1.
Shift key
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The shift key is a modifier key on a keyboard, used to type capital letters and other alternate upper characters. There are typically two shift keys, on the left and right sides of the row below the home row,2 Type-Writer of 1878, the No.1 model was capital-only. On the US layout and similar layouts, characters that typically require the use of the shift key include the parentheses, the question mark, the exclamation point. When the caps lock key is engaged, the key can be used to type lowercase letters on many operating systems. The keyboard symbol for the Shift key is given in ISO/IEC 9995-7 as symbol 1, in Unicode 6.1, the character approximating this symbol best is U+21E7 upwards white arrow. This symbol is used to denote the Shift key on modern keyboards. This symbol also is used in texts to denote the shift key, on computer keyboards, as opposed to typewriter keyboards, the shift key can have many more uses, It is sometimes used to modify the function keys. Modern Microsoft Windows keyboards typically have only 12 function keys, Shift+F1 must be used to type F13, Shift+F2 for F14 and it can modify various control and alt keys. For example, if Alt-Tab is used to cycle through open windows, holding shift while in a word processor will anchor the insertion point, such that moving the pointer and clicking the mouse to a new point will select the range of text in between. The shift key can also be used to modify the behavior on a computer. For example, holding shift while clicking on a link in a web browser might cause the page to open in a new window, in some web browsers, holding shift while scrolling will scan through previously viewed web pages. In macOS, holding shift while performing actions, such as minimising a window or enabling/disabling Dashboard or Mission Control. For some animations, holding control will make the move just slightly slower. On some keyboards, if both shift keys are held down only some letters can be typed. For example, on the Dell keyboard Model RT7D20 only 16 letters can be typed and this phenomenon is known as masking and is a fundamental limitation of the way the keyboard electronics are designed. The following is a list of actions involving the shift key for the Microsoft Windows operating system, altGr key Command key Option key StickyKeys
2.
Computer
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A computer is a device that can be instructed to carry out an arbitrary set of arithmetic or logical operations automatically. The ability of computers to follow a sequence of operations, called a program, such computers are used as control systems for a very wide variety of industrial and consumer devices. The Internet is run on computers and it millions of other computers. Since ancient times, simple manual devices like the abacus aided people in doing calculations, early in the Industrial Revolution, some mechanical devices were built to automate long tedious tasks, such as guiding patterns for looms. More sophisticated electrical machines did specialized analog calculations in the early 20th century, the first digital electronic calculating machines were developed during World War II. The speed, power, and versatility of computers has increased continuously and dramatically since then, conventionally, a modern computer consists of at least one processing element, typically a central processing unit, and some form of memory. The processing element carries out arithmetic and logical operations, and a sequencing, peripheral devices include input devices, output devices, and input/output devices that perform both functions. Peripheral devices allow information to be retrieved from an external source and this usage of the term referred to a person who carried out calculations or computations. The word continued with the same meaning until the middle of the 20th century, from the end of the 19th century the word began to take on its more familiar meaning, a machine that carries out computations. The Online Etymology Dictionary gives the first attested use of computer in the 1640s, one who calculates, the Online Etymology Dictionary states that the use of the term to mean calculating machine is from 1897. The Online Etymology Dictionary indicates that the use of the term. 1945 under this name, theoretical from 1937, as Turing machine, devices have been used to aid computation for thousands of years, mostly using one-to-one correspondence with fingers. The earliest counting device was probably a form of tally stick, later record keeping aids throughout the Fertile Crescent included calculi which represented counts of items, probably livestock or grains, sealed in hollow unbaked clay containers. The use of counting rods is one example, the abacus was initially used for arithmetic tasks. The Roman abacus was developed from used in Babylonia as early as 2400 BC. Since then, many forms of reckoning boards or tables have been invented. In a medieval European counting house, a checkered cloth would be placed on a table, the Antikythera mechanism is believed to be the earliest mechanical analog computer, according to Derek J. de Solla Price. It was designed to calculate astronomical positions and it was discovered in 1901 in the Antikythera wreck off the Greek island of Antikythera, between Kythera and Crete, and has been dated to circa 100 BC
3.
Computer program
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A computer program is a collection of instructions that performs a specific task when executed by a computer. A computer requires programs to function, and typically executes the programs instructions in a processing unit. A computer program is written by a computer programmer in a programming language. From the program in its form of source code, a compiler can derive machine code—a form consisting of instructions that the computer can directly execute. Alternatively, a program may be executed with the aid of an interpreter. A part of a program that performs a well-defined task is known as an algorithm. A collection of programs, libraries and related data are referred to as software. Computer programs may be categorized along functional lines, such as software or system software. The earliest programmable machines preceded the invention of the digital computer, in 1801, Joseph-Marie Jacquard devised a loom that would weave a pattern by following a series of perforated cards. Patterns could be weaved and repeated by arranging the cards, in 1837, Charles Babbage was inspired by Jacquards loom to attempt to build the Analytical Engine. The names of the components of the device were borrowed from the textile industry. In the textile industry, yarn was brought from the store to be milled, the device would have had a store—memory to hold 1,000 numbers of 40 decimal digits each. Numbers from the store would then have then transferred to the mill. It was programmed using two sets of perforated cards—one to direct the operation and the other for the input variables, however, after more than 17,000 pounds of the British governments money, the thousands of cogged wheels and gears never fully worked together. During a nine-month period in 1842–43, Ada Lovelace translated the memoir of Italian mathematician Luigi Menabrea, the memoir covered the Analytical Engine. The translation contained Note G which completely detailed a method for calculating Bernoulli numbers using the Analytical Engine and this note is recognized by some historians as the worlds first written computer program. In 1936, Alan Turing introduced the Universal Turing machine—a theoretical device that can model every computation that can be performed on a Turing complete computing machine and it is a finite-state machine that has an infinitely long read/write tape. The machine can move the back and forth, changing its contents as it performs an algorithm
4.
ISO/IEC 9995
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ISO/IEC9995 Information technology — Keyboard layouts for text and office systems is an ISO standard series defining layout principles for computer keyboards. It does not define specific layouts but provides the base for national, the project of this standard was adopted at ISO in Berlin in 1985 under the proposition of Dr Yves Neuville. The ISO/IEC9995 standard series dates to 1994 and has undergone several updates over the years, the figure shows the division of a keyboard into sections, which are subdivided into zones. Also, the standard does not prevent a section to be placed left of the alphanumeric section. By means of the grid, each key can be identified by a unique combination of a letter. E. g. the key containing the digit one on several layouts is identified as “Key E01”, rows above of the alphanumeric section are to be labeled from K on, and rows below the space key are to be labeled from Z downwards. The grid may be angled, or squared, the standard does not constrain the numbers of rows and columns in the alphanumeric section. The characters which can be input by the keys in the section usually are organized in levels. For two-cased scripts like Latin, the level contains lower-case letters. For characters which are not letters, no rules are given regarding their distribution among the levels, while digits are commonly in Level 1, there are exceptions. The standard allows for a third level, usually, characters in such a level are selected by the means of an AltGr key. Thus, each such group usually is subdivided into levels, common examples are layouts allowing the input of characters of different scripts, like e. g. the Japanese keyboard layout. According to ISO/IEC 9995-1, the level is indicated by the row where the character is depicted on the keytop, Level 2 above of Level 1 Level 3 below Level 1. The group is indicated by the column on the keytop, The first or “primary group” at the left keytop border The second or “secondary group” at the right keytop border Additional groups in between. Thus, on the key of the German T2 layout, in the primary group are the characters “#” in Level 1, “” in Level 2. In the secondary group, there is the lowercase letter “ə” in Level 1, for layouts containing only one group, characters in Level 3 may be depicted in the lower right corner of a keytop, to allow larger depictions for ergonomic reasons. For instance, on the key of the German T1 layout. In fact, on such layouts, the AltGr key is a “group selector” as defined in ISO/IEC 9995-2, the selection of level and group during the input of a character is specified in ISO/IEC 9995-2
5.
AltGr key
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AltGr is a modifier key found on some computer keyboards and is primarily used to type characters that are unusual for the locale of the keyboard layout, such as currency symbols and accented letters. On a typical, IBM-compatible PC keyboard, the AltGr key, in macOS, the Option key has functions similar to the AltGr key. AltGr is used similarly to the Shift key, it is held down when another key is struck in order to obtain a character other than the one that the latter normally produces, AltGr and Shift can also sometimes be combined to obtain yet another character. However, IBM states that AltGr is an abbreviation for alternate graphic, AltGr was originally introduced as a means to produce box-drawing characters, also known as pseudographics, in text user interfaces. These characters are, however, much less useful in graphical user interfaces, originally, US PC keyboards did not have an AltGr key because that was relevant to only non-US markets, they simply had left and right Alt keys. The right Alt key is usually an equivalent of the AltGr key because both the right Alt key and the AltGr key share the same scancode and are indistinguishable by software. However, on some keyboards it may not be the case, on compact keyboards like those of netbooks, the right Alt key may be missing altogether. For comparison, the US-International keyboard layout follows, note that the /~ key has been omitted, it doesnt produce any characters together with the AltGr key with or without Shift. It generally produces a \, although, applications receive a different keycode and can instead use the key to, for example. In UK & Ireland keyboard layouts, the two symbols printed on most keyboards which require the AltGr key are, € the Euro currency symbol. Either |, the bar or ¦, the broken vertical bar. Located on the /¬ key, to the left of 1. The other bar symbol is the symbol on the \ key immediately to the left of the Z key. The two latter symbols interchange places in UK keyboards according to the system in use. Using the AltGr key on Linux produces many foreign characters and international symbols, details are to be found at QWERTY § United Kingdom Layout. On South Slavic Latin keyboards, the letters and special characters are created using AltGr, On Belgian keyboards. Those shown in bold are printed on the keys, and common to all systems, ° ª º ₢ ° Some notes, The AltGr+C combination results in the symbol ₢ for the former Brazilian currency, the Brazilian cruzeiro. The AltGr+Q, AltGr+W, AltGr+E combinations are useful as a replacement for the /. key, which is physically absent on non-Brazilian keyboards
6.
