Windows WDM Drivers and You
The Whys and Wherefores of the New Windows Driver Standard

by JD Mars
Published in Digital Pro Sound in October, 2001

Just as Windows 2000 builds upon the NT kernel, or core, Microsoft's "Windows Driver Model," or WDM, draws from the NT driver model with several fundamental differences. WDM is an attempt to bridge the gap between Windows 98 and Windows 2000 driver development, with performance benefits even for users of Win98.

With the creation of this new driver standard, manufacturers are faced with support of the previous, virtual device driver model, or "VXD," for Win95/98, plus NT support, plus the development of the WDM driver which, by definition, must work in both Windows 98 and 2000. Initially, at least, that's a double-edged sword for manufacturers. One edge is the awesome task of creating a new set of drivers for a complete line of devices, and the other edge is the legacy support with inevitable obsolescence. The latter edge is perhaps not quite so sharp, and therefore ultimately more painful.
For the user and eventually for the hardware manufacturer, these are positive strides. Let's take a closer look at what this all means to the end-user.

Why WDM?
Microsoft recognized the difficulty for independent hardware vendors to develop and maintain two completely different driver standards, i.e., one for Windows 95/98 and one for Windows NT 3.51 and 4.0. With NT 5 (better known as Windows 2000) on the horizon, the WDM driver standard was introduced for Windows 98 to bridge the gap between the need to develop two separate sets of drivers and to encourage driver development for Windows 2000 (WDM will not work in Win 95, though it does contain some of the necessary elements). At the time, yet possibly less so today, legacy support for Windows 95 was a necessary evil.

The other side of the coin was Microsoft's reluctant yet eventual acknowledgment of the increasing use of the personal computer for applications where large amounts of continuous data needed to be streamed to hardware, and where the time-sensitive nature of that data required precise synchronization. Previously, Microsoft seemed to address only the more traditional business applications in a serious and efficacious way. Perhaps the cry, "Hey, what about us audio and video guys?" was finally heard. While streaming and synchronization were negatively impacted by the inefficient use of system resources and system latency coupled with an inconsistent driver model, WDM represents major advances in this area.

How WDM?
With WDM, some of the work of a device driver is offloaded onto portions of the code that are integrated into the operating system. These portions of the driver code are known as driver "classes," or the class driver. The class driver handles all of the low-level buffer management, including DMA and even Plug and Play device enumeration. The result is that the actual WDM device driver is far more streamlined with far less code, and with much greater efficiency.

For the user and eventually for the hardware manufacturer, these are positive strides. Let's take a closer look at what this all means to the end-user.

Class drivers themselves are divided into two categories, and with this simplification of the driver model, the class drivers are referred to as "miniport" and "minidriver" (not to be confused with the actress, Minnie Driver) approaches to driver operation. The port class/miniport model serves devices designed for the PCI and ISA bus, while the stream class/minidriver model addresses the needs of USB and FireWire ports. The class driver communicates with the OS through WDM, and with the rest of the world through WDM CSA.

WDM is based on the layered approach of the Windows NT driver. Built on top of WDM, is the Connection Streaming Architecture (WDM CSA), and is designed to address the higher level functions including data streaming and synchronization and delivery delays, a/k/a system latency. CSA is modular in nature, something that previous driver models lacked. This, together with the manner in which WDM CSA presents drivers as "filters" which pass streaming media (isolating the hardware issues from the streaming issues), allowed driver writers to manipulate driver behavior without having to rewrite the driver or the application.

Direct Sound
Direct Sound is the audio component of DirectX, and adds hardware acceleration and direct access from the program to the audio device within the Windows system. Direct Sound compatibility could be added to VXD drivers for Win 95/98. When Direct Sound was not or could not be added to a device driver, Windows would create an emulation mode so that the driver could at least work with an application that required Direct Sound compatibility, though performance was compromised. DirectX support was added to Windows NT in Service Pack 3 and beyond, but has always been limited to version 3 (the latest DirectX version is DirectX 8.0). Still, Direct Sound only works in Windows NT in emulation mode.

DirectX compatibility is built into the WDM driver. This creates a fundamental and major difference between the NT driver and the Win 2000 (WDM) driver, and greatly enhanced performance in the WDM driver over a Windows 98 driver with added-on DirectX (i.e., Direct Sound) compatibility. Again, due to the streamlined nature of the WDM driver model, Direct Sound support is achieved without the addition of driver code, in this case the DirectX APIs, or, "application programming interfaces."

WDM and You!
The end result is a more efficient, streamlined driver for the end-user with far fewer timing issues for the streaming media enthusiast, including synchronization between the application and the hardware and system latency that has always been a problem in the Windows system.

For the hardware manufacturer, investments in driver development can extend into the hundreds of thousands of dollars. While the transition to the single use of the WDM model may be initially troubled by legacy support of Windows 95 (and of course continued VXD driver support for Win 98 while WDM drivers are in development), it would not be unreasonable for manufacturers to soon end support of Win 95. While the WDM model should afford a seamless transition between Windows 2000 and Windows XP (and future Windows operating systems), future needs and tweaks should never result in the need to rebuild or build a new driver.

Should you upgrade your driver to WDM? If you're a Win 98 or ME user, first check with the hardware manufacturer to see if a WDM driver exists or is in development. Also check with your software manufacturer to see if your program can benefit from the use of a WDM driver with your particular capture device. If you're a Windows 2000 user, it is most likely a necessity that you use a WDM driver. Theoretically, an NT driver will work in Win 2000, but in actuality there were problems for streaming media capture and playback. Your manufacturer should have WDM drivers, but if not, some gentle lobbying is in order.

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