In computing, a device driver or software driver is a computer program allowing higher-level computer programs to interact with a hardware device.

A driver typically communicates with the device through the computer bus or communications subsystem to which the hardware is connected. When a calling program invokes a routine in the driver, the driver issues commands to the device. Once the device sends data back to the driver, the driver may invoke routines in the original calling program. Drivers are hardware-dependent and operating-system-specific. They usually provide the interrupt handling required for any necessary asynchronous time-dependent hardware interface.

Purpose

A device driver simplifies programming by acting as an abstraction layer between a hardware device and the applications or operating systems that use it. The higher-level application code can be written independently of whatever specific hardware device it will ultimately control, as it can interface with it in a standard way, regardless of the underlying hardware. Every version of a device, such as a printer, requires its own hardware-specific specialized commands. In contrast, most applications utilize devices (such as sending a file to a printer) by means of high-level device-generic commands such as PRINTLN (print a line). The device-driver accepts these generic high-level commands and breaks them into a series of low-level device-specific commands as required by the device being driven. Furthermore, drivers can provide a level of security as they can run in kernel-mode, thereby protecting the operating system from applications running in user-mode.

Design

Device drivers can be abstracted into logical and physical layers. Logical layers process data for a class of devices such as ethernet ports or disk drives. Physical layers communicate with specific device instances. For example, a serial port needs to handle standard communication protocols such as XON/XOFF that are common for all serial port hardware. This would be managed by a serial port logical layer. However, the logical layer needs to communicate with a particular serial port chip. 16550 UART hardware differs from PL-011. The physical layer addresses these chip-specific variations. Conventionally, OS requests go to the logical layer first. In turn, the logical layer calls upon the physical layer to implement OS requests in terms understandable by the hardware. Inversely, when a hardware device needs to respond to the OS, it uses the physical layer to speak to the logical layer.

In Linux, device drivers can be built either as parts of the kernel or separately as loadable modules. Makedev includes a list of the devices in Linux: ttyS (terminal), lp (parallel port), hd (disk), loop (loopback disk device), sound (these include mixer, sequencer, dsp, and audio)...

The Windows .sys files and Linux .ko modules are loadable device drivers. The advantage of loadable device drivers is that they can be loaded only when necessary and then unloaded, thus saving kernel memory.