Windows provides Application Programming Interfaces (APIs) that allow programmers to write application software that is independent of the details of specific pieces of hardware. For example, a Windows 3.1 application will use the Graphic Device Interface (GDI) to display bitmaps (such as video frames) on the video display. The application does not need to know anything about the video display card. The video display card could be a Diamond, an ATI, or a Number Nine video card. To make this work, manufacturers of video cards, sound cards, network cards, and other PC hardware must provide a device driver which allows the Windows system to access their hardware. Windows communicates with the device driver and hides the details from the application. The device driver in turn hides the details of the hardware from the Windows operating system.

Device drivers can be very complex components comprising many files. The files may be installed in different places on the hard drive, although \WINDOWS\SYSTEM is the most common. The Windows operating system needs to know the location of the drivers and sometimes configuration information for the drivers such as IRQ’s, IO ports, and other options.

Windows has the notion of a Device Driver Interface or DDI. A Windows device driver is expected to export certain functions specified in the Windows Device Driver Kits (DDKs) which the Windows operating system invokes. The DDI is often implemented in a dynamic link library with the file extension .DRV In Windows 3.1 and Windows 95, actual access to the hardware, the handling of hardware interrupts, any real-time functions that are incompatible with the Windows scheduling system, are implemented in VxDs, or Virtual Device Drivers, which operate at a very privileged level in the system. A device driver may consists of several dynamic link libraries and several VxDs.

VxD’s are not used in Windows NT. Microsoft is attempting to unify the device driver systems on “Memphis” and Windows NT 5.0 through the Win32 Driver Model (WDM).

Device drivers are extremely powerful. Bugs in device drivers can lead to crashes and very strange behavior. The symptoms may be very far removed from the problem. For example, a bug in a video card driver may lead to repeated system crashes after another application or device is installed due to a low level conflict. Based on the symptoms, a user might conclude the PC hardware is failing or the new application is at fault.

Many installation and configuration problems on Windows systems are due to bugs in device drivers, problems in installing the device drivers in the right places, and subtle conflicts between device drivers and other device drivers or applications.