDM driver for Direct3D 10-level hardware needs to implement device driver interfaces for both Direct3D 10 runtime and Direct3D 9Ex runtime in order to run legacy Direct3D applications and DWM composition engine.