Mouse (computing)

In computing, a mouse (plural mice or mouses) functions as a pointing
device by detecting two-dimensional motion relative to its supporting

surface. Physically, a mouse consists of a small case, held under one

of the user's hands, with one or more buttons. It sometimes features

other elements, such as "wheels", which allow the user to perform

various system-dependent operations, or extra buttons or features can

add more control or dimensional input. The mouse's motion typically

translates into the motion of a pointer on a display.

The name mouse, coined at the Stanford Research Institute, derives from

the resemblance of early models (which had a cord attached to the rear

part of the device, suggesting the idea of a tail) to the common mouse.

The first marketed integrated mouse — shipped as a part of a computer

and intended for personal computer navigation — came with the Xerox

8010 Star Information System in 1981.

Technologies

Early mice

Douglas Engelbart at the Stanford Research Institute invented the mouse
in 1963 after extensive usability testing. Several other experimental

pointing-devices developed for Engelbart's oN-Line System (NLS)

exploited different body movements — for example, head-mounted devices

attached to the chin or nose — but ultimately the mouse won out because

of its simplicity and convenience. The first mouse, a bulky device

(pictured) used two gear-wheels perpendicular to each other: the

rotation of each wheel translated into motion along one axis. Engelbart

received patent US3541541 on November 17, 1970 for an "X-Y Position

Indicator for a Display System". At the time, Engelbart envisaged that

users would hold the mouse continuously in one hand and type on a

five-key chord keyset with the other.

Mechanical mice

Operating a mechanical mouse.

1: moving the mouse turns the ball.
2: X and Y rollers grip the ball and transfer movement.
3: Optical encoding disks include light holes.
4: Infrared LEDs shine through the disks.
5: Sensors gather light pulses to convert to X and Y velocities.

Bill English, builder of Engelbart's original mouse,[6] invented the

so-called ball mouse in 1972 while working for Xerox PARC.[7] The

ball-mouse replaced the external wheels with a single ball that could

rotate in any direction. It came as part of the hardware package of the

Xerox Alto computer. Perpendicular chopper wheels housed inside the

mouse's body chopped beams of light on the way to light sensors, thus

detecting in their turn the motion of the ball. This variant of the

mouse resembled an inverted trackball and became the predominant form

used with personal computers throughout the 1980s and 1990s. The Xerox

PARC group also settled on the modern technique of using both hands to

type on a full-size keyboard and grabbing the mouse when required.

The ball mouse utilizes two rollers rolling against two sides of the

ball. One roller detects the horizontal motion of the mouse and other

the vertical motion. The motion of these two rollers causes two

disc-like encoder wheels to rotate, interrupting optical beams to

generate electrical signals. The mouse sends these signals to the

computer system by means of connecting wires. The driver software in

the system converts the signals into motion of the mouse pointer along

X and Y axes on the screen.

Ball mice and wheel mice were manufactured for Xerox by Jack Hawley,

doing business as The Mouse House in Berkeley, California, starting in

1975.

Based on another invention by Jack Hawley, proprietor of the Mouse

House, Honeywell produced another type of mechanical mouse.Instead of a

ball, it had two wheels rotating at off axes. Keytronic later produced

a similar product.

Modern computer mice took form at the École polytechnique fédérale de

Lausanne (EPFL) under the inspiration of Professor Jean-Daniel Nicoud

and at the hands of engineer and watchmaker André Guignard.[13] This

new design incorporated a single hard rubber mouseball and three

buttons, and remained a common design until the mainstream adoption of

the scroll-wheel mouse during the 1990s.

Another type of mechanical mouse, the "analog mouse" (now generally

regarded as obsolete), uses potentiometers rather than encoder wheels,

and is typically designed to be plug-compatible with an analog

joystick. The "Color Mouse," originally marketed by Radio Shack for

their Color Computer (but also usable on MS-DOS machines equipped with

analog joystick ports, provided the software accepted joystick input)

was the best-known example.

Optical mice

An optical mouse uses a light-emitting diode and photodiodes to detect

movement relative to the underlying surface, rather than moving some of

its parts — as in a mechanical mouse.

Early optical mice
Early optical mice, circa 1980, came in two different varieties:

Some, such as those invented by Steve Kirsch[15][16] of Mouse Systems
Corporation, used an infrared LED and a four-quadrant infrared sensor

to detect grid lines printed with infrared absorbing ink on a special

metallic surface. Predictive algorithms in the CPU of the mouse

calculated the speed and direction over the grid.
Others, invented by Richard F. Lyon and sold by Xerox, used a 16-pixel

visible-light image sensor with integrated motion detection on the same

chip and tracked the motion of light dots in a dark field of a

printed paper or similar mouse pad.
These two mouse types had very different behaviors, as the Kirsch mouse

used an x-y coordinate system embedded in the pad, and would not work

correctly when the pad was rotated, while the Lyon mouse used the x-y

coordinate system of the mouse body, as mechanical mice do.

The optical sensor from a Microsoft Wireless IntelliMouse Explorer (v.

1.0A).

Modern optical mice

Modern surface-independent optical mice work by using an optoelectronic

sensor to take successive pictures of the surface on which the mouse

operates. As computing power grew cheaper, it became possible to embed

more powerful special-purpose image-processing chips in the mouse

itself. This advance enabled the mouse to detect relative motion on a

wide variety of surfaces, translating the movement of the mouse into

the movement of the pointer and eliminating the need for a special

mouse-pad. This advance paved the way for widespread adoption of

optical mice. Optical mice illuminate the surface that they track over,

using an LED or a laser diode. Changes between one frame and the next

are processed by the image processing part of the chip and translated

into movement on the two axes using an optical flow estimation

algorithm. For example, the Avago Technologies ADNS-2610 optical mouse

sensor processes 1512 frames per second: each frame consisting of a

rectangular array of 18×18 pixels, and each pixel can sense 64

different levels of gray.

Laser mice

The laser mouse uses an infrared laser diode instead of an LED to
illuminate the surface beneath their sensor. As early as 1998, Sun
Microsystems provided a laser mouse with their Sun SPARCstation servers
and workstations.[20] However, laser mice did not enter the mainstream
market until 2004, when Logitech, in partnership with Agilent

Technologies, introduced its MX 1000 laser mouse.[21] This mouse uses a

small infrared laser instead of an LED and has significantly increased

the resolution of the image taken by the mouse. The laser enables

around 20 times more surface tracking power to the surface features

used for navigation compared to conventional optical mice, via

interference effects. While the implementation of a laser slightly

increases sensitivity and resolution, the main advantage comes from

power usage.

Power-saving in optical mice

Manufacturers often engineer their optical mice — especially

battery-powered wireless models — to save power when possible. In order

to do this, the mouse blinks the laser or LED when in standby-mode

(Each mouse has a different standby time). This function may also

increase the laser / LED life. Mice designed specifically for gamers,

such as the Logitech G5 or the Razer Copperhead, often lack this

feature in an attempt to reduce latency and to improve responsiveness.

Optical versus mechanical mice

The Logitech iFeel optical mouse uses a red LED to project light onto

the tracking surface.Unlike mechanical mice, which can become clogged

with lint, optical mice have no rolling parts; therefore, they do not

require maintenance other than removing debris that might collect under

the light emitter. However, they generally cannot track on glossy and

transparent surfaces, including some mouse-pads, sometimes causing the

cursor to drift unpredictably during operation. Mice with less

image-processing power also have problems tracking fast movement,

though high-end mice can track at 2 m/s (80 inches per second) and

faster.

Some models of laser mice can track on glossy and transparent surfaces,

and have a much higher sensitivity than either their mechanical or

optical counterparts. Such models of laser mice cost more than LED

based or mechanical mice.

As of 2006, mechanical mice have lower average power demands than their

optical counterparts. This typically has no practical impact for users

of cabled mice (except possibly those used with battery-powered

computers, such as notebook models), but has an impact on

battery-powered wireless models.

Optical models will outperform mechanical mice on uneven, slick, soft,

sticky, or loose surfaces, and generally in mobile situations lacking

mouse pads. Because optical mice render movement based on an image

which the LED illuminates, use with multi-colored mouse pads may result

in unreliable performance; however, laser mice do not suffer these

problems and will track on such surfaces. The advent of affordable

high-speed, low-resolution cameras and the integrated logic in optical

mice provides an ideal laboratory for experimentation on

next-generation input-devices. Experimenters can obtain low-cost

components simply by taking apart a working mouse and changing the

optics or by writing new software.

Inertial mice

Inertial mice use a tuning fork or other accelerometer (US Patent

4787051) to detect movement for every axis supported. Usually cordless,

they often have a switch to deactivate the movement circuitry between

use, allowing the user freedom of movement without affecting the

pointer position. A patent for an inertial mouse claims that such mice

consume less power than optically based mice, and offer increased

sensitivity, reduced weight and increased ease-of-use.

3D mice

Also known as flying mice, bats, or wands, these devices generally

function through ultrasound. Probably the best known example would be

3DConnexion/Logitech's SpaceMouse from the early 1990s.

