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 connects. 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 hardware device and the applications or
operating systems that use it. Programmers
can write the higher-level application code independently of
whatever specific hardware device it will ultimately control,
because code and device can interface in a standard way, regardless
of the software superstructure or of 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 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 environments, programmers
can build device drivers either as parts of the
kernel or separately as loadable
module.
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
Microsoft Windows .sys files
and Linux .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.
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
privilege 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.
Thus the task of writing drivers usually falls to
software engineers 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
manufacturer's interest to guarantee that their
clients can use their hardware in an optimum way. Typically, the
logical device driver (LDD) is written by the operating
system vendor, while the
physical device driver (PDD) is
implemented by the device vendor. But in recent years non-vendors
have written numerous device drivers, mainly for use with
free and open source operating systems. 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
reverse
engineering, this is much more difficult with hardware than it
is with software.
Microsoft has attempted to reduce system
instability due to poorly written device drivers by creating a new
framework for driver development, called
Windows Driver Foundation (WDF).
This includes
User-Mode
Driver Framework (UMDF) that encourages development of certain
types of drivers — primarily those that implement a
message-based protocol for
communicating with their devices — as user mode drivers. If
such drivers malfunction, they do not cause system instability. The
Kernel-Mode Driver
Framework (KMDF) model continues to allow development of
kernel-mode device drivers, but attempts to provide standard
implementations of functions that are well known to cause problems,
including cancellation of I/O operations, power management, and
plug and play device support.
Apple has an open-source framework for developing drivers on
Mac OS X called the
I/O
Kit.
Kernel-mode vs user-mode
Device drivers, particularly on Windows platforms, can run in
kernel-mode (
Ring 0) or in
user-mode (Ring 3). 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.
Applications
Because of the diversity of
hardware and
operating systems, drivers operate in many different environments.
Drivers may
interface
with:
Common levels of abstraction for device drivers include:
- for hardware:
- interfacing directly
- writing to or reading from a Device Control Register
- 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
- for software:
- allowing the operating system direct access to hardware
resources
- implementing only primitive
- 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
Virtual device drivers represent a particular variant of device
drivers. They are used to emulate a hardware device, particularly
in
virtualization environments, for
example when a
DOS program is run on a
Microsoft Windows computer or when a guest
operating system is run on, for
example, a
Xen host. Instead of enabling the
guest 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 e.g.
function calls. The virtual device driver can
also send simulated processor-level events like
interrupts into the virtual machine.
Virtual devices may also operate in a non-virtualized environment.
For example a virtual
network
adapter is used with a
virtual private network, while a
virtual
disk device is used with
iSCSI.
Open drivers
Solaris 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 socal Fiber Channel Arbitrated Loop (FCAL)
- soc: SPARC Storage Array (SSA) controllers
- socal: Serial optical controllers for FCAL (soc+)
APIs
Identifiers
See also
References
External links
for
developing Mac OS X device
drivers
Wikis
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