Unicode
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Unicode is a computing industry standard for the consistent encoding, representation, and handling of text expressed in most of the worlds writing systems. As of June 2016, the most recent version is Unicode 9.0, the standard is maintained by the Unicode Consortium. Unicodes success at unifying character sets has led to its widespread, the standard has been implemented in many recent technologies, including modern operating systems, XML, Java, and the. NET Framework. Unicode can be implemented by different character encodings, the most commonly used encodings are UTF-8, UTF-16 and the now-obsolete UCS-2. UTF-8 uses one byte for any ASCII character, all of which have the same values in both UTF-8 and ASCII encoding, and up to four bytes for other characters. UCS-2 uses a 16-bit code unit for each character but cannot encode every character in the current Unicode standard, UTF-16 extends UCS-2, using one 16-bit unit for the characters that were representable in UCS-2 and two 16-bit units to handle each of the additional characters. Many traditional character encodings share a common problem in that they allow bilingual computer processing, Unicode, in intent, encodes the underlying characters—graphemes and grapheme-like units—rather than the variant glyphs for such characters. In the case of Chinese characters, this leads to controversies over distinguishing the underlying character from its variant glyphs. In text processing, Unicode takes the role of providing a unique code point—a number, in other words, Unicode represents a character in an abstract way and leaves the visual rendering to other software, such as a web browser or word processor. This simple aim becomes complicated, however, because of concessions made by Unicodes designers in the hope of encouraging a more rapid adoption of Unicode, the first 256 code points were made identical to the content of ISO-8859-1 so as to make it trivial to convert existing western text. For other examples, see duplicate characters in Unicode and he explained that he name Unicode is intended to suggest a unique, unified, universal encoding. In this document, entitled Unicode 88, Becker outlined a 16-bit character model, Unicode could be roughly described as wide-body ASCII that has been stretched to 16 bits to encompass the characters of all the worlds living languages. In a properly engineered design,16 bits per character are more than sufficient for this purpose, Unicode aims in the first instance at the characters published in modern text, whose number is undoubtedly far below 214 =16,384. By the end of 1990, most of the work on mapping existing character encoding standards had been completed, the Unicode Consortium was incorporated in California on January 3,1991, and in October 1991, the first volume of the Unicode standard was published. The second volume, covering Han ideographs, was published in June 1992, in 1996, a surrogate character mechanism was implemented in Unicode 2.0, so that Unicode was no longer restricted to 16 bits. The Microsoft TrueType specification version 1.0 from 1992 used the name Apple Unicode instead of Unicode for the Platform ID in the naming table, Unicode defines a codespace of 1,114,112 code points in the range 0hex to 10FFFFhex. Normally a Unicode code point is referred to by writing U+ followed by its hexadecimal number, for code points in the Basic Multilingual Plane, four digits are used, for code points outside the BMP, five or six digits are used, as required. Code points in Planes 1 through 16 are accessed as surrogate pairs in UTF-16, within each plane, characters are allocated within named blocks of related characters
7.
Option key
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The Option key is a modifier key present on Apple keyboards. It is located between the Control key and Command key on a typical Mac keyboard, there are two option keys on modern Mac desktop and notebook keyboards, one on each side of the space bar. Apple commonly uses the symbol U+2325 ⌥ option key to represent the Option key, from 1980 to 1984, on the Apple II family, this key was known as the closed apple key, and had a black line drawing of a filled-in apple on it. Since the 1990s, alt typically appears on the key, as well, for use as an Alt key with non-Mac software, such as Unix, however, the Option key in a Mac operating system functions differently from the Alt key under other Unix-like systems or Microsoft Windows. Most notably, it is not used to access menus or hotkeys, but is used as a modifier for other command codes. In this regard, it is akin to the AltGr key, the options available differ depending on the keyboard input locale that the user has selected. English keyboard input, ⌥ Option+a produces the å character, and ⌥ Option+4 produces the cent sign ¢, the Option key can also provide access to dead key functionality. For example, holding down ⌥ Option while pressing will create a highlighted grave accent which will be added to the letter if possible – so if an e is then pressed. If an r is pressed instead, the two characters are not compatible so the result is r, the highlighted orange keys show the accents available from the combination of the ⌥ Option key and the keyboard characters e i n u. The accent then can be applied to associated letters both lower and uppercase, the additional characters a Mac will produce are a combination of both the ⌥ Option key and the ⇧ Shift key pressed down together. With this combination pressed the keyboard will now produce a different set or a version of the previous set in some cases. The Option key is used in conjunction with special keys like Tab ↹, delete. For example, ⌥ Option+Return typically produces a break that is not interpreted as a paragraph break. The key is used to provide for alternative menu items. Examples, Safari, Finder - the Option key causes the Close Window menu item to switch to Close All Windows when pressed down, consequently, clicking a windows close box with the option key depressed invokes close all as well. This functionality is a de facto Macintosh standard and available in other programs. IPhoto - the rotate image button toggles between a right and a rotate left. The iPhoto example is an example of a control whose behavior is governed by a preference which is inverted by holding down the Option key
8.
Internationalization and localization
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In computing, internationalization and localization are means of adapting computer software to different languages, regional differences and technical requirements of a target market. Internationalization is the process of designing a software application so that it can potentially be adapted to various languages, localization is the process of adapting internationalized software for a specific region or language by adding locale-specific components and translating text. Localization uses the infrastructure or flexibility provided by internationalization, the terms are frequently abbreviated to the numeronyms i18n and L10n for localization, due to the length of the words. Some companies, like IBM and Sun Microsystems, use the term globalization, g11n, Microsoft defines Internationalization as a combination of World-Readiness and localization. World-Readiness is a task, which enables a product to be used with multiple scripts and cultures. Hewlett-Packard and HP-UX created the system called NLS to produce localizable software, translation is typically the most time-consuming component of language localization. These differences may need to be taken account by the internationalization process in preparation for translation. Many of these differences are so regular that a conversion between languages can be easily automated, the Common Locale Data Repository by Unicode provides a collection of such differences. Its data is used by operating systems, including Microsoft Windows, OS X and Debian, and by major internet companies or projects such as Google. Examples of such include, Different scripts in different writing systems use different characters - a different set of letters, syllograms, logograms. Modern system use the Unicode standard to represent many different languages with a character encoding. Writing direction is left to right in most European languages, right-to-left in Hebrew and Arabic, complex text layout, for languages where characters change shape depending on context. Capitalization exists in some scripts and not in others, other grammar rules might also vary, e. g. genitive. Keyboard shortcuts can only use of buttons actually on the keyboard layout which is being localized for. If a shortcut corresponds to a word in a particular language, other areas often follow one of these. Specific third-party services, such as maps, weather reports, or payment service providers, might not be available worldwide from the same carriers. Time zones vary across the world, and this must be taken into account if a product originally only interacted with people in a time zone. For internationalization, UTC is often used internally and then converted into a time zone for display purposes
9.
Control-Alt-Delete
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Control-Alt-Delete is a computer keyboard command on IBM PC compatible computers, invoked by pressing the Delete key while holding the Control and Alt keys, Ctrl+Alt+Delete. The function of the key combination differs depending on the context, for instance, in pre-boot environment or in DOS, Windows 3.0 and earlier versions of Windows or OS/2, the key combination reboots the computer. Starting with Windows 3.1, the command invokes a task manager or security related component that facilitates ending a Windows session, the soft reboot function via keyboard was originally designed by David Bradley. Bill Gates remembered it as just something we were using in development, the feature, however, was detailed in IBMs technical reference documentation to the original PC and thereby revealed to the general public. Bradley viewed this work as just one small task out of many, It was five minutes,10 minutes of activity, and then I moved on to the next of the 100 things that needed to get done. During a question and answer presentation on 21 September 2013, Gates said it was a mistake, Gates stated he would have preferred a single button to trigger the same actions, but could not get IBM to add the extra button into the keyboard layout. By default, when the system is running in real mode. The BIOS reacts by performing a soft reboot, examples of such operating systems include DOS, Windows 3.0 in Standard Mode as well as earlier versions of Windows. In Windows 9x and Windows 3.0 running in 386 Enhanced mode, according to the value of the LocalReboot option in the section of system. ini, Windows performs one of several actions in response. The text of this rudimentary task manager was written by Steve Ballmer, Windows 9x temporarily halts the entire system and displays the Close Program dialog box, a window which lists currently running processes and allows the user to end them. The user can press Control+Alt+Delete again to perform a soft reboot, if LocalReboot=Off, Windows performs a soft reboot. Windows NT family of operating system, whose members do not have NT in their names since Windows 2000, the key combination always invokes Windows Security in all versions and editions of Windows NT family except Windows XP. Starting with Windows Vista, Windows Security became full-screen, to thwart this attack, Windows NT implements an optional security measure in which Ctrl+Alt+Delete acts as a secure attention key combination. Once the protection is activated, Windows requires the user to press Ctrl+Alt+Delete each time before logging on or unlocking the computer, since the key combination is intercepted by Windows itself and malicious software cannot mimic this behavior, the trick is thwarted. Unless the Windows computer is part of a Windows domain network, Windows XP behavior Windows XP introduces Welcome Screen, a redesigned logon interface. The Welcome Screen of Windows XP, however, does not support the secure attention scenario and it may be disabled in favor of the classic plain logon screen, either explicitly by the user or as a consequence of the Windows XP computer becoming part of a Windows domain network. With that in mind, Windows XP uses the three-finger salute in the following scenarios, At a logon prompt. When a user is logged on to a Windows XP computer and Welcome Screen is enabled, Windows Vista and the next versions of Windows NT did not inherit any of the above
10.