In the late 1990s Kantek introduced the 3D RingMouse. This wireless

mouse was worn on a ring around a finger, which enabled the thumb to

access three buttons. The mouse was tracked in three dimensions by a

base station.Despite a certain appeal, it was finally discontinued

because it did not provide sufficient resolution.

A recent consumer 3D pointing device is the Wii Remote. While primarily

a motion-sensing device (that is, it can determine its orientation and

direction of movement), Wii Remote can also detect its spatial position

by comparing the distance and position of the lights from the IR

emitter using its integrated IR camera (since the nunchuk lacks a

camera, it can only tell its current heading and orientation). The

obvious drawback to this approach is that it can only produce spatial

coordinates while its camera can see the sensor bar.

Double mouse

Double mouse allow for two mice to be used by both hands as input

devices such as when operating various graphics and multimedia

applications.

Connectivity and communication protocols

To transmit their input, typical cabled mice use a thin electrical cord

terminating in a standard connector, such as RS-232C, PS/2, ADB or USB.

Cordless mice instead transmit data via infrared radiation (see IrDA)

or radio (including Bluetooth or WiFi), although many such cordless

interfaces are themselves connected through the aforementioned wired

serial buses.

While the electrical interface and the format of the data transmitted

by commonly available mice is currently standardized on USB, in the

past it varied between different manufacturers. A bus mouse used a

dedicated interface card for connection to an IBM PC or compatible

computer.

Serial interface and protocol

Standard PC mice once used the RS-232C serial port via a D-subminiature
connector, which provided power to run the mouse's circuits as well as

data on mouse movements. The Mouse Systems Corporation version used a

five-byte protocol and supported three buttons. The Microsoft version

used an incompatible three-byte protocol and only allowed for two

buttons. Due to the incompatibility, some manufacturers sold serial

mice with a mode switch: "PC" for MSC mode, "MS" for Microsoft

mode.

PS/2 interface and protocol

With the arrival of the IBM PS/2 personal-computer series in 1987, IBM

introduced the eponymous PS/2 interface for mice and keyboards, which

other manufacturers rapidly adopted. The most visible change was the

use of a round 6-pin mini-DIN, in lieu of the former 5-pin connector.

In default mode (called stream mode) a PS/2 mouse communicates motion,

and the state of each button, by means of 3-byte packets.[28] For any

motion, button press or button release event, a PS/2 mouse sends, over

a bi-directional serial port, a sequence of three bytes, with the

following format:

Bit 7 Bit 6 Bit 5 Bit 4 Bit 3 Bit 2 Bit 1 Bit 0
Byte 1 YV XV YS XS 1 MB RB LB
Byte 2 X movement
Byte 3 Y movement

Here, XS and YS represent the sign bits of the movement vectors, XV and

YV indicate an overflow in the respective vector component, and LB, MB

and RB indicate the status of the left, middle and right mouse buttons

(1 = pressed). PS/2 mice also understand several commands for reset and

self-test, switching between different operating modes, and changing

the resolution of the reported motion vectors.

IntelliMouse and others

A Microsoft IntelliMouse relies on an extension of the PS/2 protocol:

the ImPS/2 or IMPS/2 protocol (the abbreviation combines the concepts

of "IntelliMouse" and "PS/2"). It initially operates in standard PS/2

format, for backwards compatibility. After the host sends a special

command sequence, it switches to an extended format in which a fourth

byte carries information about wheel movements. The IntelliMouse

Explorer works analogously, with the difference that its 4-byte packets

also allow for two additional buttons (for a total of five).

The Typhoon mouse uses 6-byte packets which can appear as a sequence of

two standard 3-byte packets, such that ordinary PS/2 driver can handle

them.

Mouse-vendors also use other extended formats, often without providing

public documentation.

For 3D or 6DOF input, vendors have made many extensions both to the

hardware and to software. In the late 90's Logitech created ultrasound

based tracking which gave 3D input to a few millimeters accuracy, which

worked well as an input device but failed as a money making product.

Apple Desktop Bus

Apple Macintosh Plus mice, 1986.In 1986 Apple first implemented the

Apple Desktop Bus allowing the daisy-chaining together of up to 16

devices, including arbitrarily many mice and other devices on the same

bus with no configuration whatsoever. Featuring only a single data pin,

the bus used a purely polled approach to computer/mouse communications

and survived as the standard on mainstream models (including a number

of non-Apple workstations) until 1998 when iMac began the industry-wide

switch to using USB. Beginning with the "Bronze Keyboard" PowerBook G3

in May 1999, Apple dropped the external ADB port in favor of USB, but

retained an internal ADB connection in the PowerBook G4 for

communication with its built-in keyboard and trackpad until early 2005.

Tactile mice

In 2000, Logitech introduced the "tactile mouse", which contained a
small actuator that made the mouse vibrate. Such a mouse can augment

user-interfaces with haptic feedback, such as giving feedback when

crossing a window boundary. To surf by touch requires the user to be

able to feel depth or hardness; this ability was realized with the

first electrorheological tactile mice[31] but never marketed.

Other unusual variants have included a mouse that a user holds freely

in the hand, rather than on a flat surface, and that detects six

dimensions of motion (the three spatial dimensions, plus rotation on

three axes). Its vendor marketed it for business presentations in which

the speaker stands or walks around. So far, these mice have not

achieved widespread popularity.


Buttons

In contrast to the motion-sensing mechanism, the mouse's buttons have
changed little over the years, varying mostly in shape, number, and

placement. Engelbart's very first mouse had a single button; Xerox PARC

soon designed a three-button model, but reduced the count to two for

Xerox products. After experimenting with 4-button prototypes Apple

reduced it back to one button with the Macintosh in 1984, while Unix

workstations from Sun and others used three buttons. OEM bundled mice

usually have between one and three buttons, although in the aftermarket

many mice have always had five or more.


Apple Mighty Mouse with capacitance triggered buttonsThe three-button

scrollmouse has become the most commonly available design. As of 2007

(and roughly since the late 1990s), users most commonly employ the

second button to invoke a contextual menu in the computer's software

user interface, which contains options specifically tailored to the

interface element over which the mouse pointer currently sits. By

default, the primary mouse button sits located on the left-hand side of

the mouse, for the benefit of right-handed users; left-handed users can

usually reverse this configuration via software.

On systems with three-button mice, pressing the center button (a middle

click) typically opens a system-wide noncontextual menu. In the X

Window System, middle-clicking by default pastes the contents of the

primary buffer at the pointer's position. Many users of two-button mice

emulate a three-button mouse by clicking both the right and left

buttons simultaneously.

Additional buttons

Aftermarket manufacturers have long built mice with five or more

buttons. Depending on the user's preferences and software environment,

the extra buttons may allow forward and backward web-navigation,

scrolling through a browser's history, or other functions, including

mouse related functions like quick-changing the mouse's

resolution/sensitivity. As with similar features in keyboards, however,

not all software supports these functions. The additional buttons

become especially useful in computer games, where quick and easy access

to a wide variety of functions (for example, weapon-switching in

first-person shooters) can give a player an advantage. Because software

can map mouse-buttons to virtually any function, keystroke, application

or switch, extra buttons can make working with such a mouse more

efficient and easier.

In the matter of the number of buttons, Douglas Engelbart favored the

view "as many as possible". The prototype that popularised the idea of

three buttons as standard had that number only because "we could not

find anywhere to fit any more switches".

Wheels

The scroll wheel, a notably different form of mouse-button, consists of

a small wheel that the user can rotate to provide immediate

one-dimensional input. Usually, this input translates into "scrolling"

up or down within the active window or GUI-element . The scroll wheel

can provide convenience, especially when navigating a long document.

The scroll wheel nearly always includes a third (center) button. Under

many Microsoft Windows applications, appropriate pressure on the wheel

activates autoscrolling, and in conjunction with the control key (Ctrl)

may give the capability of zooming in and out; applications that

support this feature include Adobe Reader, Microsoft Word, Internet

Explorer, Opera, Mozilla Firefox and Mulberry. Some applications also

allow the user to scroll left and right by pressing the shift key while

using the mouse wheel.

Note that scrollwheels almost always function more as two switches,

rotating only in discrete "clicks" rather than actually acting as a

third analog axis.

Manufacturers may refer to scroll-wheels by different names for

branding purposes; Genius, for example, usually brand their

scroll-wheel-equipped products "Netscroll".

Mouse Systems introduced the scroll-wheel commercially in 1995,[32]

marketing it as the Mouse Systems ProAgio and Genius EasyScroll.

However, mainstream adoption of the scroll wheel mouse did not occur

until Microsoft released the Microsoft IntelliMouse in 1996. It became

a commercial success in 1997 when their Microsoft Office application

suite and their Internet Explorer browser started supporting its

wheel-scrolling feature.[33] Since then the scroll wheel has become a

standard feature of many mouse models.

Some mouse models have two wheels, separately assigned to horizontal

and vertical scrolling. Designs exist which make use of a "rocker"

button instead of a wheel — a pivoting button that a user can press at

the top or bottom, simulating "up" and "down" respectively. A peculiar

early example was a mouse by Saitek which had a joystick-style

hatswitch on it.

