A device driver, often called a driver for short, is a computer program that is intended to allow another program (typically, an operating system) to interact with a hardware device.
Because of the diversity of modern hardware and operating systems, many ways exist in which drivers can be used. Drivers are used for interfacing with:
Common levels of abstraction
for device drivers are:
- On the hardware side:
- Interfacing directly
- Using some higher-level interface (e.g. Video BIOS)
- Using another lower-level device driver (e.g. file system drivers)
- Simulating work with hardware, while doing something entirely different
- On the software side:
- Allowing the operating system direct access to hardware resources
- Implementing only primitivess
- Implementing an interface for non-driver software (e.g. TWAIN)
- Implementing a language, sometimes quite high-level, e.g. PostScript
Writing a device driver is considered a challenge in most cases, as it requires an in-depth understanding of how a given platform
functions, both at the hardware and the software level. In contrast to most types of user-level software running under modern operating systems, which can be stopped without greatly affecting the rest of the system, a bug in a device driver means in many cases that the whole system can stop functioning in a way which can severely damage the data or even the hardware of the computer system
. Moreover, debugging device drivers is a difficult skill as it often involves monitoring hardware itself - which by definition behaves in a non-deterministic way.
All of this means that the people most likely to write device drivers come from the companies that develop the hardware - since they have more complete access to information about the design of their hardware than most outsiders. Moreover, it was traditionally considered in the hardware manufacturer's interest to guarantee that their clients would be able to use their hardware in an optimum way. However, in recent years non-vendors too have written numerous device drivers, mainly for use under free operating systems. In such cases, co-operation on behalf of the vendor is still important, however, as reverse engineering is much more difficult with hardware than it is with software, meaning it may take a long time to learn to operate hardware that has an unknown interface.