Apple Inc.
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Apple is an American multinational technology company headquartered in Cupertino, California that designs, develops, and sells consumer electronics, computer software, and online services. Apples consumer software includes the macOS and iOS operating systems, the media player, the Safari web browser. Its online services include the iTunes Store, the iOS App Store and Mac App Store, Apple Music, Apple was founded by Steve Jobs, Steve Wozniak, and Ronald Wayne in April 1976 to develop and sell personal computers. It was incorporated as Apple Computer, Inc. in January 1977, Apple joined the Dow Jones Industrial Average in March 2015. In November 2014, Apple became the first U. S. company to be valued at over US$700 billion in addition to being the largest publicly traded corporation in the world by market capitalization. The company employs 115,000 full-time employees as of July 2015 and it operates the online Apple Store and iTunes Store, the latter of which is the worlds largest music retailer. Consumers use more than one billion Apple products worldwide as of March 2016, Apples worldwide annual revenue totaled $233 billion for the fiscal year ending in September 2015. This revenue accounts for approximately 1. 25% of the total United States GDP.1 billion, the corporation receives significant criticism regarding the labor practices of its contractors and its environmental and business practices, including the origins of source materials. Apple was founded on April 1,1976, by Steve Jobs, Steve Wozniak, the Apple I kits were computers single-handedly designed and hand-built by Wozniak and first shown to the public at the Homebrew Computer Club. The Apple I was sold as a motherboard, which was less than what is now considered a personal computer. The Apple I went on sale in July 1976 and was market-priced at $666.66, Apple was incorporated January 3,1977, without Wayne, who sold his share of the company back to Jobs and Wozniak for $800. Multimillionaire Mike Markkula provided essential business expertise and funding of $250,000 during the incorporation of Apple, during the first five years of operations revenues grew exponentially, doubling about every four months. Between September 1977 and September 1980 yearly sales grew from $775,000 to $118m, the Apple II, also invented by Wozniak, was introduced on April 16,1977, at the first West Coast Computer Faire. It differed from its rivals, the TRS-80 and Commodore PET, because of its character cell-based color graphics. While early Apple II models used ordinary cassette tapes as storage devices, they were superseded by the introduction of a 5 1/4 inch floppy disk drive and interface called the Disk II. The Apple II was chosen to be the platform for the first killer app of the business world, VisiCalc. VisiCalc created a market for the Apple II and gave home users an additional reason to buy an Apple II. Before VisiCalc, Apple had been a distant third place competitor to Commodore, by the end of the 1970s, Apple had a staff of computer designers and a production line
11.
MacOS
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Within the market of desktop, laptop and home computers, and by web usage, it is the second most widely used desktop OS after Microsoft Windows. Launched in 2001 as Mac OS X, the series is the latest in the family of Macintosh operating systems, Mac OS X succeeded classic Mac OS, which was introduced in 1984, and the final release of which was Mac OS9 in 1999. An initial, early version of the system, Mac OS X Server 1.0, was released in 1999, the first desktop version, Mac OS X10.0, followed in March 2001. In 2012, Apple rebranded Mac OS X to OS X. Releases were code named after big cats from the release up until OS X10.8 Mountain Lion. Beginning in 2013 with OS X10.9 Mavericks, releases have been named after landmarks in California, in 2016, Apple rebranded OS X to macOS, adopting the nomenclature that it uses for their other operating systems, iOS, watchOS, and tvOS. The latest version of macOS is macOS10.12 Sierra, macOS is based on technologies developed at NeXT between 1985 and 1997, when Apple acquired the company. The X in Mac OS X and OS X is pronounced ten, macOS shares its Unix-based core, named Darwin, and many of its frameworks with iOS, tvOS and watchOS. A heavily modified version of Mac OS X10.4 Tiger was used for the first-generation Apple TV, Apple also used to have a separate line of releases of Mac OS X designed for servers. Beginning with Mac OS X10.7 Lion, the functions were made available as a separate package on the Mac App Store. Releases of Mac OS X from 1999 to 2005 can run only on the PowerPC-based Macs from the time period, Mac OS X10.5 Leopard was released as a Universal binary, meaning the installer disc supported both Intel and PowerPC processors. In 2009, Apple released Mac OS X10.6 Snow Leopard, in 2011, Apple released Mac OS X10.7 Lion, which no longer supported 32-bit Intel processors and also did not include Rosetta. All versions of the system released since then run exclusively on 64-bit Intel CPUs, the heritage of what would become macOS had originated at NeXT, a company founded by Steve Jobs following his departure from Apple in 1985. There, the Unix-like NeXTSTEP operating system was developed, and then launched in 1989 and its graphical user interface was built on top of an object-oriented GUI toolkit using the Objective-C programming language. This led Apple to purchase NeXT in 1996, allowing NeXTSTEP, then called OPENSTEP, previous Macintosh operating systems were named using Arabic numerals, e. g. Mac OS8 and Mac OS9. The letter X in Mac OS Xs name refers to the number 10 and it is therefore correctly pronounced ten /ˈtɛn/ in this context. However, a common mispronunciation is X /ˈɛks/, consumer releases of Mac OS X included more backward compatibility. Mac OS applications could be rewritten to run natively via the Carbon API, the consumer version of Mac OS X was launched in 2001 with Mac OS X10.0. Reviews were variable, with praise for its sophisticated, glossy Aqua interface
12.
Display server
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A display server or window server is a program whose primary task is to coordinate the input and output of its clients to and from the rest of the operating system, the hardware, and each other. The display server communicates with its clients over the display server protocol, a communications protocol, the display server is a key component in any graphical user interface, specifically the windowing system. One example of a server is the X. Org Server. It receives data from evdev and passes it to one of its clients, the display server also receives data from its clients, it processes the data, it does the compositing and passes the data to one of three kernel components – DRM, gem or KMS driver. The component writes the data into the framebuffer and content of the framebuffer is transmitted to the connected screen, one of the implementations of display server concept is X Window System, in particular its actually used version – X. Org Server and Xlib and XCB client libraries. The X. Org Server is a server, but in its current implementation it relies on a second program. One tool called xev is a Unix command which creates a window, display servers that implement the Wayland display server protocol, are called Wayland compositors. Like any display server, a Wayland compositor is responsible for handling input and output for its clients, examples are Weston, Mutter, KWin or Enlightenment. Wayland compositors communicate with Wayland clients over the Wayland display server protocol and this protocol defines that clients can directly write data into the framebuffer using the EGL rendering API. The display server still gets to decide which window is on top and thus visible to the user, Wayland is used to a certain degree in some Linux desktop distributions, such as Fedora. It is also suited for mobile computing and has been adopted, for example, by the smartphone- and tablet-focused projects Tizen. An implementation of Wayland is available under the MIT License, the libwayland-client, the Mir display server comes with its own Mir display server protocol different from the protocols X11 or Wayland. Mir will additionally support the X11 and maybe the Wayland protocol and it is developed by Canonical and is intended to be the display server of choice for Ubuntu. There is an implementation of the Mir display server available under the GPLv3, the libmir-server, yet another Android-specific solution is Gralloc. Gralloc handles device memory i. e. it does allocation, arbitration, Gralloc competes with other solutions like e. g. Mesas Generic Buffer Management or Nvidias EGLStreams. The gralloc hardware abstraction layer is used to allocate the buffers that underly surfaces, for compositing in Android, Surfaces are sent to SurfaceFlinger, which uses OpenGL|ES to do the compositing. Hardware Composer HAL was introduced in Android 3.0 and has evolved steadily over the years and its primary purpose is to determine the most efficient way to composite buffers with the available hardware. As a HAL, its implementation is device-specific and usually done by the display hardware OEM, quartz Compositor is the name of the component that is fulfilling the tasks of a display server and of a window manager in the windowing system
13.
X Window System
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The X Window System is a windowing system for bitmap displays, common on UNIX-like computer operating systems. X provides the framework for a GUI environment, drawing and moving windows on the display device and interacting with a mouse. X does not mandate the user interface – this is handled by individual programs, as such, the visual styling of X-based environments varies greatly, different programs may present radically different interfaces. X originated at the Massachusetts Institute of Technology in 1984, the protocol has been version 11 since September 1987. The X. Org Foundation leads the X project, with the current reference implementation, X. Org Server, available as free and open source software under the MIT License, X is an architecture-independent system for remote graphical user interfaces and input device capabilities. Each person using a terminal has the ability to interact with the display with any type of user input device. X provides the framework, or primitives, for building such GUI environments, drawing and moving windows on the display and interacting with a mouse. X does not mandate the user interface, individual client programs handle this, programs may use Xs graphical abilities with no user interface. As such, the styling of X-based environments varies greatly. Unlike most earlier display protocols, X was specifically designed to be used over network connections rather than on an integral or attached display device. X features network transparency, which means an X program running on a computer somewhere on a network can display its user interface on an X server running on other computer on the network. The fact that the server is applied to the software in front of the user is often surprising to users accustomed to their programs being clients to services on remote computers. Xs network protocol is based on X command primitives and this approach allows both 2D and 3D operations by an X client application which might be running on a different computer to still be fully accelerated on the X servers display. X provides no support for audio, several projects exist to fill this niche. X uses a model, an X server communicates with various client programs. The server accepts requests for graphical output and sends back user input, a client and server can even communicate securely over the Internet by tunneling the connection over an encrypted network session. An X client itself may emulate an X server by providing services to other clients. This is known as X nesting, open-source clients such as Xnest and Xephyr support such X nesting
14.