A more recent form of mouse wheel is the tilt-wheel. Tilt wheels are

essentially conventional mouse wheels that have been modified with a

pair of sensors articulated to the tilting mechanism. These sensors are

mapped, by default, to horizontal scrolling.

A third variety of built-in scrolling device, the scroll ball,

essentially consists of a trackball embedded in the upper surface of

the mouse. The user can scroll in all possible directions in very much

the same way as with the actual mouse, and in some mice, can use it as

a trackball. Mice featuring a scroll ball include Apple's Mighty Mouse

and the IOGEAR 4D Web Cruiser Optical Scroll Ball Mouse. IBM's

ergonomics laboratory designed a mouse with a pointing stick in it,[34]

envisioned to be used for scrolling, zooming or (with appropriate

software) controlling a second mouse cursor.

Button techniques

Rollover
Drag
Click
(left) Single-click
(left) Double-click
(left) Triple-click
Right-click
Rocker
Combination of right-click then left-click or keyboard letter
Combination of left-click then right-click or keyboard letter
Combination of left or right-click and the mouse wheel

Common button operations

Select
Launch an application
Display a menu
Drag and drop
Cut/copy to the clipboard
Paste from the clipboard

Mouse speed

The computer industry often measures mouse sensitivity in terms of

counts per inch (CPI), commonly expressed less correctly as dots per

inch (DPI) — the number of steps the mouse will report when it moves

one inch. In early mice, this specification was called pulses per inch

(ppi).If the default mouse-tracking condition involves moving the

pointer by one screen-pixel or dot on-screen per reported step, then

the CPI does equate to DPI: dots of pointer motion per inch of mouse

motion. The CPI or DPI as reported by manufacturers depends on how they

make the mouse; the higher the CPI, the faster the pointer moves with

mouse movement. However, software can adjust the mouse sensitivity,

making the cursor move faster or slower than its DPI. Current software

can change the speed of the pointer dynamically, taking into account

the mouse's absolute speed and the movement from the last stop-point.

Different software may name the settings "acceleration" or "speed" —

referring respectively to "threshold" and "pointer precision".

For simple software, when the mouse starts to move, the software will

count the number of "counts" received from the mouse and will move the

pointer across the screen by that number of pixels (or multiplied by a

factor f1=1,2,3). So, the pointer will move slowly on the screen,

having a good precision. When the movement of the mouse reaches the

value set for "threshold", the software will start to move the pointer

more quickly; thus for each number n of counts received from the mouse,

the pointer may move (f2 x n) pixels, where f2=2,3...10. Usually, the

user can set the value of f2 by changing the "acceleration" setting.

Operating systems sometimes apply acceleration, referred to as

"ballistics", to the motion reported by the mouse. For example,

versions of Windows prior to Windows XP doubled reported values above a

configurable threshold, and then optionally doubled them again above a

second configurable threshold. These doublings applied separately in

the X and Y directions, resulting in very nonlinear response. For

example one can see how the things work in Microsoft Windows NT.

Starting with Windows XP OS version of Microsoft and many OS versions

for Apple Macintosh, computers use a smoother ballistics calculation

that compensates for screen-resolution and has better linearity.

Etymology and plural

The first known publication of the word "mouse" is in Bill English's

1965 publication "Computer-Aided Display Control".

The Compact Oxford English Dictionary (third edition) and the fourth

edition of The American Heritage Dictionary of the English Language

endorse both computer mice and computer mouses as correct plural forms

for computer mouse. The form mice, however, appears most commonly,

while some authors of technical documents may prefer either mouse

devices or the more generic pointing devices. The plural mouses treats

mouse as a "headless noun."


Accessories

Mousepad

Englebart's original mouse did not require a mousepad;[36] the mouse

had two large wheels which could roll on virtually any surface.

However, most subsequent mice starting with the steel roller ball mouse

have needed mousepads in order to perform effectively.

The mousepad, the most common mouse accessory, appears most commonly in

conjunction with mechanical mice, because in order to roll smoothly,

the ball requires more friction than common desk surfaces usually

provide. So-called "hard mousepads" for gamers or optical/laser mice

also exist.

Although most optical and laser mice do not require a pad, some users

find that using a mousepad provides more comfort and less jitter of the

pointer on the display.Whether to use a hard or soft

mousepad with an optical mouse is largely a matter of personal

preference. One exception occurs when the desk surface creates problems

for the optical or laser tracking. Other cases may involve keeping desk

or table surfaces free of scratches and deterioration; when the grain

pattern on the surface causes inaccurate tracking of the pointer, or

when the mouse-user desires a more comfortable mousing surface to work

on and reduced collection of debris under the mouse.

Foot covers

Mouse foot-covers (or foot-pads) consists of low-friction or polished

plastic. This makes the mouse glide with less resistance over a

surface. Some higher quality models have teflon feet to reduce friction

even further.

Mice in the marketplace

Around 1981 Xerox included mice with its Xerox Star, based on the mouse
used in the 1970s on the Alto computer at Xerox PARC. Sun Microsystems,

Symbolics, Lisp Machines Inc., and Tektronix also shipped workstations

with mice, starting in about 1981. Later, inspired by the Star, Apple

Computer released the Apple Lisa, which also used a mouse. However,

none of these products achieved large-scale success. Only with the

release of the Apple Macintosh in 1984 did the mouse see widespread

use.

The Macintosh design, commercially successful and technically

influential, led many other vendors to begin producing mice or

including them with their other computer products (in 1985, Atari ST,

Commodore Amiga, Windows 1.0, and GEOS for the Commodore 64). The

widespread adoption of graphical user interfaces in the software of the

1980s and 1990s made mice all but indispensable for controlling

computers.

Alternative pointing devices

Trackball – the user rolls a ball mounted in a fixed base.

Touchpad – detects finger movement about a sensitive surface — the norm

for modern laptop computers. At least one physical button normally

comes with the touchpad, but users can also (configurably) generate a

click by tapping on the pad. Advanced features include detection of

finger pressure, and scrolling by moving one's finger along an edge.

Pointing stick – a pressure sensitive nub used like a joystick on

laptops, usually found between the g, h, and b keys on the keyboard.
Consumer touchscreen devices exist that resemble monitor shields.

Framed around the monitor, they use software-calibration to match

screen and cursor positions. Many firms that integrate touchscreen

equipment into existing displays and all-in-one devices (such as

portables PCs) for a reasonable fee are also in operation.

Mini-mouse – a small egg-sized mouse for use with laptop computers —

usually small enough for use on a free area of the laptop body itself.

It is generally optical, includes a retractable cord and uses a USB

port to save battery.

Palm mouse – held in the palm and operated with only two buttons; the

movements across the screen correspond to a feather touch, and pressure

increases the speed of movement.

Footmouse – a mouse variant for those who do not wish to or cannot use

the hands (see carpal tunnel) or the head; instead, it provides

footclicks.

Graphics tablet – a tablet with a pen or stylus used for pointing. The

user holds the device like a normal pen and moves it across a special

pad. The thumb usually controls the clicking via a two-way button on

the top of the pen, or by tapping.
Similar to a mouse is a puck, in which rather than tracking the speed

of the device, it tracks the absolute position of a point on the device

(typically a set of crosshairs painted on a transparent plastic tab

sticking out from the top of the puck). Pucks are typically used for

tracing in CAD/CAM/CAE work, and are often accessories for larger

graphics tablets.

Eyeball-controlled – A mouse controlled by the user's eyeball/retina

movements, allowing cursor-manipulation without touch.
Finger-mouse – An extremely small mouse controlled by two fingers only;

the user can hold it in any position

Gyroscopic mouse - A gyroscope senses the movement of the mouse as it

moves through the air. Users can operate a gyroscopic mouse when they

have no room for a regular mouse or must give commands while standing

up. This input device needs no cleaning and can have many extra

buttons, in fact, some laptops doubling as TVs come with gyroscopic

mice that resemble, and double as, remotes with LCD screens built in.
Some high-degree-of-freedom input devices

Applications of mice in user-interfaces

Computer-users usually utilize a mouse to control the motion of a

cursor in two dimensions in a graphical user interface. Clicking or

hovering can select files, programs or actions from a list of names, or

(in graphical interfaces) through pictures called "icons" and other

elements. For example, a text file might be represented by a picture of

a paper notebook, and clicking while the pointer hovers this icon might

cause a text editing program to open the file in a window. (See also

point-and-click)

Users can also employ mice gesturally; meaning that a stylized motion

of the mouse cursor itself, called a "gesture", can issue a command or

map to a specific action. For example, in a drawing program, moving the

mouse in a rapid "x" motion over a shape might delete the shape.

