In
computing, a device driver is a computer program that operates or
controls a particular type of device that is attached to a computer. 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 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
the end-user is using. 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 physical
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. Conversely,
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 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 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 works for a
given platform function. Drivers operate in a 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.
The task of writing drivers thus
usually falls to software engineers or computer 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 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
modern. Microsoft Windows platforms, can run in kernel-mode (Ring 0 on x86
CPUs) or in user-mode (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.
Applications
Because of the diversity of modern
hardware and operating systems, drivers operate in many different
environments. Drivers may interface with:
·
printers
·
video adapters
·
Network cards
·
Sound cards
·
Local buses of various sorts—in particular, for bus mastering on modern
systems
·
Low-bandwidth I/O buses of various sorts (for pointing devices such as mice,
keyboards, USB, etc.)
·
Computer storage devices such as hard disk, CD-ROM, and floppy disk buses
(ATA, SATA, SCSI)
·
Implementing support for different file systems
·
Image scanners
·
Digital cameras
Common levels of abstraction for
device drivers include:
·
For hardware:
o
Interfacing directly
o
Writing to or reading from a
device control register
o
Using some higher-level interface
(e.g. Video BIOS)
o
Using another lower-level device
driver (e.g. file system drivers using disk drivers)
o
Simulating work with hardware,
while doing something entirely different
·
For software:
o
Allowing the operating system
direct access to hardware resources
o
Implementing only primitives
o
Implementing an interface for
non-driver software (e.g., TWAIN)
o
Implementing a language, sometimes
quite high-level (e.g., PostScript)
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 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. A good example for virtual device drivers can be "Daemon Tools".
There are several variants of
virtual device drivers..
Open driversPrinters: CUPS
·
RAIDs: CCISS (Compaq Command Interface for SCSI-3 Support)
·
Scanners: SANE
·
Video: Vidix, Direct Rendering Infrastructure
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 social Fiber Channel Arbitrated Loop (FCAL)
·
soc: SPARC Storage Array (SSA) controllers
·
social: Serial optical controllers for FCAL (soc+)
APIs
Windows Display Driver Model (WDDM) – the graphic display driver
architecture for Windows Vista, Windows 7 and Windows 8.
·
Windows Driver Foundation (WDF)
·
Windows Driver Model (WDM)
·
Network Driver Interface Specification (NDIS) – a standard network card
driver API
·
Advanced Linux Sound Architecture (ALSA) – as of 2009 the standard Linux
sound-driver interface
·
Scanner Access Now Easy (SANE) – a public-domain interface to raster-image
scanner-hardware
·
I/O Kit – an open-source framework from Apple for developing Mac OS X device
drivers
·
Installable File System (IFS) – a filesystem API for IBM OS/2 and Microsoft
Windows NT
·
Open Data-Link Interface (ODI) – a network card API similar to NDIS
·
Uniform Driver Interface (UDI) – a cross-platform driver interface project
·
Dynax Driver Framework (dxd) – C++ open source cross-platform driver
framework for KMDF and IOKit.
Identifiers
A device on the PCI bus or USB is
identified by two IDs which consist of 4 hexadecimal 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. |