The first feature is actually nothing else but multi-tasking, but for the GPU rather than the CPU. This allows running several threads of code generated by different processes simultaneously on a single GPU. It’s like with the CPU: the WDDM scheduler distributes GPU resources among the different applications that request them. The scheduling can be basic or extended. Within the WGF 1.0 framework only the first, basic level of graphical multi-tasking is implemented. It has a number of limitations. Particularly, the execution of a shader program or the rendering of a graphics primitive cannot be interrupted and must be completed before the GPU can switch to another process. This is not an optimal solution in terms of overall performance, so WGF 2.0 offers an extended GPU scheduling mode in which the execution of a program can be interrupted. As a result, the simultaneous processing of multiple tasks can be performed more effectively. The Window Manager in Vista uses the basic GPU scheduling mode so, theoretically, conflicts with user software that makes intensive use of the graphics pipeline are possible. We didn’t see such problems in practice, though.

The memory virtualization technique is alike to the memory paging mechanism in the OS, but works with the graphics card. It allows allotting graphics memory for data needed at the moment and unloading unnecessary data into system RAM or, if there is no free space in system RAM, to the hard drive (a considerable performance hit is unavoidable in the latter case, of course). So, graphics memory virtualization helps avoid problems arising due to shortage of graphics memory and provides for a more efficient distribution of available resources among the competing graphics applications. This concept is somewhat similar to the GART mode of the AGP bus that allowed storing textures in system memory rather than in the graphics card’s onboard memory.

The shared use of DirectX surfaces is perhaps the most important WDDM component because it is this feature that allows applications to render into off-screen buffers so that the contents of the buffers could then be combined and output on the display. This works with all types of windows: GDI, DirectX or mixed. Describing this mechanism in detail is beyond the scope of this review, but we’d want to note that the virtualization of all processes in WDDM prohibits direct access to the primary surface, i.e. to the display. If an application tries to access it, DWM shuts down and the Aero interface becomes disabled, too, until the application that has provoked the exception quits.

Another key feature is that WDDM supports checking of commands and parameters given to the graphics subsystem for correctness. This feature help improve stability of 3D applications. If supported by the driver, this check is performed by the graphics hardware to save CPU resources.

Summarizing this part of the review, we should again say that WDDM is one of the most important parts of the Windows Vista infrastructure because you can’t utilize all the features of the new OS from Microsoft without an appropriate graphics driver. We’ve also told you why Windows Vista will have to be installed on every gaming computer: the new generation of games using WGF 1.0 and, later on, WGF 2.0 will arrive sooner or later, and they will only run in a Vista environment with WDDM drivers installed.

The sequel to the popular sci-fi shooter Halo, scheduled for a January 2007 release, will be the first game of that new generation. Even the full name of the game indicates its OS requirement: Halo 2 for Windows Vista. We don’t know if the game will use WGF 2.0 – most likely not – but it is announced as a game to be run exclusively under the new OS from Microsoft.

On the other hand, next-generation graphics cards with unified shader architecture will be utterly useless outside the Windows Vista environment. They will have Windows XP drivers, but the capabilities of the new graphics architecture won’t be used fully in Windows XP because Direct3D 10 (WGF 2.0) will not be released separately for installation into Windows XP/2000 environments.

In other words, a PC gamer has to transition to Windows Vista not only because this OS offers enhanced graphics-related capabilities, but also because of the unique nature of those capabilities: future games will not be able to run under Windows XP which will lack support for WDDM and Direct3D 10. The transition to the new OS will obviously call for a considerable overhaul of your computer. You may need to enlarge the amount of system RAM and, later on, to install a WGF 2.0-compatible graphics card. But that’s the cost of progress we all have to pay for.