Gestural interfaces occur more rarely than plain pointing-and-clicking;

and people often find them more difficult to use, because they require

finer motor-control from the user. However, a few gestural conventions

have become widespread, including the drag-and-drop gesture, in which:

The user presses the mouse button while the mouse cursor hovers over an

interface object
The user moves the cursor to a different location while holding the

button down
The user releases the mouse button
For example, a user might drag-and-drop a picture representing a file

onto a picture of a trash-can, thus instructing the system to delete

the file.

Other uses of the mouse's input occur commonly in special

application-domains. In interactive three-dimensional graphics, the

mouse's motion often translates directly into changes in the virtual

camera's orientation. For example, in the first-person shooter genre of

games (see below), players usually employ the mouse to control the

direction in which the virtual player's "head" faces: moving the mouse

up will cause the player to look up, revealing the view above the

player's head.

When mice have more than one button, software may assign different

functions to each button. Often, the primary (leftmost in a

right-handed configuration) button on the mouse will select items, and

the secondary (rightmost in a right-handed) button will bring up a menu

of alternative actions applicable to that item. For example, on

platforms with more than one button, the Mozilla web browser will

follow a link in response to a primary button click, will bring up a

contextual menu of alternative actions for that link in response to a

secondary-button click, and will often open the link in a new tab or

window in response to a click with the tertiary (middle) mouse button.


One, two or three buttons?

One button mouseThe issue of whether pack-in bundled mice "should" have

exactly one button or more than one has attracted an enormous amount of

controversy. From the first Macintosh until late 2005 Apple shipped

every computer with a single-button mouse, whereas most other platforms

used multi-button mice. Apple and its advocates promoted single-button

mice as more user-friendly, and portrayed multi-button mice as

confusing for novice users. The Macintosh user interface, by design,

always has and still does make all functions available with a

single-button mouse. Apple's Human Interface Guidelines still specify

that all software-providers need to make functions available with a

single button mouse. However, X Window System applications, which Mac

OS X can also run, have developed with the use of two-button or even

three-button mice in mind, causing even simple operations like "cut and

paste" to become awkward.

While there has always been an aftermarket for mice with two, three, or

more buttons among experienced Macintosh users and extensive

configurable support to complement such devices in all major software

packages on the platform, Mac OS X shipped with hardcoded support for

multi-button mice. On August 2, 2005, Apple introduced their Mighty

Mouse multi-button mouse, which has four independently-programmable

buttons and a trackball-like "scroll ball" which allows the user to

scroll in any direction. Since the mouse uses touch-sensitive

technology, users can treat it as a one-, two-, three-, or four-button

mouse, as desired.

Advocates of multiple-button mice argue that support for a

single-button mouse often leads to clumsy workarounds in interfaces

where a given object may have more than one appropriate action. One

workaround was the double click, first used on the Apple Lisa, to allow

both the "select" and "open" operation to be performed with a single

button. Several common workarounds exist, and some are specified by the

Apple Human Interface Guidelines.


Three-button mouseOne such workaround (that favored on Apple platforms)

has the user hold down one or more keys on the keyboard before pressing

the mouse button (typically control on a Macintosh for contextual

menus). This has the disadvantage that it requires that both the user's

hands be engaged. It also requires that the user perform actions on

completely separate devices in concert; that is, holding a key on the

keyboard while pressing a button on the mouse. This can be a difficult

task for a disabled user, although can be remedied by allowing keys to

stick so that they do not need to be pressed down.

Another involves the press-and-hold technique. In a press-and-hold, the

user presses and holds the single button. After a certain period,

software perceives the button press not as a single click but as a

separate action. This has two drawbacks: first, a slow user may

press-and-hold inadvertently. Second, the user must wait for the

software to detect the click as a press-and-hold, otherwise the system

might interpret the button-depression as a single click. Furthermore,

the remedies for these two drawbacks conflict with each other: the

longer the lag time, the more the user must wait; and the shorter the

lag time, the more likely it becomes that some user will accidentally

press-and-hold when meaning to click. Studies have found all of the

above workarounds less usable than additional mouse buttons for

experienced users.

Most machines running Unix or a Unix-like operating system run the X

Window System which almost always encourages a three-button mouse. X

numbers the buttons by convention. This allows user instructions to

apply to mice or pointing devices that do not use conventional button

placement. For example, a left handed user may reverse the buttons,

usually with a software setting. With non-conventional button

placement, user directions that say "left mouse button" or "right mouse

button" are confusing. The ground-breaking Xerox Parc Alto and Dorado

computers from the mid-1970s used three-button mice, and each button

was assigned a color. Red was used for the left (or primary) button,

yellow for the middle (secondary), and blue for the right (meta or

tertiary). This naming convention lives on in some SmallTalk

environments, such as Squeak, and can be less confusing than the right,

middle and left designations.

Acorn's RISC OS based computers necessarily use all three mouse buttons

throughout their WIMP based GUI. RISC OS refers to the three buttons

(from left to right) as Select, Menu and Adjust. Select functions in

the same way as the "Primary" mouse button in other operating systems.

Menu will bring up a context-sensitive menu appropriate for the

position of the mouse pointer, and this often provides the only means

of activating this menu. This menu in most applications equates to the

"Application Menu" found at the top of the screen in Mac OS, and

underneath the window title under Microsoft Windows. Adjust serves for

selecting multiple items in the "Filer" desktop, and for altering

parameters of objects within applications — although its exact function

usually depends on the programmer.


Mice in gaming

Mice often function as an interface for PC-based computer games and

sometimes for video game consoles. They often appear in combination

with the keyboard.


First-person shooters

Logitech G5 Laser Mouse designed for gaming.Due to the cursor-like

nature of the crosshairs in shooter games, a combination of mouse and

keyboard provides a popular way to play first-person shooter (FPS)

games. Players use the X-axis of the mouse for looking (or turning)

left and right, leaving the Y-axis for looking up and down. The left

button usually controls primary fire. Many gamers prefer this over a

gamepad or joystick because it allows them to look around easily,

quickly and accurately and also as a consequence aim without auto-aim

assist. If the game supports multiple fire-modes, the right button

often provides secondary fire from the selected weapon. Secondary

weapons include grenades, knives, etc. The right button may also

provide bonus options for a particular weapon, such as allowing access

to the scope of a sniper rifle or allowing the mounting of a bayonet or

silencer or sometimes even jumping.

Gamers can use a scroll wheel for changing weapons, or for controlling

scope-zoom magnification. On most FPS games, programming may also

assign more functions to additional buttons on mice with more than

three controls. A keyboard usually controls movement (for example,

WASD, for moving forward, left, backward and right, respectively) and

other functions such as changing posture. Since the mouse serves for

aiming, a mouse that tracks movement accurately and with less lag

(latency) will give a player an advantage over players with less

accurate or slower mice.

An early technique of players, circle-strafing, saw a player

continuously strafing while aiming and shooting at an opponent by

walking in circle around the opponent with the opponent at the center

of the circle. Players could achieve this by holding down a key for

strafing while continuously aiming the mouse towards the opponent.

Games using mice for input have such a degree of popularity that many

manufacturers, such as Logitech, and Razer USA Ltd, make peripherals

such as mice and keyboards specifically for gaming. Such devices

frequently feature (in the case of mice) adjustable weights,

high-resolution optical or laser components, additional buttons,

ergonomic shape, and other features such as adjustable DPI.


Invert mouse setting

Many games, such as first- or third-person shooters, have a setting

named "invert mouse" or similar (not to be confused with "button

inversion", sometimes performed by left-handed users) which allows the

user to look downward by moving the mouse forward and upward by moving

the mouse backward (the opposite of non-inverted movement). This

control system resembles that of aircraft control sticks, where pulling

back causes pitch up and pushing forward causes pitch down; computer

joysticks also typically emulate this control-configuration.

After id Software's Doom, the game that popularized FPS games but which

did not support vertical aiming with a mouse (the y-axis served for

forward/backward movement), competitor 3D Realms' Duke Nukem 3D became

one of the first games that supported using the mouse to aim up and

down. It and other games using the Build engine had an option to invert

the Y-axis. The "invert" feature actually made the mouse behave in a

manner that users now regard as non-inverted (by default, moving mouse

forward resulted in looking down). Soon after, id Software released

Quake, which introduced the invert feature as users now know it. Other

games using the Quake engine have come on the market following this

standard, likely due to the overall popularity of Quake.

Home consoles

In 1988 the educational video game system, the VTech Socrates, featured

a wireless mouse with an attached mouse pad as an optional controller

used for some games. In the early 1990s the Super Nintendo

Entertainment System video game system featured a mouse in addition to

its controllers. The Mario Paint game in particular used the mouse's

capabilities, as did its successor on the N64. Sony Computer

Entertainment released an official mouse product for the PlayStation

console, and included one along with the Linux for PlayStation 2 kit.

However, users can attach virtually any USB mouse to the PlayStation 2

console. In addition the PlayStation 3, and Xbox 360 also support USB

mice. Recently the Wii also has this latest development added on in a

recent software update.

