Device Driver
A device driver (sometimes called simply a driver) is a device-specific program that permits a computer's operating system to communicate with a particular hardware component or peripheral device such as a diskette drive, scanner, or printer. A device driver translates the input/output instructions of a computer's operating system to messages that the driver's specific device can understand. For example, a printer driver is a device driver that translates data into a language understood by a specific printer. The driver that works for one model of Epson printer, say, may not work for another Epson model, or for a Hewlett-Packard or Apple printer, because they all require slightly different instruction signals.
Programmers construct a driver to match each device's characteristics. This device driver software contains the instructions necessary to perform all functions requested by any computer application that may wish to activate the device in question. For example, many applications offer the user the option to print a document: if the user chooses to print, the application activates the printer driver. One important responsibility of a device driver is to access the hardware registers (high-speed memory locations) of the central processing unit. Drivers often include an interrupt handler. An interrupt handler is a routine that is executed when a low-level "interrupt" event--a signal requiring immediate response from the computer, such as a mouse click--is received from a peripheral device.
Every device attached to a computer must use a device driver. Many devices, such as the keyboard and the mouse, are purchased as part of the total computer package, and their associated device drivers are therefore included with the computer's operating system. Such built-in drivers accomplish many essential functions, such as scheduling tasks, allocating data storage, and coordinating data transmission between the computer, connected devices, and other computers. For some devices, especially those bought separately, new "loadable" drivers are usually installed when the devices (e.g., display adapters, printers, sound cards) are first connected to the computer. In other cases, the computer's existing operating system may already contain a driver appropriate to the new device. In any case, when a device driver has been properly installed it will accept generic commands from a program (such as any program that might offer a "Print Document" option) and translate them into specialized commands for the device (e.g., the printer).
In a personal computer, device drivers that manage such basic components as the keyboard, floppy drive, and internal hard drive, are included in the system BIOS (basic input/output system). Additional drivers may be loaded into working (random-access) memory by the operating system during start-up of the computer. Device drivers often exist as part of the core or kernel of the operating system. The kernel is responsible for such functions as managing memory, files, peripheral devices, resource allocation, low-level hardware interfaces, and security. Device drivers are normally linked to their associated devices when the kernel is built (loaded from disk into working memory). In DOS (Disk Operating System, an IBM-PC type operating system), driver files names end with the extension .sys. In the Windows operating system, drivers use extensions called .drv. In Macintosh operating systems, the drivers are also system extensions, but are known by the name of the device they control, such as "LaserWriter."
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