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 a translator between a device and the applications or operating systems that use it. The higher-level code can be written independently of whatever specific hardware device it may control. Every version of a device, such as a printer, requires its own specialized commands. In contrast, most applications access devices (such as sending a file to a printer) by using high-level, generic commands, such as PRINTLN. The driver accepts these generic statements and converts them into the low-level commands required by the device.

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.

Development

Writing a device driver requires an in-depth understanding of how the hardware and the software of a given platform function. Drivers "...operate in a highly privileged environment and can cause disaster if they get things wrong..." In contrast, most user-level software on modern operating systems can be stopped without greatly affecting the rest of the system. Even drivers executing in user mode can crash a system if the device is erroneously programmed. These factors make it more difficult and dangerous to diagnose problems.