HARD DISK DRIVES

A hard disk drive (HDD), commonly referred to as a hard drive, hard disk or fixed disk drive,[1] is a non-volatile storage device which stores digitally encoded data on rapidly rotating platters with magnetic surfaces. Strictly speaking, "drive" refers to a device distinct from its medium, such as a tape drive and its tape, or a floppy disk drive and its floppy disk. Early HDDs had removable media; however, an HDD today is typically a sealed unit with fixed media.[2]

HDDs were originally developed for use with computers. In the 21st century, applications for HDDs have expanded beyond computers to include digital video recorders, digital audio players, personal digital assistants, digital cameras and video game consoles. In 2005 the first mobile phones to include HDDs were introduced by Samsung and Nokia.[3] The need for large-scale, reliable storage, independent of a particular device, led to the introduction of configurations such as RAID arrays, network attached storage (NAS) systems and storage area network (SAN) systems that provide efficient and reliable access to large volumes of data.

Technology

HDDs record data by magnetizing a ferromagnetic material directionally, to represent either a 0 or a 1 binary digit. They read the data back by detecting the magnetization of the material. A typical HDD design consists of a spindle which holds one or more flat circular disks called platters, onto which the data is recorded. The platters are made from a non-magnetic material, usually glass or aluminum, and are coated with a thin layer of magnetic material. Older disks used iron(III) oxide as the magnetic material, but current disks use a cobalt-based alloy.


A hard disk drive with the disks and spindle motor hub removed. In the center, the internal structure of the spindle motor can be seen. To the left of center is the actuator arm with a read-write head under the tip of its very end (near center); the orange wires along the side of the arm are part of the path the signals take to and from the read-write head. The flexible, somewhat 'U'-shaped, ribbon cable barely visible below and to the left of the actuator arm is another part of its path connecting the head to the controller board on the opposite side.
A cross section of the magnetic surface in action. In this case the binary data encoded using frequency modulation:The platters are spun at very high speeds. Information is written to a platter as it rotates past mechanisms called read-and-write heads that operate very close over the magnetic surface. The read-and-write head is used to detect and modify the magnetization of the material immediately under it. There is one head for each magnetic platter surface on the spindle, mounted on a common arm. An actuator arm (or access arm) moves the heads on an arc (roughly radially) across the platters as they spin, allowing each head to access almost the entire surface of the platter as it spins. The arm is moved using a voice coil actuator or (in older designs) a stepper motor.

The magnetic surface of each platter is divided into many small sub-micrometre-sized magnetic regions, each of which is used to encode a single binary unit of information. In today's HDDs each of these magnetic regions is composed of a few hundred magnetic grains. Each magnetic region forms a magnetic dipole which generates a highly localized magnetic field nearby. The write head magnetizes a magnetic region by generating a strong local magnetic field nearby. Early HDDs used an electromagnet both to generate this field and to read the data by using electromagnetic induction. Later versions of inductive heads included metal in Gap (MIG) heads and thin film heads. In today's heads, the read and write elements are separate but in close proximity on the head portion of an actuator arm. The read element is typically magneto-resistive while the write element is typically thin-film inductive.[4]

In modern drives, the small size of the magnetic regions creates the danger that their magnetic state be lost because of thermal effects. To counter this, the platters are coated with two parallel magnetic layers, separated by a 3-atom-thick layer of the non-magnetic element ruthenium, and the two layers are magnetized in opposite orientation, thus reinforcing each other.[5] Another technology used to overcome thermal effects to allow greater recording densities is perpendicular recording, which has been used in some hard drives as of 2006.

Hard disk drives are sealed to prevent dust and other sources of contamination from interfering with the operation of the hard disks heads. The hard drives are not air tight, but rather utilize an extremely fine air filter, to allow for air inside the hard drive enclosure. The spinning of the disks causes the air to circulate forcing any particulates to become trapped on the filter. The same air currents also act as a gas bearing which enables the heads to float on a cushion of air above the surfaces of the disks.

Hard drives are precise devices, moving at very high speed, and a number of analogies have been made to try to describe this. One states:

“ As an analogy, a magnetic head slider flying over a disk surface with a flying height of 25 nm with a relative speed of 20 meters/second is equivalent to an aircraft flying at a physical spacing of 0.2 µm at 900 kilometers/hour. This is what a disk drive experiences during its operation.


Capacity and access speed

Using rigid disks and sealing the unit allows much tighter tolerances than in a floppy disk drive. Consequently, hard disk drives can store much more data than floppy disk drives and can access and transmit it faster. In 2007, a typical “enterprise”, i.e. workstation HDD, might store between 160 GB and 1 TB of data (as of local US market by July 2007), rotate at 7,200 or 10,000 revolutions per minute (RPM) and have a media transfer rate of over 1 Gbit/s or higher.[6] The fastest “enterprise” HDDs spin at 15,000 rpm, and can achieve sequential media transfer speeds above 1.6 Gbit/s.[7] Mobile, i.e., laptop HDDs, which are physically smaller than their desktop and enterprise counterparts, tend to be slower and have less capacity. In the 1990s, most spun at 4,200 rpm.[8] In 2007, a typical mobile HDD spins at 5,400 rpm, with 7,200 rpm models available for a slight price premium.

The exponential increases in disk space and data access speeds of HDDs have enabled the commercial viability of consumer products that require large storage capacities, such as digital video recorders and digital audio players.[9] In addition, the availability of vast amounts of cheap storage has made viable a variety of web-based services with extraordinary capacity requirements, such as free-of-charge web search and email (Google, Yahoo!, etc.).

The main way to decrease access time is to increase rotational speed, while the main way to increase throughput and storage capacity is to increase areal density. A vice president of Seagate Technology projects a future growth in disk density of 40% per year.[10] Access times have not kept up with throughput increases, which themselves have not kept up with growth in storage capacity.

As of 2006, some disk drives use perpendicular recording technology to increase recording density and throughput.[11]

The first 3.5" HDD marketed as able to store 1 TB was the Hitachi Deskstar 7K1000. It contains five platters at approximately 200 GB each, providing 935.5 GiB of usable space.[12] Hitachi has since been joined by Samsung (Samsung SpinPoint F1, which has 3 × 334 GB platters), Seagate and Western Digital in the 1 TB drive market.[13][14]

Form factor Width Largest capacity Platters (Max)

5.25" FH 146 mm 47 GB[15] (1998) 14
5.25" HH 146 mm 19.3 GB[16] (1998) 4[17]
3.5" 102 mm 1 TB[12] (2007) 5
2.5" 69.9 mm 320 GB[18] (2007) 3
1.8" (PCMCIA) 54 mm 160 GB[19] (2007)
1.8" (ATA-7 LIF) 53.8 mm


Capacity measurements

A disassembled and labeled 1997 hard drive.The capacity of an HDD can be calculated by multiplying the number of cylinders by the number of heads by the number of sectors by the number of bytes/sector (most commonly 512). Drives with ATA interface bigger and more than eight gigabytes behave as if they were structured into 16383 cylinders, 16 heads, and 63 sectors, for compatibility with older operating systems. Unlike in the 1980s, the cylinder, head, sector counts reported to the CPU by a modern ATA drive are no longer actual physical parameters since the reported numbers are constrained by historic operating-system interfaces and with zone bit recording the actual number of sectors varies by zone. Disks with SCSI interface address each sector with a unique integer number; the operating system remains ignorant of their head or cylinder count.

Hard disk drive manufacturers specify disk capacity using the SI prefixes mega-, giga- and tera-, and their abbreviations M, G and T. Byte is typically abbreviated B.

Some operating-system tools report capacity using the same abbreviations but actually use binary prefixes. For instance, the prefix mega-, which normally means 106 (1,000,000), in the context of data storage can mean 220 (1,048,576), which is nearly 5% more. Similar usage has been applied to prefixes of greater magnitude. This results in a discrepancy between the disk manufacturer's stated capacity and the apparent capacity of the drive when examined through some operating-system tools. The difference becomes with 7% even more noticeable for a gigabyte. For example, Microsoft Windows reports disk capacity both in decimal-based units to 12 or more significant digits and with binary-based units to three significant digits. Thus a disk specified by a disk manufacturer as a 30 GB disk might have its capacity reported by Windows 2000 both as "30,065,098,568 bytes" and "28.0 GB". The disk manufacturer used the SI definition of "giga", 109 to arrive at 30 GB; however, because the utilities provided by Windows define a gigabyte as 1,073,741,824 bytes (230 bytes, often referred to as a gibibyte, or GiB), the operating system reports capacity of the disk drive as (only) 28.0 GB.


Form factors

5¼" full height 110 MB HDD,
2½" 8.5 mm 6495 MB HDD,
US/UK pennies for comparisonThe earliest “form factor” hard disk drives inherited their dimensions from floppy-disk drives (FDDs), so that either could be mounted in chassis slots, and thus the HDD form factors became colloquially named after the corresponding FDD types. "Form factor" compatibility continued after the 3½ in size even though floppy disk drives with new smaller dimensions ceased to be offered.