Alt-Tab
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Alt+Tab ↹ is the common name for a keyboard shortcut that has been in Microsoft Windows since Windows 3.0. This shortcut switches between top-level windows without using the mouse, hence it was named Task Switcher. Perhaps the most common use of Alt+Tab ↹ is to alternate between a window and the desktop, but it can also be used to switch to any running program that has a top-level window. Thus, it can be used alternate between the two most recent tasks, the Alt+Tab ↹ keyboard combination has also been incorporated in other operating systems and desktop environments such as macOS and iOS, KDE, Xfce, and GNOME. Holding Alt continuously while pressing Tab ↹ repeatedly, releasing Alt has an immediate effect, it closes the task switcher and switches to the selected task. There are many subtleties to the behavior of Alt+Tab ↹, the behavior follows these rules, If there is more than one top-level window, the task list appears as soon as Tab ↹ is pressed for the first time while Alt is being held down. The task list remains open until Alt is released, Tab ↹ moves the cursor forward in the list, ⇧ Shift+Tab ↹ moves it backward. Tab ↹ or ⇧ Shift+Tab ↹ will autorepeat if held down, with the initial press of Tab ↹ or ⇧ Shift+Tab ↹, the selection cursor starts on the window immediately following or immediately preceding the active one. If there are no topmost windows above the window, an initial ⇧ Shift+Tab ↹ wraps the cursor around to the end of the list. Using the mouse to click on an icon in the task window switches to that task. Pressing Esc or clicking the mouse outside of the window while Alt is still down cancels the switch. The windows are listed by their Z-order, Any windows that are always on top are placed at the front of the Z-order sequence, followed by the current window and the windows underneath it. The desktop is given a window just like it was a top-level window and this no longer works on Windows 10. Switching to a window moves it to the front of the Z-order, with the exception that always on top windows remain topmost, when the Alt+Tab ↹ task switcher window is not active, Alt+Esc places the active window at the bottom of the Z-order. In Windows 8 the behavior has changed, the window will be moved one level down the Z-order instead of going to the end, Alt+⇧ Shift+Esc is equivalent to one Alt+⇧ Shift+Tab ↹ except that minimized windows are selected without being restored. Minimizing a window also sends it to the back of the Z-order in the way as Alt+Esc. Alt+⇧ Shift+Tab ↹ can restore the most recently minimized window, pressing Alt+Tab ↹-Tab ↹ performs the same quick switch back and forth, but between three programs. Any number of Tab ↹ presses can be used to achieve this with any number of windows, when the task list is initially activated by pressing Alt+Tab ↹, the list is populated in this order, Any always-on-top top-level windows according to Z-order, front-to-back
15.
Microsoft Windows
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Microsoft Windows is a metafamily of graphical operating systems developed, marketed, and sold by Microsoft. It consists of families of operating systems, each of which cater to a certain sector of the computing industry with the OS typically associated with IBM PC compatible architecture. Active Windows families include Windows NT, Windows Embedded and Windows Phone, defunct Windows families include Windows 9x, Windows 10 Mobile is an active product, unrelated to the defunct family Windows Mobile. Microsoft introduced an operating environment named Windows on November 20,1985, Microsoft Windows came to dominate the worlds personal computer market with over 90% market share, overtaking Mac OS, which had been introduced in 1984. Apple came to see Windows as an encroachment on their innovation in GUI development as implemented on products such as the Lisa. On PCs, Windows is still the most popular operating system, however, in 2014, Microsoft admitted losing the majority of the overall operating system market to Android, because of the massive growth in sales of Android smartphones. In 2014, the number of Windows devices sold was less than 25% that of Android devices sold and this comparison however may not be fully relevant, as the two operating systems traditionally target different platforms. As of September 2016, the most recent version of Windows for PCs, tablets, smartphones, the most recent versions for server computers is Windows Server 2016. A specialized version of Windows runs on the Xbox One game console, Microsoft, the developer of Windows, has registered several trademarks each of which denote a family of Windows operating systems that target a specific sector of the computing industry. It now consists of three operating system subfamilies that are released almost at the time and share the same kernel. Windows, The operating system for personal computers, tablets. The latest version is Windows 10, the main competitor of this family is macOS by Apple Inc. for personal computers and Android for mobile devices. Windows Server, The operating system for server computers, the latest version is Windows Server 2016. Unlike its clients sibling, it has adopted a strong naming scheme, the main competitor of this family is Linux. Windows PE, A lightweight version of its Windows sibling meant to operate as an operating system, used for installing Windows on bare-metal computers. The latest version is Windows PE10.0.10586.0, Windows Embedded, Initially, Microsoft developed Windows CE as a general-purpose operating system for every device that was too resource-limited to be called a full-fledged computer. The following Windows families are no longer being developed, Windows 9x, Microsoft now caters to the consumers market with Windows NT. Windows Mobile, The predecessor to Windows Phone, it was a mobile operating system
16.
Num lock
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Num Lock or Numeric Lock is a key on the numeric keypad of most computer keyboards. It is a key, like Caps Lock and Scroll Lock. Its state affects the function of some of the keys, and is displayed by an LED built into the keyboard. The Num Lock key exists because earlier 84-key IBM PC keyboards did not have control or arrows separate from the numeric keypad. Most earlier computer keyboards had separate number keys and cursor keys, however, to reduce cost. Num Lock would be used to choose between the two functions, on some laptop computers, the Num Lock key is used to convert part of the main keyboard to act as a numeric keypad rather than letters. On some laptop computers, the Num Lock key is absent, since Apple keyboards never had a combination of arrow keys and numeric keypad, Apple has keyboards with a separate numeric keypad but no functional Num Lock key. Instead, these include a Clear key
17.
Macintosh
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The Macintosh (/ˈmækᵻntɒʃ/ MAK-in-tosh, is a series of personal computers designed, developed, and marketed by Apple Inc. Steve Jobs introduced the original Macintosh computer on January 24,1984 and this was the companys first mass-market personal computer featuring an integral graphical user interface and mouse. This first model was renamed to Macintosh 128k for uniqueness amongst a populous family of subsequently updated models which are also based on Apples same proprietary architecture. Since 1998, Apple has largely phased out the Macintosh name in favor of Mac, Macintosh systems still found success in education and desktop publishing and kept Apple as the second-largest PC manufacturer for the next decade. In the 1990s, improvements in the rival Wintel platform, notably with the introduction of Windows 3.0, then Windows 95, gradually took market share from the more expensive Macintosh systems. The performance advantage of 68000-based Macintosh systems was eroded by Intels Pentium, even after a transition to the superior PowerPC-based Power Macintosh line in 1994, the falling prices of commodity PC components and the release of Windows 95 saw the Macintosh user base decline. In 1998, after the return of Steve Jobs, Apple consolidated its multiple consumer-level desktop models into the all-in-one iMac G3, since their transition to Intel processors in 2006, the complete lineup is entirely based on said processors and associated systems. Its current lineup comprises three desktops, and three laptops and its Xserve server was discontinued in 2011 in favor of the Mac Mini and Mac Pro. Apple also develops the operating system for the Mac, currently macOS version 10.12 Sierra, Macs are currently capable of running non-Apple operating systems such as Linux, OpenBSD, and Microsoft Windows with the aid of Boot Camp or third-party software. Apple does not license macOS for use on computers, though it did license previous versions of the classic Mac OS through their Macintosh clone program from 1995 to 1997. The Macintosh project was begun in 1979 by Jef Raskin, an Apple employee who envisioned an easy-to-use, in 1978 Apple began to organize the Apple Lisa project, aiming to build a next-generation machine similar to an advanced Apple III or the yet-to-be-introduced IBM PC. In 1979, Steve Jobs learned of the work on graphical user interfaces taking place at Xerox PARC. He arranged a deal in which Xerox received Apple stock options in return for which Apple would license their designs, the basic layout of the Lisa was largely complete by 1982, at which point Jobs continual suggestions for improvements led to him being kicked off the project. At the same time that the Lisa was becoming a GUI machine in 1979, the design at that time was for a low-cost, easy-to-use machine for the average consumer. Raskin was authorized to start hiring for the project in September 1979 and his initial team would eventually consist of himself, Howard, Joanna Hoffman, Burrell Smith, and Bud Tribble. Smiths design used fewer RAM chips than the Lisa, which production of the board significantly more cost-efficient. Though there were no memory slots, its RAM was expandable to 512 kB by means of soldering sixteen IC sockets to accept 256 kb RAM chips in place of the factory-installed chips. The final products screen was a 9-inch, 512x342 pixel monochrome display, burrels innovative design, combining the low production cost of an Apple II with the computing power of Lisas Motorola 68000 CPU, began to receive Jobs attentions
18.
