The Windows Driver Model has two separate but equally important aspects. First, the core model describes the standard structure for device drivers. Second, Microsoft provides a series of bus and class drivers for common types of devices.
The core WDM model describes how device drivers are installed and started, and how they should service user requests and interact with hardware. A WDM device driver must fit into the Plug and Play (PnP) system that lets users plug in devices that can be configured in software.
Microsoft provides a series of system drivers that have all the basic functionality needed to service many standard types of device. The first type of system driver supports different types of bus, such as the Universal Serial Bus (USB), IEEE 1394 (FireWire) and Audio port devices. Other class drivers implement standard Windows facilities such as Human Input Devices (HID) and kernel streaming. Finally, the Still Image Architecture (STI) provides a framework for handling still images, scanners, etc.
These system class drivers can make it significantly easier to write some types of device driver. For example, the USB system drivers handle all the low-level communications across this bus. A well defined interface is made available to other drivers. This makes it fairly straightforward to issue requests to the USB bus.
Conclusion
Conclusion
Windows provides a well-defined hierarchy and structure for drivers. Go with the grain and learn how to fit in nicely with the rest of the operating system. Do not forget to cope with multiprocessor machines.
Windows provides a generic model of different processors that may be running. Drivers have routines that run at PASSIVE_LEVEL, DISPATCH_LEVEL, and DIRQL interrupt levels. Use system memory carefully and access user memory using the relevant kernel routines.
The kernel calls a driver in many ways. Most driver processing is in response to I/O Request Packets (IRPs). Conversely, a driver can make use of a whole host of kernel routines. Many system drivers have specific interfaces defined to handle particular I/O control codes. Drivers should use these interfaces wherever possible.
Enough prevarication, let's start on the first real device driver. The following chapters describe the Wdm1 virtual device driver in detail, how it is implemented and used, and how to test and debug drivers.