"8 inch" drive: (9.5 in x 4.624 in x 14.25 in = 241.3 mm x 117.5 mm x 362 mm)
In 1979, Shugart Associates' SA1000 was the first form factor compatible HDD, having the same dimensions and a compatible interface to the 8" FDD. Both "full height" and "half height" (2.313 in) versions were available.
"5¼ inch" drive: (5.75 in x 1.63 in x 8 in = 146.1 mm x 41.4 mm x 203 mm)
This smaller form factor, first used in an HDD by Seagate in 1980, was the same size as full height 5¼-inch diameter FDD, i.e., 3.25 inches high. This is twice as high as commonly used today; i.e., 1.63 in = 41.4 mm (“half height”). Most desktop models of drives for optical 120 mm disks (DVD, CD) use the half height 5¼" dimension, but it fell out of fashion for HDDs. The Quantum “Bigfoot” HDD was the last to use it in the late 1990s, with “low-profile” (~25 mm) and “ultra-low-profile” (~20 mm) high versions.
"3½ inch" drive: (4 in x 1 in x 5.75 in = 101.6 mm x 25.4 mm x 146 mm)
This smaller form factor, first used in an HDD by Rodime in 1984, was the same size as the "half height" 3½ FDD, 1.e., 1.63 inches high. Today has been largely superseded by 1-inch high “slimline” or “low-profile” versions of this form factor which is used by most desktop HDDs.
"2½ inch" drive: ((2.75 in x 0.374 in x 3.945 in = 69.85 mm x 9.5 mm x 100 mm)
This smaller form factor was introduced by PrairieTek in 1988; there is no corresponding FDD. It is widely used today for hard-disk drives in mobile devices (laptops, music players, etc.). Today, the dominant height of this form factor is 9.5 mm, but there were also 19 mm, 17 mm, and 12.5 mm high variants in use.
"1.8 inch" drive: (54 mm × 8 mm × 71 mm)
This form factor, originally introduced by Integral Peripherals in 1993, has evolved into the ATA-7 LIF with dimensions as stated. It is increasingly used in digital audio players and subnotebooks. An original variant exists for 2–5 GB sized HDDs that fit directly into a PC card expansion slot.
"1 inch" drive: (42.8 mm × 5 mm × 36.4 mm)
This form factor was introduced in 1999 as IBM's Microdrive to fit inside a CF Type II slot.
"0.85 inch" drive: (24 mm × 5 mm × 32 mm)
Toshiba announced this form factor in January 2004[20] for use in mobile phones and similar applications, including SD/MMC slot compatible HDDs optimized for video storage on 4G handsets. Toshiba currently sells a 4 GB (MK4001MTD) and 8 GB (MK8003MTD) version[3] and holds the Guinness World Record for the smallest harddisk drive.[21]
Major manufacturers discontinued the development of new products for the 1-inch and 0.85 inch form factors in 2007, due to falling prices of flash memory.[22]

The inch-based nickname of all these form factors usually do not indicate any actual product dimension (which are for more recent form factors specified in millimeters), but just roughly indicate a size relative to disk diameters, in the interest of historic continuity.


Other characteristics

Capacity of a hard disk drive is usually quoted in gigabytes. Older HDDs quoted their smaller capacities in megabytes.

The data transfer rate at the inner zone ranges from 44.2 MB/s to 74.5 MB/s, while the transfer rate at the outer zone ranges from 74.0 MB/s to 111.4 MB/s. An HDD's random access time ranges from 5 ms to 15 ms.

Integrity

An IBM HDD head resting on a disk platter. Since the drive is not in operation, the head is simply pressed against the disk by the suspension.
Close-up of a hard disk head resting on a disk platter, and its suspension. A reflection of the head and suspension are visible beneath on the mirror-like disk.Due to the extremely close spacing between the heads and the disk surface, any contamination of the read-write heads or platters can lead to a head crash — a failure of the disk in which the head scrapes across the platter surface, often grinding away the thin magnetic film and causing data loss. Head crashes can be caused by electronic failure, a sudden power failure, physical shock, wear and tear, corrosion, or poorly manufactured platters and heads.

The HDD's spindle system relies on air pressure inside the enclosure to support the heads at their proper flying height while the disk rotates. An HDD requires a certain range of air pressures in order to operate properly. The connection to the external environment and pressure occurs through a small hole in the enclosure (about 0.5 mm in diameter), usually with a carbon filter on the inside (the breather filter, see below). If the air pressure is too low, then there is not enough lift for the flying head, so the head gets too close to the disk, and there is a risk of head crashes and data loss. Specially manufactured sealed and pressurized disks are needed for reliable high-altitude operation, above about 10,000 feet (3,000 m). Note that modern commercial aircraft have a pressurized cabin, whose pressure altitude does not normally exceed 8,500 feet - thus, ordinary hard drives can safely be used in flight. Modern disks include temperature sensors and adjust their operation to the operating environment. Breather holes can be seen on all disks — they usually have a sticker next to them, warning the user not to cover the holes. The air inside the operating disk is constantly moving too, being swept in motion by friction with the spinning platters. This air passes through an internal recirculation (or "recirc") filter to remove any leftover contaminants from manufacture, any particles or chemicals that may have somehow entered the enclosure, and any particles or outgassing generated internally in normal operation. Very high humidity for extended periods can corrode the heads and platters.

For giant magnetoresistive (GMR) heads in particular, a minor head crash from contamination (that does not remove the magnetic surface of the disk) still results in the head temporarily overheating, due to friction with the disk surface, and can render the data unreadable for a short period until the head temperature stabilizes (so called "thermal asperity," a problem which can partially be dealt with by proper electronic filtering of the read signal).

The hard disk's electronics control the movement of the actuator and the rotation of the disk, and perform reads and writes on demand from the disk controller. Modern disk firmware is capable of scheduling reads and writes efficiently on the platter surfaces and remapping sectors of the media which have failed.


Disk failures and their metrics

Most major hard disk and motherboard vendors now support self-monitoring, analysis and reporting technology (S.M.A.R.T.), which attempts to alert users to impending failures.

However, not all failures are predictable. Normal use eventually can lead to a breakdown in the inherently fragile device, which makes it essential for the user to periodically back up the data onto a separate storage device. Failure to do so can lead to the loss of data. While it may be possible to recover lost information, it is normally an extremely costly procedure, and it is not possible to guarantee success. A 2007 study published by Google suggested very little correlation between failure rates and either high temperature or activity level.[26] While several S.M.A.R.T. parameters have an impact on failure probability, a large fraction of failed drives do not produce predictive S.M.A.R.T. parameters.[26] S.M.A.R.T. parameters alone may not be useful for predicting individual drive failures.[26]

SCSI, SAS and FC drives are typically more expensive and are traditionally used in servers and disk arrays, whereas inexpensive ATA and SATA drives evolved in the home computer market and were perceived to be less reliable. This distinction is now becoming blurred.

The mean time between failures (MTBF) of SATA drives is usually about 600,000 hours (some drives such as Western Digital Raptor have rated 1.2 million hours MTBF), while SCSI drives are rated for upwards of 1.5 million hours.[citation needed] However, independent research indicates that MTBF is not a reliable estimate of a drive's longevity.[27] MTBF is conducted in laboratory environments in test chambers and is an important metric to determine the quality of a disk drive before it enters high volume production. Once the drive product is in production, the more valid metric is annualized failure rate (AFR). AFR is the percentage of real-world drive failures after shipping.

SAS drives are comparable to SCSI drives, with high MTBF and high reliability.

Enterprise SATA drives designed and produced for enterprise markets, unlike standard SATA drives, have reliability comparable to other enterprise class drives.

Typically enterprise drives (all enterprise drives, including SCSI, SAS, enterprise SATA and FC) experience between .70%-.78% annual failure rates from the total installed drives.

HISTOY OF COMPUTER

It is difficult to identify any one device as the earliest computer, partly because the term "computer" has been subject to varying interpretations over time.

Originally, the term "computer" referred to a person who performed numerical calculations (a human computer), often with the aid of a mechanical calculating device. Examples of early mechanical computing devices included the abacus, the slide rule and arguably the astrolabe and the Antikythera mechanism (which dates from about 150-100 BC). The end of the Middle Ages saw a re-invigoration of European mathematics and engineering, and Wilhelm Schickard's 1623 device was the first of a number of mechanical calculators constructed by European engineers.

However, none of those devices fit the modern definition of a computer because they could not be programmed. In 1801, Joseph Marie Jacquard made an improvement to the textile loom that used a series of punched paper cards as a template to allow his loom to weave intricate patterns automatically. The resulting Jacquard loom was an important step in the development of computers because the use of punched cards to define woven patterns can be viewed as an early, albeit limited, form of programmability.

In 1837, Charles Babbage was the first to conceptualize and design a fully programmable mechanical computer that he called "The Analytical Engine".[3] Due to limited finance, and an inability to resist tinkering with the design, Babbage never actually built his Analytical Engine.

Large-scale automated data processing of punched cards was performed for the U.S. Census in 1890 by tabulating machines designed by Herman Hollerith and manufactured by the Computing Tabulating Recording Corporation, which later became IBM. By the end of the 19th century a number of technologies that would later prove useful in the realization of practical computers had begun to appear: the punched card, Boolean algebra, the vacuum tube (thermionic valve) and the teleprinter.