Ubuntu (operating system)
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Ubuntu is published by Canonical Ltd, who offer commercial support. It uses Unity as its user interface for the desktop. Ubuntu is the most popular operating system running in hosted environments, so–called clouds, development of Ubuntu is led by UK-based Canonical Ltd. a company of South African entrepreneur Mark Shuttleworth. Canonical generates revenue through the sale of technical support and other related to Ubuntu. The Ubuntu project is committed to the principles of open-source software development, people are encouraged to use free software, study how it works, improve upon it. Ubuntu is built on Debians architecture and infrastructure, to provide Linux server, desktop, phone, tablet, the first release was in October 2004. Starting with Ubuntu 6.06, every release, one release every two years, receives long-term support. Long-term support includes updates for new hardware, security patches and updates to the Ubuntu stack. The first LTS releases were supported for three years on the desktop and five years on the server, since Ubuntu 12.04 LTS, LTS releases get regular point releases with support for new hardware and integration of all the updates published in that series to date. Ubuntu packages are based on packages from Debians unstable branch, both distributions use Debians deb package format and package management tools. Debian and Ubuntu packages are not necessarily compatible with each other, however. Many Ubuntu developers are also maintainers of key packages within Debian, Ubuntu cooperates with Debian by pushing changes back to Debian, although there has been criticism that this does not happen often enough. Ian Murdock, the founder of Debian, had expressed concern about Ubuntu packages potentially diverging too far from Debian to remain compatible, before release, packages are imported from Debian unstable continuously and merged with Ubuntu-specific modifications. One month before release, imports are frozen, and packagers then work to ensure that the frozen features interoperate well together, Ubuntu is currently funded by Canonical Ltd. On 8 July 2005, Mark Shuttleworth and Canonical announced the creation of the Ubuntu Foundation, the purpose of the foundation is to ensure the support and development for all future versions of Ubuntu. Mark Shuttleworth describes the goal as to ensure the continuity of the Ubuntu project. On 12 March 2009, Ubuntu announced developer support for 3rd-party cloud management platforms, Unity has become the default GUI for Ubuntu Desktop, although following the release of Ubuntu 17.10 it will move to the GNOME3 desktop instead as work on Unity ends. A default installation of Ubuntu contains a range of software that includes LibreOffice, Firefox, Thunderbird, Transmission
19.
Internet Archive
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The Internet Archive is a San Francisco–based nonprofit digital library with the stated mission of universal access to all knowledge. As of October 2016, its collection topped 15 petabytes, in addition to its archiving function, the Archive is an activist organization, advocating for a free and open Internet. Its web archive, the Wayback Machine, contains over 150 billion web captures, the Archive also oversees one of the worlds largest book digitization projects. Founded by Brewster Kahle in May 1996, the Archive is a 501 nonprofit operating in the United States. It has a budget of $10 million, derived from a variety of sources, revenue from its Web crawling services, various partnerships, grants, donations. Its headquarters are in San Francisco, California, where about 30 of its 200 employees work, Most of its staff work in its book-scanning centers. The Archive has data centers in three Californian cities, San Francisco, Redwood City, and Richmond, the Archive is a member of the International Internet Preservation Consortium and was officially designated as a library by the State of California in 2007. Brewster Kahle founded the Archive in 1996 at around the time that he began the for-profit web crawling company Alexa Internet. In October 1996, the Internet Archive had begun to archive and preserve the World Wide Web in large quantities, the archived content wasnt available to the general public until 2001, when it developed the Wayback Machine. In late 1999, the Archive expanded its collections beyond the Web archive, Now the Internet Archive includes texts, audio, moving images, and software. It hosts a number of projects, the NASA Images Archive, the contract crawling service Archive-It. According to its web site, Most societies place importance on preserving artifacts of their culture, without such artifacts, civilization has no memory and no mechanism to learn from its successes and failures. Our culture now produces more and more artifacts in digital form, the Archives mission is to help preserve those artifacts and create an Internet library for researchers, historians, and scholars. In August 2012, the Archive announced that it has added BitTorrent to its file download options for over 1.3 million existing files, on November 6,2013, the Internet Archives headquarters in San Franciscos Richmond District caught fire, destroying equipment and damaging some nearby apartments. The nonprofit Archive sought donations to cover the estimated $600,000 in damage, in November 2016, Kahle announced that the Internet Archive was building the Internet Archive of Canada, a copy of the archive to be based somewhere in the country of Canada. The announcement received widespread coverage due to the implication that the decision to build an archive in a foreign country was because of the upcoming presidency of Donald Trump. Kahle was quoted as saying that on November 9th in America and it was a firm reminder that institutions like ours, built for the long-term, need to design for change. For us, it means keeping our cultural materials safe, private and it means preparing for a Web that may face greater restrictions
20.
IBM PC keyboard
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The keyboards for IBM PC compatible computers are standardized. However, during the more than 30 years of PC architecture being constantly updated, the PC keyboard changed over the years, often at the launch of new IBM PC versions. The IBM PC layout, particularly the Model M, has been extremely influential and this results in the Emacs pinky problem. The magazine reported in 1982 that it received more letters to its Wish List column asking for the ability to determine the status of the three lock keys than on any other topic. He reported that the layout nearly drove science-fiction editor Jim Baen crazy, and it praised the keyboard as bar none, the best. On any microcomputer and described the unusual Shift key locations as compared to some of the gigantic mistakes made on almost every other microcomputer keyboard. I wasnt thrilled with the placement of keys, either, IBMs Don Estridge stated in 1983 and he defended the layout, however, stating that every place you pick to put them is not a good place for somebody. Theres no consensus, and claimed that if we were to change it now we would be in hot water, the PC keyboard with its various keys has a long history of evolution reaching back to teletypewriters. In addition to the old standard keys, the PC keyboard has accumulated several special keys over the years, ⇧ Shift selects the upper character, or upper case of letters. The Shift key in typewriters was attached to a lever moved the character types so that the uppercase characters could be printed in the paper. Unlike mechanical typewriters, PC keyboards do not capitalize all letters properly when both keys are engaged simultaneously. ⇪ Caps Lock selects upper case, or if shift is pressed, in mechanical typewriters, it worked like the Shift key, but also used a lock to keep the Shift key depressed. The lock was released by pressing the Shift key, enter wraps to the next line or activates the default or selected option. ASCII keyboards were labeled CR or Return, typewriters used a lever that would return the cylinder with the paper to the start of the line. Tab ↹ produces an ASCII tab character, moving to the tab stop. Ctrl shifts the value of letters and numbers from the ASCII graphics range, for example, CTRL-S is XOFF CTRL-Q is XON. Esc produces an ASCII escape character and it may be used to exit menus or modes. ~ is the tilde, an accent backspaced and printed over other letters for non-English languages. The single quote is used for an acute accent. ^ is a circumflex
21.
Esc key
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On computer keyboards, the Esc key is a key used to generate the escape character. It is now placed at the top left corner of the keyboard. The keyboard symbol for the ESC key is standardized in ISO/IEC 9995-7 as symbol 29 and this symbol is encoded in Unicode as U+238B broken circle with northwest arrow. The Escape keys creation is credited to Bob Bemer, a programmer who worked for IBM. He created the key in 1960 to allow programmers working with machines to switch from one type of code to another. Much later printers and computer terminals that would use escape sequences often would take more than one following byte as part of a special sequence and this key combination still works as of Windows 10. Microsoft Windows makes use of Esc for many key shortcuts, many of these shortcuts have been present since Windows 3.0, through Windows XP, Windows Vista, Windows 7, Windows 8, and Windows 10. In OS X, Esc usually closes or cancels a dialog box or sheet, the ⌘ Command+⌥ Option+⎋ Esc combination opens the Force Quit dialog box, allowing users to end non-responsive programs. Another use for the Esc key, in combination with the Command key, is switching to Front Row, if installed. In most computer games, the key is used as a pause button and/or as a way to bring up the in-game menu. In the vi family of text editors, escape is used to switch modes and this usage is due to escape being conveniently placed in what is now the tab position on the ADM-3A terminal keyboard used to develop vi, though it is now inconveniently placed. Although such devices are long out of use, standard processing of ANSI Escape sequences very similar to 1970s VT100, is implemented in both ANSI
22.
Function key
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On some keyboards/computers, function keys may have default actions, accessible on power-on. Function keys may have default actions printed on/besides them, or they may have the more common F-number designations, the Singer/Friden 2201 Flexowriter Programatic, introduced in 1965, had a cluster of 13 function keys, labeled F1 to F13 to the right of the main keyboard. Although the Flexowriter could be used as a terminal, this electromechanical typewriter was primarily intended as a stand-alone word processing system. The interpretation of the keys was determined by the programming of a plugboard inside the back of the machine. Soft keys date to avionics multi-function displays of military planes of the late 1960s/early 1970s, the HP 9830A was an early desktop computer, and one of the earliest specifically computing uses. HP continued its use of keys in the HP2640, which used screen-labeled function keys, placing the keys close to the screen. NECs PC-8001, introduced in 1979, featured five function keys at the top of the keyboard and their modern use may have been popularized by IBM keyboards, first the IBM3270 terminals, then the IBM PC. Later models replaced this with a keypad, and moved the function keys to 24 keys at the top of the keyboard. The original IBM PC keyboard had 10 function keys in a 2×5 matrix at the left of the keyboard, this was replaced by 12 keys in 3 blocks of 4 at the top of the keyboard in the Model M. Since the introduction of the Apple Extended Keyboard with the Macintosh II, however, keyboards with function keys have been available and they have not traditionally been a major part of the Mac user interface, however, and are generally only used on cross-platform programs. According to the Macintosh Human Interface Guidelines, they are reserved for customization by the user, current Mac keyboards include specialized function keys for controlling sound volume. The most recent Mac keyboards include 19 function keys, but keys F1–F4 and F7–F12 by default control features such as volume, media control, former keyboards and Apple Keyboard with numeric keypad has the F1–F19 keys. Apple Macintosh notebooks, Function keys were not standard on Apple notebook hardware until the introduction of the PowerBook 5300, for the most part, Mac laptops have keys F1 through F12, with pre-defined actions for some, including controlling sound volume and screen brightness. Atari 8-bit family, four dedicated keys at the hand side or on the top of the keyboard. Atari 1200XL had four additional keys labeled F1 through F4 with pre-defined actions, atari ST, ten parallelogram-shaped keys in a horizontal row across the top of the keyboard, inset into the keyboard frame instead of popping up like normal keys. BBC Micro, red/orange keys F0 to F9 in a row above the number keys on top of the computer/keyboard. The break, arrow, and copy keys could function as F10–F15, the case included a transparent plastic strip above them to hold a function key reference card. Coleco Adam, six dark brown keys in a row above the number keys
23.
