Device drivers and Windows

A Windows device driver is a bit like a DLL: except that instead of an application dynamic linking/loading it, it's the O/S that dynamic links/loads it.

Registry entries tell the O/S what device drivers exist (so that the O/S knows which device drivers to dynamic-link/load).

The device drivers run in ring 0. In ring zero, they (device drivers) don't have access to (can't link to or use) Windows APIs: instead they have access to various NT kernel APIs.

But if it does call device driver, does it mean, that all device drivers which can be called by winAPI function, must have routines named in some specific way? I mean, when I have new sound card, must its drivers have functions named same as the old one? So Windows can actually call the same function from its perspective?

Basically yes. All the device drivers within a given type or class (e.g. all video drivers, or all disk drivers) have a similar API, which is invoked by the O/S (and/or invoked by higher-level drivers, for example disk drivers are used/invoked by file system drivers).

The Windows Device Driver Kit defines the various APIs and includes sample drivers for the various types of device.

But if Windows have predefined sets of functions requored by device drivers, that it cannot use new drivers that doesent existed before last version of OS came out.

The O/S is dynamic-linking to the device driver functions: because device driver APIs are predefined, device drivers are interchangeable as far as the O/S is concerned; new device drivers can be written, provided they support (are backward-compatible with) the standard device driver API.

The dynamic-linking mechanism is very similar to the way in which COM objects or C++ classes implement any predefined pure-abstract interface: a header file in the DDK declares the pure-abstract interface (like virtual functions), device drivers implement these functions, and the O/S loads the drivers and invokes these functions.

The basics:Please note that this explanation is simplified and sometime only true for most cases and not all.

Most HW devices you will ever encounter will have these basic operations:Write to memory(or Registers) on them.Read from memory(or Registers) on them.

This is enough to control the HW, to give it the data it needs, and to get the data you want from it.

These memory areas are mapped by the BIOS and/or the OS to the Physical memory range on your PC (which may in turn be accessed by your driver.)

So we now have two operations READ and WRITE that the device driver knows to do.

In addition, the driver can read and write in a manner that does not involve the cpu. This is called Direct Memory Access (DMA) and usually performed by your HW.

The last type of operation is called INTERRUPTS and is meant for your HW to notify your driver of something that just happend. This is usually done by the HW interrupting the CPU and calling your driver to perform some operation in high priority. For example: an image is ready in the HW to be read by the driver.