During the first half of the 20th century, many scientific computing needs were met by increasingly sophisticated analog computers, which used a direct mechanical or electrical model of the problem as a basis for computation. However, these were not programmable and generally lacked the versatility and accuracy of modern digital computers.


A succession of steadily more powerful and flexible computing devices were constructed in the 1930s and 1940s, gradually adding the key features that are seen in modern computers. The use of digital electronics (largely invented by Claude Shannon in 1937) and more flexible programmability were vitally important steps, but defining one point along this road as "the first digital electronic computer" is difficult (Shannon 1940). Notable achievements include:


EDSAC was one of the first computers to implement the stored program (von Neumann) architecture.Konrad Zuse's electromechanical "Z machines". The Z3 (1941) was the first working machine featuring binary arithmetic, including floating point arithmetic and a measure of programmability. In 1998 the Z3 was proved to be Turing complete, therefore being the world's first operational computer.
The non-programmable Atanasoff–Berry Computer (1941) which used vacuum tube based computation, binary numbers, and regenerative capacitor memory.
The secret British Colossus computer (1944), which had limited programmability but demonstrated that a device using thousands of tubes could be reasonably reliable and electronically reprogrammable. It was used for breaking German wartime codes.
The Harvard Mark I (1944), a large-scale electromechanical computer with limited programmability.
The U.S. Army's Ballistics Research Laboratory ENIAC (1946), which used decimal arithmetic and is sometimes called the first general purpose electronic computer (since Konrad Zuse's Z3 of 1941 used electromagnets instead of electronics). Initially, however, ENIAC had an inflexible architecture which essentially required rewiring to change its programming.
Several developers of ENIAC, recognizing its flaws, came up with a far more flexible and elegant design, which came to be known as the stored program architecture or von Neumann architecture. This design was first formally described by John von Neumann in the paper "First Draft of a Report on the EDVAC", published in 1945. A number of projects to develop computers based on the stored program architecture commenced around this time, the first of these being completed in Great Britain. The first to be demonstrated working was the Manchester Small-Scale Experimental Machine (SSEM) or "Baby". However, the EDSAC, completed a year after SSEM, was perhaps the first practical implementation of the stored program design. Shortly thereafter, the machine originally described by von Neumann's paper—EDVAC—was completed but did not see full-time use for an additional two years.

Nearly all modern computers implement some form of the stored program architecture, making it the single trait by which the word "computer" is now defined. By this standard, many earlier devices would no longer be called computers by today's definition, but are usually referred to as such in their historical context. While the technologies used in computers have changed dramatically since the first electronic, general-purpose computers of the 1940s, most still use the von Neumann architecture. The design made the universal computer a practical reality.


Microprocessors are miniaturized devices that often implement stored program CPUs.Vacuum tube-based computers were in use throughout the 1950s, but were largely replaced in the 1960s by transistor-based devices, which were smaller, faster, cheaper, used less power and were more reliable. These factors allowed computers to be produced on an unprecedented commercial scale. By the 1970s, the adoption of integrated circuit technology and the subsequent creation of microprocessors such as the Intel 4004 caused another leap in size, speed, cost and reliability. By the 1980s, computers had become sufficiently small and cheap to replace simple mechanical controls in domestic appliances such as washing machines. Around the same time, computers became widely accessible for personal use by individuals in the form of home computers and the now ubiquitous personal computer. In conjunction with the widespread growth of the Internet since the 1990s, personal computers are becoming as common as the television and the telephone and almost all modern electronic devices contain a computer of some kind.


Stored program architecture

The defining feature of modern computers which distinguishes them from all other machines is that they can be programmed. That is to say that a list of instructions (the program) can be given to the computer and it will store them and carry them out at some time in the future.

In most cases, computer instructions are simple: add one number to another, move some data from one location to another, send a message to some external device, etc. These instructions are read from the computer's memory and are generally carried out (executed) in the order they were given. However, there are usually specialized instructions to tell the computer to jump ahead or backwards to some other place in the program and to carry on executing from there. These are called "jump" instructions (or branches). Furthermore, jump instructions may be made to happen conditionally so that different sequences of instructions may be used depending on the result of some previous calculation or some external event. Many computers directly support subroutines by providing a type of jump that "remembers" the location it jumped from and another instruction to return to the instruction following that jump instruction.

Program execution might be likened to reading a book. While a person will normally read each word and line in sequence, they may at times jump back to an earlier place in the text or skip sections that are not of interest. Similarly, a computer may sometimes go back and repeat the instructions in some section of the program over and over again until some internal condition is met. This is called the flow of control within the program and it is what allows the computer to perform tasks repeatedly without human intervention.

Comparatively, a person using a pocket calculator can perform a basic arithmetic operation such as adding two numbers with just a few button presses. But to add together all of the numbers from 1 to 1,000 would take thousands of button presses and a lot of time—with a near certainty of making a mistake. On the other hand, a computer may be programmed to do this with just a few simple instructions. For example:

mov #0,sum ; set sum to 0
mov #1,num ; set num to 1
loop: add num,sum ; add num to sum
add #1,num ; add 1 to num
cmp num,#1000 ; compare num to 1000
ble loop ; if num <= 1000, go back to 'loop'
halt ; end of program. stop running
Once told to run this program, the computer will perform the repetitive addition task without further human intervention. It will almost never make a mistake and a modern PC can complete the task in about a millionth of a second.[4]

However, computers cannot "think" for themselves in the sense that they only solve problems in exactly the way they are programmed to. An intelligent human faced with the above addition task might soon realize that instead of actually adding up all the numbers one can simply use the equation


and arrive at the correct answer (500,500) with little work.[5] In other words, a computer programmed to add up the numbers one by one as in the example above would do exactly that without regard to efficiency or alternative solutions.

PROGRAMS

In practical terms, a computer program might include anywhere from a dozen instructions to many millions of instructions for something like a word processor or a web browser. A typical modern computer can execute billions of instructions every second and nearly never make a mistake over years of operation.

Large computer programs may take teams of computer programmers years to write and the probability of the entire program having been written completely in the manner intended is unlikely. Errors in computer programs are called bugs. Sometimes bugs are benign and do not affect the usefulness of the program, in other cases they might cause the program to completely fail (crash), in yet other cases there may be subtle problems. Sometimes otherwise benign bugs may be used for malicious intent, creating a security exploit. Bugs are usually not the fault of the computer. Since computers merely execute the instructions they are given, bugs are nearly always the result of programmer error or an oversight made in the program's design.[6]

In most computers, individual instructions are stored as machine code with each instruction being given a unique number (its operation code or opcode for short). The command to add two numbers together would have one opcode, the command to multiply them would have a different opcode and so on. The simplest computers are able to perform any of a handful of different instructions, the more complex computers have several hundred to choose from—each with a unique numerical code. Since the computer's memory is able to store numbers, it can also store the instruction codes. This leads to the important fact that entire programs (which are just lists of instructions) can be represented as lists of numbers and can themselves be manipulated inside the computer just as if they were numeric data. The fundamental concept of storing programs in the computer's memory alongside the data they operate on is the crux of the von Neumann, or stored program, architecture. In some cases, a computer might store some or all of its program in memory that is kept separate from the data it operates on. This is called the Harvard architecture after the Harvard Mark I computer. Modern von Neumann computers display some traits of the Harvard architecture in their designs, such as in CPU caches.

While it is possible to write computer programs as long lists of numbers (machine language) and this technique was used with many early computers,[7] it is extremely tedious to do so in practice, especially for complicated programs. Instead, each basic instruction can be given a short name that is indicative of its function and easy to remember—a mnemonic such as ADD, SUB, MULT or JUMP. These mnemonics are collectively known as a computer's assembly language. Converting programs written in assembly language into something the computer can actually understand (machine language) is usually done by a computer program called an assembler. Machine languages and the assembly languages that represent them (collectively termed low-level programming languages) tend to be unique to a particular type of computer. For instance, an ARM architecture computer (such as may be found in a PDA or a hand-held videogame) cannot understand the machine language of an Intel Pentium or the AMD Athlon 64 computer that might be in a PC.[8]

Though considerably easier than in machine language, writing long programs in assembly language is often difficult and error prone. Therefore, most complicated programs are written in more abstract high-level programming languages that are able to express the needs of the computer programmer more conveniently (and thereby help reduce programmer error). High level languages are usually "compiled" into machine language (or sometimes into assembly language and then into machine language) using another computer program called a compiler.[9] Since high level languages are more abstract than assembly language, it is possible to use different compilers to translate the same high level language program into the machine language of many different types of computer. This is part of the means by which software like video games may be made available for different computer architectures such as personal computers and various video game consoles.

The task of developing large software systems is an immense intellectual effort. It has proven, historically, to be very difficult to produce software with an acceptably high reliability, on a predictable schedule and budget. The academic and professional discipline of software engineering concentrates specifically on this problem.