Print screen
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Print screen is a key present on most PC keyboards. It is typically situated in the section as the break key. The print screen may share the key as system request. Under command-line based operating systems such as MS-DOS, this key causes the contents of the current text mode screen memory buffer to be copied to the printer port. In essence, whatever was currently on the screen when the key was pressed will be printed, pressing the Ctrl key in combination with Prt Sc turns on and off the printer echo feature. When echo is in effect, any conventional text output to the screen will be copied to the printer, there is also a Unicode character for print screen, U+2399 ⎙ print screen symbol. Newer-generation operating systems using a graphical interface tend to save a bitmap image of the current screen, or screenshot, some shells allow modification of the exact behavior using modifier keys such as the control key. In Microsoft Windows, pressing Prt Sc will capture the entire screen, the captured image can then be pasted into an editing program such as a graphics program or even a word processor. Pressing Prt Sc with both the left Alt key and left ⇧ Shift pressed turns on a high contrast mode, since Windows 8, pressing the ⊞ Win key in combination with Prt Sc will save the captured image to disk. This behavior is therefore compatible with users who learned Print Screen actions under operating systems such as MS-DOS. Sending the image to many services, or even screen recording, is built-in too, macintosh does not use a print screen key, instead, key combinations are used that start with ⌘ Cmd+⇧ Shift. These key combinations are used to more functionality including the ability to select screen objects. ⌘ Cmd+⇧ Shift+3 captures the screen, while ⌘ Cmd+⇧ Shift+4 allows for part of the screen to be selected. The standard print screen functions described above save the image to the desktop, however, using any of the key sequences described above, but additionally pressing the Ctrl will modify the behavior to copy the image to the system clipboard instead. On the IBM Model F keyboard, the key is labeled PrtSc and is located under the Enter key, on the IBM Model M, it is located next to the F12 key and is labeled Print Screen
24.
System request
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System request is a key on personal computer keyboards that has no standard use. A special BIOS routine — software interrupt 0x15, subfunction 0x85 — was added to signal the OS when SysRq was pushed or released, unlike most keys, when it is pressed nothing is stored in the keyboard buffer. The specific low level function that the SysRq key was meant for was to switch between operating systems, when the original IBM-PC was created in 1980, there were three leading competing operating systems, PC DOS, CP/M-86, and UCSD p-System, while Xenix was added in 1983-1984. The SysRq key was added so that multiple operating systems could be run on the same computer, a special key was needed because most software of the day operated at a low level, often bypassing the OS entirely, and typically made use of many hotkey combinations. The use of Terminate and Stay Resident programs further complicated matters, to implement a task switching or multitasking environment, it was thought that a special, separate key was needed. This is similar to the way “Control-Alt-Delete” is used under Windows NT, on 84-key keyboards, SysRq was a key of its own. On the later 101-key keyboard, it shares a key with the Print Screen key function. One must hold down the Alt key while pressing this “dual-function” key to invoke SysRq, the default BIOS keyboard routines simply ignore SysRq and return without taking action. So did the MS-DOS input routines, the keyboard routines in libraries supplied with many high-level languages followed suit. Although it is included on most PC keyboards manufactured, and though it is used by some debugging software. On the Hyundai/Hynix Super-16 computer, pressing Ctrl+SysRq will hard boot the system, in Linux, the kernel can be configured to provide functions for system debugging and crash recovery. This use is known as the “Magic SysRq key”, microsoft has also used SysRq for various OS- and application-level debuggers. In the CodeView debugger, it was used to break into the debugging during program execution. For the Windows NT remote kernel debugger, it can be used to force the system into the debugger, in embedded systems, SysRq key is usually used to assert low-level on RESET# signal. Print Screen Serial console Magic SysRq key Break key Scroll Lock
25.
Scroll lock
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Scroll lock is a lock key on most IBM-compatible computer keyboards. Depending on the system, it may be used for different purposes. The key is not frequently used, and therefore some reduced or specialized keyboards lack Scroll Lock altogether, pressing Ctrl+Scroll Lock performs the same function as pressing Ctrl+Break. This behavior is a remnant of the original IBM PC keyboards, instead, they assigned the Pause function to Ctrl+Num Lock and the Break function to Ctrl+Scroll Lock. The Scroll Lock key was meant to lock all scrolling techniques, in the original design, Scroll Lock was intended to modify the behavior of the arrow keys. When the Scroll Lock mode was on, the keys would scroll the contents of a text window instead of moving the cursor. In this usage, Scroll Lock is a lock key like Num Lock or Caps Lock. Today, this use of Scroll Lock is rare. Modern programs honoring this behavior include IBM Lotus Notes, Forté Agent, Image-Line FL Studio, Renoise, Microsoft Excel, LibreOffice Calc, some text editors exhibit similar behavior when the arrow keys are used with a Ctrl depressed. Most GUI environments neglect Scroll Lock, which means scrolling must be accomplished with a mouse, often, the middle or the wheel mouse button works as a toggle determining if mouse movements will move the mouse cursor or scroll the contents in scroll window. Some web browsers including Firefox and Internet Explorer support a Caret browsing mode which can be toggled by F7, when Scroll Lock is pressed again the screen is unfrozen and all text generated during the freeze is printed at once. This allows the user to pause the display and read long messages that scroll through the screen too quickly to read, for example when the system is booting up. If not configured otherwise, Ctrl+S and Ctrl+Q can be used instead of Scroll Lock, in any terminal in Linux and this behaviour emulates the Hold Screen key or similar flow control mechanisms on computer terminals. The behaviour must be distinguished from pressing the Pause key during POST or under DOS, on the FreeBSD system console, the Scroll Lock key additionally enables scrolling, after pressing Scroll Lock, one can use Page Up and Page Down to scroll up or down. Pressing Scroll Lock again disables this mode and moves back to the bottom of the output, the Linux console implements this behaviour by using ⇧ Shift+Page Up and ⇧ Shift+Page Down, not requiring Scroll Lock to be active. Automatic scrolling is suspended for as long as Scroll Lock remains activated, dR-DOS7.02 and higher provides a CONFIG. SYS directive named SCROLLOCK=ON|OFF to change the Scroll Lock status on the fly. The operating system, however, will temporarily suspend any automatic timeouts in CONFIG. SYS trace and this can be accomplished by setting the REG_DWORD registry key HKEY_LOCAL_MACHINE\SYSTEM\CurrentControlSet\Services\i8042prt\Parameters\CrashOnCtrlScroll to 1. Once set, the user can hold the RCtrl key and tap the Scroll Lock key twice to trigger a Blue Screen of Death
26.
Break key
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The Break key of a computer keyboard refers to breaking a telegraph circuit, and originated with 19th century practice. Because the break function is combined with the pause function on one key since the introduction of the IBM Model M 101-key keyboard in 1985. It can be used to some computer games. A standard telegraph circuit connects all the keys, sounders and batteries in a single series loop, thus the sounders actuate only when both keys are down. So the receiving operator has to hold his key down, or close a built-in shorting switch, as a consequence the receiving operator could interrupt the sending operator by opening his key, breaking the circuit and forcing it into a spacing condition. Both sounders stop responding to the senders keying, alerting the sender, the teleprinter operated in a very similar fashion except that the sending station kept the loop closed even during short pauses between characters. Holding down a special break key opened the loop, forcing it into a continuous logic 0 or spacing condition, when this occurred, the teleprinter mechanisms continually actuated without printing anything as the all-0s character is the non-printing NUL in both Baudot and ASCII. The resulting noise got the sending operators attention and this practice carried over to teleprinter use on time-sharing computers. A continuous spacing condition violates the rule that every character has to end with one or more logic 1 stop bits. The computer recognized this as a special condition and generated an interrupt that typically stopped a running program or forced the operating system to prompt for a login. Although asynchronous serial telegraphy is now rare, the Break key once used with terminal emulators can still be used by software for similar purposes, on the Sinclair ZX80 and ZX81 computers, the Break is accessed by pressing Space. On the Sinclair ZX Spectrum it is accessed by Caps Shift+Space, the Spectrum+ and later computers have a dedicated Break key. It does not trigger an interrupt but will halt any running BASIC program, an interrupted BASIC program can usually be resumed with the CONTINUE command. The Sinclair QL computer, without a Break key, maps the function to Ctrl+Space, on a BBC Micro computer, the Break key generates an interrupt which would normally cause a warm restart of the computer. A cold restart is triggered by typing Ctrl+Break, if a DFS ROM is installed, ⇧ Shift+Break will cause the computer to search for and load a file called. Boot on the floppy disk in drive 0. The latter two behaviours were inherited by the successor to Acorn MOS, RISC OS, on many modern PCs, Pause interrupts screen output by BIOS until another key is pressed. This is effective during boot in text mode and in a DOS box in Windows safe mode with 50 lines. The Pause key is different from all keys in that it sends no scancodes at all on release
27.
