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Device Drivers
Definition
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In computing, a device driver or software
driver is a computer program allowing higherlevel computer programs to interact with a
hardware device.
A device driver simplifies programming by
acting as translator between a hardware device
and the applications or operating system that
use it. Programmers can write the higher-level
application code independently of whatever
specific hardware device.
Why Do We Need Device Drivers?
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In the early days of computers, programmers wrote
programs that worked directly at the hardware level. If
the program needed to access information from an
external piece of machinery such as a card reader for
instance, the program had to deal directly with that card
reader in a highly detailed way. The same program
might not work with a different card reader, even when
running on the same type of computer, meaning that the
program would have to be rewritten. Obviously, this
caused a lot of duplication of programming effort,
resulting in higher cost software and non-portable
programs.
Functions
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A device driver processes the request of an
application and then sends the instruction to the
hardware device to produce the output. This is
how the display information is sent from the
operating system to the display or monitor. A
device driver typically has a configuration
interface that can be accessed so that the user
can make adjustments to the hardware device.
Design
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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 ports needs to handle standard
communication protocols such as XON/OFF 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.
The Structure Of A Device Driver
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A device driver by its nature has a very
rigid structure. DOS must be able to work
properly with any driver, so all drivers
must follow an agreed upon design.
Every driver is made up of three parts: the
device header, the strategy routine, and
the interrupt routine, and are everything
that DOS needs to use the driver.
Development
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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.
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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.
File Types
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A device driver can have a file type of DLL or
EXE depending on what type or program is
being used. Many software programs that
communicate with a device use a combination of
DLL and EXE files in order for the device to
function properly. An example of this would be a
TV Tuner card. Software needs to be installed to
display the video on the screen or send audio to
the speakers.
Kernel-mode vs user-mode
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Device drivers, particularly on modern Windows
platforms, can run in kernel-mode0 (Ring ) 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/kernelmode transitions usually impose a considerable
performance overhead, thereby prohibiting user
mode-drivers for low latency and high
throughput requirements.
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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
 Devices such as Hard Disk, CD-ROM and Floppy
Disk buses (ATA, SATA, SCSI)
 Image Scanners
 Digital Cameras etc
Installation
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A device driver is typically installed when
the installation CD for a new hardware
device is used to run the setup program.
The installation CD contains the device
drivers as well as any additional software
applications that are needed to use the
new hardware device. A device driver will
stay on the system until it is removed or
uninstalled
Updating
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Device drivers can be updated on a system
using a variety of methods. Typically, a device
driver can be downloaded from the
manufacturer if an update is available. Many
times a manufacturer will update a device driver
to add additional functionality or to fix a bug in
the program. A device driver can also be
updated by installing updates to the operating
system or running the setup program from a
device's installation CD.
Conflicts
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Sometimes a device driver can conflict
with other components on a system. The
Device Manager in Windows can be used
to see if there is any sort of conflict with
an installed device. Conflicts can usually
be resolved by repairing corrupt files or
removing a device and attempting to
reinstall it on the system.
Virtual device drivers
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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.
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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.
Thank You
Bibliography
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Wikipedia
www.ehow.com
www.bbc.org
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