Device driver

From Freepedia

A device driver, often called a driver for short, is a computer program that enables another program, typically, an operating system (OS) (e.g., Windows, Linux, FreeBSD) to interact with a hardware device. A driver is essentially an instruction manual that provides the operating system with the information on how to control and communicate with a particular piece of hardware. In layman's terms, a driver is an important, vital piece to a program application; the main ingredients of the system.

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Device driver philosophy

The key design goal of device drivers is abstraction. Every model of hardware (even within the same class of device) is different. Newer models also are released by manufacturers that provide more reliable or better performance and these newer models are often controlled differently.

Computers and their operating systems cannot be expected to know how to control every device, both now and in the future. To solve this problem, OSes essentially dictate how every type of device should be controlled. The function of the device driver is then to translate these OS mandated function calls into device specific calls. In theory a new device, which is controlled in a new manner, should function correctly if a suitable driver is available. This new driver will ensure that the device appears to operate as usual from the computer's point of view.

Device driver development

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. This makes it particularly difficult to diagnose the problem and can even result in damage to the system. Moreover, debugging device drivers is a difficult skill as it often involves monitoring hardware itself which often acts in an unpredictable manner.

All of this means that the engineers 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.

Device driver applications

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 using disk 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 primitives
    • Implementing an interface for non-driver software (e.g. TWAIN)
    • Implementing a language, sometimes quite high-level, e.g. PostScript

Choosing and installing the correct device drivers for given hardware is often a key component of computer system configuration.

Virtual device drivers

A particular variant of device drivers are virtual device drivers. They are used in virtualization environments, for example when an MS-DOS program is run on a Microsoft Windows computer or when a guest operating system is run inside eg. VMware. Instead of enabling the host operating system to dialog with hardware, virtual device drivers take the opposite role and emulate a piece of hardware, so that the guest operating system and its drivers running inside a virtual machine can have the illusion of accessing real hardware. Attempts by the guest operating system to access the hardware are routed to the virtual device driver in the host operating system as eg. function calls. The virtual device driver can also send simulated processor-level events like interrupts into the virtual machine.

Open drivers

See also

External links

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