Tilde
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The tilde is a grapheme with several uses. The name of the character came into English from Spanish, which in came from the Latin titulus. The reason for the name was that it was written over a letter as a scribal abbreviation, as a mark of suspension. Thus the commonly used words Anno Domini were frequently abbreviated to Ao Dñi, such a mark could denote the omission of one letter or several letters. This saved on the expense of the labour and the cost of vellum. Medieval European charters written in Latin are largely made up of such abbreviated words with suspension marks and other abbreviations, the tilde has since been applied to a number of other uses as a diacritic mark or a character in its own right. These are encoded in Unicode at U+0303 ◌̃ Combining Tilde and U+007E ~ Tilde, in lexicography, the latter kind of tilde and the swung dash are used in dictionaries to indicate the omission of the entry word. This symbol informally means approximately, about, or around, such as ~30 minutes before and it can mean similar to, including of the same order of magnitude as, such as, x ~ y meaning that x and y are of the same order of magnitude. The tilde is used to indicate equal to or approximately equal to by placing it over the = symbol, like so. The text of the Domesday Book of 1086, relating for example, the text with abbreviations expanded is as follows, Mollande tempore regis Edwardi geldabat pro iiii hidis et uno ferling. In dominio sunt iii carucae et x servi et xxx villani et xx bordarii cum xvi carucis, ibi xii acrae prati et xv acrae silvae. Pastura iii leugae in longitudine et latitudine, elwardus tenebat tempore regis Edwardi pro manerio et geldabat pro dimidia hida. Ibi sunt v villani cum i servo, valet xx solidos ad pensam et arsuram. Eidem manerio est injuste adjuncta Nimete et valet xv solidos, ipsi manerio pertinet tercius denarius de Hundredis Nortmoltone et Badentone et Brantone et tercium animal pasturae morarum. The incorporation of the tilde into ASCII is a result of its appearance as a distinct character on mechanical typewriters in the late nineteenth century. Any good typewriter store had a catalog of alternative keyboards that could be specified for machines ordered from the factory, at that time, the tilde was used only in Spanish and Portuguese typewriters. In Modern Spanish, the tilde is used only with n and N, both were conveniently assigned to a single mechanical typebar, which sacrificed a key that was felt to be less important, usually the 1⁄2— 1⁄4 key. Portuguese, however, uses not ñ but nh and it uses the tilde on the vowels a and o
28.
Exclamation mark
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The exclamation mark or exclamation point is a punctuation mark usually used after an interjection or exclamation to indicate strong feelings or high volume, and often marks the end of a sentence. Similarly, an exclamation mark is often used in warning signs. Other uses include, In mathematics it denotes the factorial operation, at the beginning of an expression to denote logical negation, e. g. A means the logical negation of A, also called not A. Graphically the exclamation mark is represented as a stop point with a vertical line above. One theory of its origin is that it is derived from a Latin exclamation of joy, the modern graphical representation is believed to have been born in the Middle Ages. Medieval copyists wrote the Latin word io at the end of a sentence to indicate joy, over time, the i moved above the o, and the o became smaller, becoming a point. The exclamation mark did not have its own dedicated key on standard manual typewriters before the 1970s, instead, one typed a period, backspaced, and typed an apostrophe. In the 1950s, secretarial dictation and typesetting manuals in America referred to the mark as bang, appeared in dialogue balloons to represent a gun being fired, although the nickname probably emerged from letterpress printing. This bang usage is behind the names of the interrobang, an unconventional character, and a shebang line. In the printing world, the mark can be called a screamer, a gasper. In hacker culture, the mark is called bang, shriek, or, in the British slang known as Commonwealth Hackish. For example, the password communicated in the spoken phrase Your password is em-nought-pee-aitch-pling-en-three is m0ph. n3, the exclamation mark is common to languages using the Latin alphabet, although usage varies slightly between languages. The exclamation mark was adopted in languages written in other scripts, such as Greek, Russian, Arabic, Hebrew, Chinese, Korean, Japanese and Devanagari. A sentence ending in an exclamation mark may be an exclamation, or an imperative, or may indicate astonishment or surprise, They were the footprints of a gigantic hound. Exclamation marks are occasionally placed mid-sentence with a similar to a comma, for dramatic effect, although this usage is obsolescent, On the walk. Informally, exclamation marks may be repeated for emphasis. The exclamation mark is used in conjunction with the question mark. This can be in protest or astonishment, a few writers replace this with a single, nonstandard punctuation mark, the interrobang, which is the combination of a question mark and an exclamation mark
29.
At sign
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The at sign, @, normally read aloud as at, also commonly called the at symbol or commercial at, was originally an accounting and commercial invoice abbreviation meaning at a rate of. In contemporary use, the at sign is most commonly used in email addresses and it is now universally included on computer keyboards. The mark is encoded as U+0040 @ Commercial AT, the earliest yet discovered reference to the @ symbol is a religious one, it features in a Bulgarian translation of a Greek chronicle written by Constantinos Manasses in 1345. Held today in the Vatican Apostolic Library, it features the @ symbol in place of the letter alpha A in the word Amen. Why it was used in context is still a mystery. In terms of the character of the at sign, there are several theories pending verification. One theory is that the developed as a mercantile shorthand symbol of each at. For example, the cost of 12 apples @ $1 would be $12, whereas the cost of 12 apples at $1 would be $1, another theory is that medieval monks abbreviated the Latin word ad next to a numeral. One reason for the abbreviation saving space and ink, a theory concerning this graphic puts forward the idea that the form derives from the Latin word ad, using the older form of lower case d, ∂, which persists as the partial derivative symbol. It has been theorized that it was originally an abbreviation of the Greek preposition ανά, meaning at the rate of or per. It is also used like this in Modern French, Swedish or Czech, in view, the at-symbol is a stylised form of à. The compromise between @ and à in French handwriting is found in street market signs, an Italian academic claims to have traced the @ symbol to the 16th century, in a mercantile document sent by Florentine Francesco Lapi from Seville to Rome on May 4,1536. The document is about commerce with Pizarro, in particular the price of an @ of wine in Peru, in Italian, the symbol was interpreted to mean amphora. Currently, the word means both the at-symbol and a unit of weight. Until now the first historical document containing a symbol resembling a @ as a one is the Spanish Taula de Ariza. Even though the oldest fully developed modern @ sign is the one found on the above-mentioned Florentine letter, in contemporary English usage, @ is a commercial symbol, called at site or at rate meaning at and at the rate of. It has rarely used in financial documents or grocers price tags. Since 23 October 2012, the At-sign is registered as a mark by the German Patent
30.
Dollar sign
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The dollar sign is a symbol primarily used to indicate the various units of currency around the world. The symbol can interchangeably have one or two vertical strokes, note that the two-stroked version is not the same symbol as the cifrão. In common usage, the sign appears to the left of the amount specified, the best documented explanation holds that the sign evolved out of the Spanish and Spanish American scribal abbreviation pˢ for pesos. A study of late eighteenth- and early nineteenth-century manuscripts shows that the s gradually came to be written over the p, a variation, though less plausible, of this hypothesis derives the sign from a combination of the Greek character psi and S. There are a number of hypotheses about the origin of the symbol, some with a measure of academic acceptance. Kingdom of Sicily Denaro minted in the Kingdom in Sicily at least by 1258–1266 had what could be construed as a dollar symbol. These coins were widely circulated due to the Crusades – and specifically the Crusade that targeted Tunis, several alternative hypotheses relate specifically to the dollar sign drawn with two vertical lines. A common hypothesis holds that the sign derives from the Spanish coat of arms, in 1492, Ferdinand II of Aragon adopted the symbol of the Pillars of Hercules and added the Latin warning Non plus ultra meaning nothing further beyond, indicating this is the end of the world. But when Christopher Columbus came to America, the legend was changed to Plus ultra, the Pillars of Hercules wrapped in a banner thus became a symbol of the New World. In this example the right-hand pillar clearly resembles the dollar sign, the symbol was adopted by Charles V and was part of his coat of arms representing Spains American possessions. The symbol was later stamped on coins minted in gold and silver, the coin, also known as Spanish dollar, was the first global currency used in the entire world since the Spanish Empire was the first global Empire. These coins, depicting the pillars over two hemispheres and a small S-shaped ribbon around each, were spread throughout America, Europe and Asia. According to this, traders wrote signs that, instead of saying Spanish dollar, had this made by hand. When the US got independence from the UK, they created the American dollar, a dollar sign with two vertical lines could have started off as a monogram of USA, used on money bags issued by the United States Mint. The letters U and S superimposed resemble the historical double-stroke dollar sign, robert Morris was such a zealous patriot – known as the Financier of the Revolution in the West – that conjecture does not overstep its bounds in purporting this hypothesis as viable. A similar idea claims that the letters U and S would stand for unit of silver, referencing pieces of eight again, another hypothesis is that it derives from the symbol used on a German Thaler. This refers to the Bible, Numbers, Chapter 21, a similar symbol, constructed by superposition of S and I or J, was used to denote German Joachimsthaler. It was known in the English-speaking world by the 17th century, robert Morris was the first to use that symbol in official documents and in official communications with Oliver Pollock
31.
Percent sign
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The percent sign is the symbol used to indicate a percentage, a number or ratio as a fraction of 100. Related signs include the permille sign ‰ and the permyriad sign ‱, english style guides prescribe writing the number and percent sign without any space between. In Finnish, the percent sign is always spaced, and a case suffix can be attached to it using the colon, in French, the percent sign must be spaced with a non-breaking space. In Italian, the percent sign is never spaced, in Spanish, the percent sign must always be spaced now, as almost every other symbol. In traditional Russian typography, the percent sign is never spaced, but it is not that common in Russia today. In Chinese, the percent sign is almost never spaced, probably because Chinese does not use spaces to separate characters or words at all, according to the Swedish Language Council, the percent sign should be preceded by a space in Swedish, as all other units. In German, the space is prescribed by the body in the national standard DIN5008. In Persian and Turkish, the percent sign precedes rather than follows the number and it is often recommended that the percent sign only be used in tables and other places with space restrictions. In running text, it should be spelled out as percent or per cent, for example, not Sales increased by 24% over 2006, but rather Sales increased by 24 percent over 2006. Prior to 1425 there is no evidence of a special symbol being used for percentage. The Italian term per cento, for a hundred, was used as well as several different abbreviations, examples of this can be seen in the 1339 arithmetic text depicted below. The letter p with its crossed by a horizontal or diagonal strike conventionally stood for per, por, par, or pur in Mediaeval. At some point a scribe of some sort used the abbreviation pc with a loop or circle This appears in some additional pages of a 1425 text which were probably added around 1435. The pc with a loop eventually evolved into a fraction sign by 1650. In 1925 D. E. Smith wrote, The solidus form is modern, the ASCII code for the percent character is 37, or 0x25 in hexadecimal. Names for the percent sign include percent sign, mod, grapes, in the textual representation of URIs, a % immediately followed by a 2-digit hexadecimal number denotes an octet specifying a character that might otherwise not be allowed in URIs. In SQL, the percent sign is a character in LIKE expressions. In TeX and PostScript, a % denotes a line comment, in BASIC, a trailing % after a variable name marks it as an integer
32.
