This article is about device drivers for hardware. For other software drivers, see .
In computing, a device driver is a that operates or controls a particular type of device that is attached to a computer. A driver provides a to hardware devices, enabling operating systems and other computer programs to access hardware functions without needing to know precise details about the hardware being used.
A driver communicates with the through the or communications subsystem to which the hardware connects. When a calling program invokes a 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 -specific. They usually provide the handling required for any necessary asynchronous time-dependent hardware interface.
The main purpose of device drivers is to provide abstraction by acting as a translator between a hardware device and the applications or that use it. Programmers can write the higher-level application code independently of whatever specific hardware the end-user is using.
For example, a high-level application for interacting with a may simply have two functions for "send data" and "receive data". At a lower level, a device driver implementing these functions would communicate to the particular serial port controller installed on a user's computer. The commands needed to control a are much different from the commands needed to control an serial port converter, but each hardware-specific device driver these details into the same (or similar) software interface.
Writing a device driver requires an in-depth understanding of how the hardware and the software works for a given function. Because drivers require low-level access to hardware functions in order to operate, drivers typically operate in a highly environment and can cause system operational issues if something goes wrong. In contrast, most user-level software on modern can be stopped without greatly affecting the rest of the system. Even drivers executing in can crash a system if the device is erroneously programmed. These factors make it more difficult and dangerous to diagnose problems.
The task of writing drivers thus usually falls to or who work for hardware-development companies. This is because they have better information than most outsiders about the design of their hardware. Moreover, it was traditionally considered in the hardware 's interest to guarantee that their clients can use their hardware in an optimum way. Typically, the (LDD) is written by the operating system vendor, while the (PDD) is implemented by the device vendor. But in recent years non-vendors have written numerous device drivers, mainly for use with . In such cases, it is important that the hardware manufacturer provides information on how the device communicates. Although this information can instead be learned by , this is much more difficult with hardware than it is with software.
has attempted to reduce system instability due to poorly written device drivers by creating a new framework for driver development, called (WDF). This includes (UMDF) that encourages development of certain types of drivers—primarily those that implement a for communicating with their devices—as user-mode drivers. If such drivers malfunction, they do not cause system instability. The (KMDF) model continues to allow development of kernel-mode device drivers, but attempts to provide standard implementations of functions that are known to cause problems, including cancellation of I/O operations, power management, and plug and play device support.
has an open-source framework for developing drivers on called the .
In environments, programmers can build device drivers as parts of the , separately as loadable , or as user-mode drivers (for certain types of devices where kernel interfaces exist, such as for USB devices). includes a list of the devices in Linux: ttyS (terminal), lp (), hd (disk), loop, sound (these include , , , and audio)...
The files and .ko modules contain 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.
Kernel mode vs. user mode
Device drivers, particularly on modern platforms, can run in () or in (Ring 3 on x86 CPUs). The primary benefit of running a driver in user mode is improved stability, since a poorly written user mode device driver cannot crash the system by overwriting kernel memory. On the other hand, user/kernel-mode transitions usually impose a considerable performance overhead, thereby prohibiting user-mode drivers for low latency and high throughput requirements.
Kernel space can be accessed by user module only through the use of system calls. End user programs like the UNIX shell or other GUI-based applications are part of the user space. These applications interact with hardware through kernel supported functions.
Because of the diversity of modern hardware and operating systems, drivers operate in many different environments. Drivers may with:
- Local of various sorts—in particular, for on modern systems
- Low- buses of various sorts (for such as , , , etc.)
- devices such as , , and buses (, , )
- Implementing support for different
Common levels of abstraction for device drivers include:
- For hardware:
- Interfacing directly
- Writing to or reading from a
- Using some higher-level interface (e.g. )
- Using another lower-level device driver (e.g. file system drivers using disk drivers)
- Simulating work with hardware, while doing something entirely different
- For software:
- Allowing the operating system direct access to hardware resources
- Implementing only
- Implementing an interface for non-driver software (e.g. )
- Implementing a language, sometimes quite high-level (e.g. )
So choosing and installing the correct device drivers for given hardware is often a key component of computer system configuration.
Virtual device drivers
Virtual device drivers represent a particular variant of device drivers. They are used to emulate a hardware device, particularly in environments, for example when a program is run on a computer or when a guest is run on, for example, a host. Instead of enabling the guest operating system to dialog with hardware, virtual device drivers take the opposite role and emulates a piece of hardware, so that the guest operating system and its drivers running inside a 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 e.g., . The virtual device driver can also send simulated processor-level events like into the virtual machine.
Virtual devices may also operate in a non-virtualized environment. For example, a virtual is used with a , while a virtual device is used with . A good example for virtual device drivers can be .
There are several variants of virtual device drivers, such as , , and VDDs.
descriptions of commonly used device drivers:
- fas: Fast/wide SCSI controller
- hme: Fast (10/100 Mbit/s) Ethernet
- isp: Differential SCSI controllers and the SunSwift card
- glm: (Gigabaud Link Module) UltraSCSI controllers
- scsi: Small Computer Serial Interface (SCSI) devices
- sf: soc+ or social Fiber Channel Arbitrated Loop (FCAL)
- soc: SPARC Storage Array (SSA) controllers and the control device
- social: Serial optical controllers for FCAL (soc+)
A device on the or USB is identified by two IDs which consist of 4 numbers each. The vendor ID identifies the vendor of the device. The device ID identifies a specific device from that manufacturer/vendor.
A PCI device has often an ID pair for the main chip of the device, and also a subsystem ID pair which identifies the vendor, which may be different from the chip manufacturer.
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