EXAMPLE
Suppose a computer is being employed to drive a traffic light. A simple stored program might say:

Turn off all of the lights
Turn on the red light
Wait for sixty seconds
Turn off the red light
Turn on the green light
Wait for sixty seconds
Turn off the green light
Turn on the yellow light
Wait for two seconds
Turn off the yellow light
Jump to instruction number (2)
With this set of instructions, the computer would cycle the light continually through red, green, yellow and back to red again until told to stop running the program.

However, suppose there is a simple on/off switch connected to the computer that is intended be used to make the light flash red while some maintenance operation is being performed. The program might then instruct the computer to:

Turn off all of the lights
Turn on the red light
Wait for sixty seconds
Turn off the red light
Turn on the green light
Wait for sixty seconds
Turn off the green light
Turn on the yellow light
Wait for two seconds
Turn off the yellow light
If the maintenance switch is NOT turned on then jump to instruction number 2
Turn on the red light
Wait for one second
Turn off the red light
Wait for one second
Jump to instruction number 11
In this manner, the computer is either running the instructions from number (2) to (11) over and over or its running the instructions from (11) down to (16) over and over, depending on the position of the switch.[10]


How computers work
Main articles: Central processing unit and Microprocessor
A general purpose computer has four main sections: the arithmetic and logic unit (ALU), the control unit, the memory, and the input and output devices (collectively termed I/O). These parts are interconnected by busses, often made of groups of wires.

The control unit, ALU, registers, and basic I/O (and often other hardware closely linked with these) are collectively known as a central processing unit (CPU). Early CPUs were composed of many separate components but since the mid-1970s CPUs have typically been constructed on a single integrated circuit called a microprocessor.

Control unit

The ALU is capable of performing two classes of operations: arithmetic and logic.

The set of arithmetic operations that a particular ALU supports may be limited to adding and subtracting or might include multiplying or dividing, trigonometry functions (sine, cosine, etc) and square roots. Some can only operate on whole numbers (integers) whilst others use floating point to represent real numbers—albeit with limited precision. However, any computer that is capable of performing just the simplest operations can be programmed to break down the more complex operations into simple steps that it can perform. Therefore, any computer can be programmed to perform any arithmetic operation—although it will take more time to do so if its ALU does not directly support the operation. An ALU may also compare numbers and return boolean truth values (true or false) depending on whether one is equal to, greater than or less than the other ("is 64 greater than 65?").

Logic operations involve Boolean logic: AND, OR, XOR and NOT. These can be useful both for creating complicated conditional statements and processing boolean logic.

Superscalar computers contain multiple ALUs so that they can process several instructions at the same time. Graphics processors and computers with SIMD and MIMD features often provide ALUs that can perform arithmetic on vectors and matrices.

MEMORY

A computer's memory can be viewed as a list of cells into which numbers can be placed or read. Each cell has a numbered "address" and can store a single number. The computer can be instructed to "put the number 123 into the cell numbered 1357" or to "add the number that is in cell 1357 to the number that is in cell 2468 and put the answer into cell 1595". The information stored in memory may represent practically anything. Letters, numbers, even computer instructions can be placed into memory with equal ease. Since the CPU does not differentiate between different types of information, it is up to the software to give significance to what the memory sees as nothing but a series of numbers.

In almost all modern computers, each memory cell is set up to store binary numbers in groups of eight bits (called a byte). Each byte is able to represent 256 different numbers; either from 0 to 255 or -128 to +127. To store larger numbers, several consecutive bytes may be used (typically, two, four or eight). When negative numbers are required, they are usually stored in two's complement notation. Other arrangements are possible, but are usually not seen outside of specialized applications or historical contexts. A computer can store any kind of information in memory as long as it can be somehow represented in numerical form. Modern computers have billions or even trillions of bytes of memory.

The CPU contains a special set of memory cells called registers that can be read and written to much more rapidly than the main memory area. There are typically between two and one hundred registers depending on the type of CPU. Registers are used for the most frequently needed data items to avoid having to access main memory every time data is needed. Since data is constantly being worked on, reducing the need to access main memory (which is often slow compared to the ALU and control units) greatly increases the computer's speed.

Computer main memory comes in two principal varieties: random access memory or RAM and read-only memory or ROM. RAM can be read and written to anytime the CPU commands it, but ROM is pre-loaded with data and software that never changes, so the CPU can only read from it. ROM is typically used to store the computer's initial start-up instructions. In general, the contents of RAM is erased when the power to the computer is turned off while ROM retains its data indefinitely. In a PC, the ROM contains a specialized program called the BIOS that orchestrates loading the computer's operating system from the hard disk drive into RAM whenever the computer is turned on or reset. In embedded computers, which frequently do not have disk drives, all of the software required to perform the task may be stored in ROM. Software that is stored in ROM is often called firmware because it is notionally more like hardware than software. Flash memory blurs the distinction between ROM and RAM by retaining data when turned off but being rewritable like RAM. However, flash memory is typically much slower than conventional ROM and RAM so its use is restricted to applications where high speeds are not required.[13]

In more sophisticated computers there may be one or more RAM cache memories which are slower than registers but faster than main memory. Generally computers with this sort of cache are designed to move frequently needed data into the cache automatically, often without the need for any intervention on the programmer's part.

Input/output (I/O)

I/O is the means by which a computer receives information from the outside world and sends results back. Devices that provide input or output to the computer are called peripherals. On a typical personal computer, peripherals include input devices like the keyboard and mouse, and output devices such as the display and printer. Hard disk drives, floppy disk drives and optical disc drives serve as both input and output devices. Computer networking is another form of I/O.

Often, I/O devices are complex computers in their own right with their own CPU and memory. A graphics processing unit might contain fifty or more tiny computers that perform the calculations necessary to display 3D graphics. Modern desktop computers contain many smaller computers that assist the main CPU in performing I/O.

Multitasking

While a computer may be viewed as running one gigantic program stored in its main memory, in some systems it is necessary to give the appearance of running several programs simultaneously. This is achieved by having the computer switch rapidly between running each program in turn. One means by which this is done is with a special signal called an interrupt which can periodically cause the computer to stop executing instructions where it was and do something else instead. By remembering where it was executing prior to the interrupt, the computer can return to that task later. If several programs are running "at the same time", then the interrupt generator might be causing several hundred interrupts per second, causing a program switch each time. Since modern computers typically execute instructions several orders of magnitude faster than human perception, it may appear that many programs are running at the same time even though only one is ever executing in any given instant. This method of multitasking is sometimes termed "time-sharing" since each program is allocated a "slice" of time in turn.

Before the era of cheap computers, the principle use for multitasking was to allow many people to share the same computer.

Seemingly, multitasking would cause a computer that is switching between several programs to run more slowly - in direct proportion to the number of programs it is running. However, most programs spend much of their time waiting for slow input/output devices to complete their tasks. If a program is waiting for the user to click on the mouse or press a key on the keyboard, then it will not take a "time slice" until the event it is waiting for has occurred. This frees up time for other programs to execute so that many programs may be run at the same time without unacceptable speed loss

Multiprocessing
Some computers may divide their work between one or more separate CPUs, creating a multiprocessing configuration. Traditionally, this technique was utilized only in large and powerful computers such as supercomputers, mainframe computers and servers. However, multiprocessor and multi-core (multiple CPUs on a single integrated circuit) personal and laptop computers have become widely available and are beginning to see increased usage in lower-end markets as a result.

Supercomputers in particular often have highly unique architectures that differ significantly from the basic stored-program architecture and from general purpose computers.[14] They often feature thousands of CPUs, customized high-speed interconnects, and specialized computing hardware. Such designs tend to be useful only for specialized tasks due to the large scale of program organization required to successfully utilize most of a the available resources at once. Supercomputers usually see usage in large-scale simulation, graphics rendering, and cryptography applications, as well as with other so-called "embarrassingly parallel" tasks.

Networking and the Internet

Computers have been used to coordinate information in multiple locations since the 1950s, with the U.S. military's SAGE system the first large-scale example of such a system, which led to a number of special-purpose commercial systems like Sabre.

In the 1970s, computer engineers at research institutions throughout the United States began to link their computers together using telecommunications technology. This effort was funded by ARPA (now DARPA), and the computer network that it produced was called the ARPANET. The technologies that made the Arpanet possible spread and evolved. In time, the network spread beyond academic and military institutions and became known as the Internet. The emergence of networking involved a redefinition of the nature and boundaries of the computer. Computer operating systems and applications were modified to include the ability to define and access the resources of other computers on the network, such as peripheral devices, stored information, and the like, as extensions of the resources of an individual computer. Initially these facilities were available primarily to people working in high-tech environments, but in the 1990s the spread of applications like e-mail and the World Wide Web, combined with the development of cheap, fast networking technologies like Ethernet and ADSL saw computer networking become almost ubiquitous. In fact, the number of computers that are networked is growing phenomenally. A very large proportion of personal computers regularly connect to the Internet to communicate and receive information. "Wireless" networking, often utilizing mobile phone networks, has meant networking is becoming increasingly ubiquitous even in mobile computing environments