Ampersand
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The ampersand is the logogram &, representing the conjunction word and. It originated as a ligature of the et, Latin for. The word ampersand is a corruption of the phrase and per se &, meaning and intrinsically the word, traditionally, when reciting the alphabet in English-speaking schools, any letter that could also be used as a word in itself was repeated with the Latin expression per se. This habit was useful in spelling where a word or syllable was repeated after spelling, e. g. d, o, g—dog would be clear but simply saying a—a would be confusing without the clarifying per se added. It was also common practice to add the & sign at the end of the alphabet as if it were the 27th letter, pronounced as the Latin et or later in English as and. As a result, the recitation of the alphabet would end in X, Y, Z and this last phrase was routinely slurred to ampersand and the term had entered common English usage by 1837. Through popular etymology, it has been claimed that André-Marie Ampère used the symbol in his widely read publications. The ampersand can be traced back to the 1st century A. D. during the following development of the Latin script that led up to the Carolingian minuscule the use of ligatures in general diminished. The et-ligature, however, continued to be used and gradually became more stylized, the modern italic type ampersand is a kind of et ligature that goes back to the cursive scripts developed during the Renaissance. After the advent of printing in Europe in 1455, printers made extensive use of both the italic and Roman ampersands, since the ampersands roots go back to Roman times, many languages that use a variation of the Latin alphabet make use of it. The ampersand often appeared as a letter at the end of the Latin alphabet, similarly, & was regarded as the 27th letter of the English alphabet, as used by children. An example may be seen in M. B, moores 1863 book The Dixie Primer, for the Little Folks. The popular Apple Pie ABC finishes with the lines X, Y, Z, the ampersand should not be confused with the Tironian et, which is a symbol similar to the numeral 7. Both symbols have their roots in the antiquity, and both signs were used up through the Middle Ages as a representation for the Latin word et. However, while the ampersand was in origin a common ligature in the everyday script, the Tironian et is found in old Irish language script, a Latin-based script generally only used for decorative purposes today, where it signifies agus in Irish. This symbol may have entered the language by way of monastic influence in the time of the early Christian church in Ireland. In everyday handwriting, the ampersand is sometimes simplified in design as a large lowercase epsilon or a backwards numeral 3 superimposed by a vertical line. The ampersand is also shown as a backwards 3 with a vertical line above and below it or a dot above
33.
Asterisk
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An asterisk is a typographical symbol or glyph. It is so called because it resembles a conventional image of a star, computer scientists and mathematicians often vocalize it as star. In English, an asterisk is usually five-pointed in sans-serif typefaces, six-pointed in serif typefaces and it can be used as censorship. It is also used on the Internet to correct ones spelling, the asterisk is derived from the need of the printers of family trees in feudal times for a symbol to indicate date of birth. The original shape was seven-armed, each arm like a shooting from the center. In computer science, the asterisk is used as a wildcard character, or to denote pointers, repetition. Origin Adamantius is known to have used the asteriskos to mark missing Hebrew lines from his Hexapla. The asterisk evolved in shape over time, but its meaning as a used to correct defects remained. In the Middle Ages, the asterisk was used to emphasize a part of text. However, an asterisk was not always used, one hypothesis to the origin of the asterisk is that it stems from the five thousand year old Sumerian character dingir,
34.
Parenthesis
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A bracket is a tall punctuation mark typically used in matched pairs within text, to set apart or interject other text. The matched pair may be described as opening and closing, or left, forms include round, square, curly, and angle brackets, and various other pairs of symbols. Chevrons were the earliest type of bracket to appear in written English, desiderius Erasmus coined the term lunula to refer to the rounded parentheses, recalling the shape of the crescent moon. Some of the names are regional or contextual. Sometimes referred to as angle brackets, in cases as HTML markup. Occasionally known as broken brackets or brokets, ⸤ ⸥, ｢ ｣ – corner brackets ⟦ ⟧ – double square brackets, white square brackets Guillemets, ‹ › and « », are sometimes referred to as chevrons or angle brackets. The characters ‹ › and « », known as guillemets or angular quote brackets, are actually quotation mark glyphs used in several European languages, which one of each pair is the opening quote mark and which is the closing quote varies between languages. In English, typographers generally prefer to not set brackets in italics, however, in other languages like German, if brackets enclose text in italics, they are usually set in italics too. Parentheses /pəˈrɛnθᵻsiːz/ contain material that serves to clarify or is aside from the main point, a milder effect may be obtained by using a pair of commas as the delimiter, though if the sentence contains commas for other purposes, visual confusion may result. In American usage, parentheses are considered separate from other brackets. Parentheses may be used in writing to add supplementary information. They can also indicate shorthand for either singular or plural for nouns and it can also be used for gender neutral language, especially in languages with grammatical gender, e. g. he agreed with his physician. Parenthetical phrases have been used extensively in informal writing and stream of consciousness literature, examples include the southern American author William Faulkner as well as poet E. E. Cummings. Parentheses have historically been used where the dash is used in alternatives, such as parenthesis) is used to indicate an interval from a to c that is inclusive of a. That is, [5, 12) would be the set of all numbers between 5 and 12, including 5 but not 12. The numbers may come as close as they like to 12, including 11.999 and so forth, in some European countries, the notation [5, 12[ is also used for this. The endpoint adjoining the bracket is known as closed, whereas the endpoint adjoining the parenthesis is known as open, if both types of brackets are the same, the entire interval may be referred to as closed or open as appropriate. Whenever +∞ or −∞ is used as an endpoint, it is considered open
35.
Plus and minus signs
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The plus and minus signs are mathematical symbols used to represent the notions of positive and negative as well as the operations of addition and subtraction. Their use has extended to many other meanings, more or less analogous. Plus and minus are Latin terms meaning more and less, respectively, though the signs now seem as familiar as the alphabet or the Hindu-Arabic numerals, they are not of great antiquity. In Europe in the early 15th century the letters P and M were generally used, the symbols appeared for the first time in Luca Pacioli’s mathematics compendium, Summa de arithmetica, geometria, proportioni et proportionalità, first printed and published in Venice in 1494. The + is a simplification of the Latin et, the − may be derived from a tilde written over m when used to indicate subtraction, or it may come from a shorthand version of the letter m itself. In his 1489 treatise Johannes Widmann referred to the symbols − and + as minus and mer, was − ist, das ist minus, und das + ist das mer. They werent used for addition and subtraction here, but to indicate surplus and deficit, the plus sign is a binary operator that indicates addition, as in 2 +3 =5. It can also serve as an operator that leaves its operand unchanged. This notation may be used when it is desired to emphasize the positiveness of a number, the plus sign can also indicate many other operations, depending on the mathematical system under consideration. Many algebraic structures have some operation which is called, or is equivalent to and it is conventional to use the plus sign to only denote commutative operations. Subtraction is the inverse of addition, directly in front of a number and when it is not a subtraction operator it means a negative number. For instance −5 is negative 5, a unary operator that acts as an instruction to replace the operand by its additive inverse. For example, if x is 3, then −x is −3, similarly, − is equal to 2. The above is a case of this. All three uses can be referred to as minus in everyday speech, further, some textbooks in the United States encourage −x to be read as the opposite of x or the additive inverse of x to avoid giving the impression that −x is necessarily negative. However, in programming languages and Microsoft Excel in particular, unary operators bind strongest, so in those cases −5^2 is 25. Some elementary teachers use raised plus and minus signs before numbers to show they are positive or negative numbers. For example, subtracting −5 from 3 might be read as positive three take away negative 5 and be shown as 3 − −5 becomes 3 +5 =8, in grading systems, the plus sign indicates a grade one level higher and the minus sign a grade lower
36.
Backspace
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In some typewriters, a typist would, for example, type a lowercase letter A with acute accent by typing a lowercase letter A, backspace, and then the acute accent key. This technique is the basis for such spacing modifiers in computer character sets such as the ASCII caret, some software like TeX or Microsoft Windows use the opposite method for diacritical marks, namely positioning the accent first, and then the base letter on its position. A dedicated symbol for backspace exists as U+232B ⌫ but its use as a label is moderately rare. The backspace is distinct from the key, which in paper media for computers would punch out all the holes to strike out a character. Also, the key often works as a generic command to remove an object. In modern systems, the key is often mapped to the delete character. The backspace key is used to go back a page or up one level in graphical web or file browsers. Pressing the backspace key on a terminal would generate the ASCII code 08, BS or Backspace. That control code could also be accessed by pressing Control-H, as H is the letter of the Latin alphabet. Even if a terminal did interpret backspace by deleting the preceding character, then, the senders screen would show a message without the supposedly deleted text, while that text, and the deletion codes, would be visible to the recipient. Example, Be nice to this fool^H^H^H^Hgentleman, hes visiting from corporate HQ, an alternative sometimes seen is ^W, which is the shortcut to delete the previous word in the Berkeley Unix terminal line discipline. This shortcut has also made it into the mode of the Vi text editor. In a mainframe environment, to backspace means to move a magnetic tape backwards, typically to the previous block
