ASCII (/ˈæskiː/ⓘASS-kee),[3]: 6 an acronym forAmerican Standard Code for Information Interchange, is acharacter encoding standard for electronic communication. ASCII codes represent text in computers,telecommunications equipment, and other devices. ASCII has just 128code points, of which only 95 areprintable characters, which severely limit its scope. The set of available punctuation had significant impact on the syntax of computer languages and text markup. ASCII hugely influenced the design of character sets used by modern computers; for example the first 128 code points ofUnicode are the same as ASCII.
ASCII was developed in part fromtelegraph code. Its first commercial use was in theTeletype Model 33 and the Teletype Model 35 as a seven-bitteleprinter code promoted by Bell data services.[when?] Work on the ASCII standard began in May 1961, with the first meeting of the American Standards Association's (ASA) (now theAmerican National Standards Institute or ANSI) X3.2 subcommittee. The first edition of the standard was published in 1963,[5][6] underwent a major revision during 1967,[7][8] and experienced its most recent update during 1986.[9] Compared to earlier telegraph codes, the proposed Bell code and ASCII were both ordered for more convenient sorting (i.e., alphabetization) of lists and added features for devices other than teleprinters.[9]
The use of ASCII format for Network Interchange was described in 1969.[10] That document was formally elevated to an Internet Standard in 2015.[11]
Originally based on the (modern)English alphabet, ASCII encodes 128 specifiedcharacters into seven-bit integers as shown by the ASCII chart in this article.[12] Ninety-five of the encoded characters are printable: these include the digits0 to9, lowercase lettersa toz, uppercase lettersA toZ, andpunctuation symbols. In addition, the original ASCII specification included 33 non-printingcontrol codes which originated withTeletype models; most of these are now obsolete,[13] although a few are still commonly used, such as thecarriage return,line feed, andtab codes.
For example, lowercasei would be represented in the ASCII encoding bybinary 1101001 =hexadecimal 69 (i is the ninth letter) =decimal 105.
Despite being an American standard, ASCII does not have a code point for thecent (¢). It also does not supportEnglish terms with diacritical marks such asrésumé andjalapeño, orproper nouns with diacritical marks such asBeyoncé (although on certain devices characters could be combined with punctuation such asTilde (~) andBacktick (`) to approximate such characters.)
ASCII (1963).Control Pictures of equivalent controls are shown where they exist, or a grey dot otherwise.
The American Standard Code for Information Interchange (ASCII) was developed under the auspices of a committee of the American Standards Association (ASA), called the X3 committee, by its X3.2 (later X3L2) subcommittee, and later by that subcommittee's X3.2.4 working group (nowINCITS). The ASA later became the United States of America Standards Institute (USASI)[3]: 211 and ultimately became theAmerican National Standards Institute (ANSI).
With the other special characters and control codes filled in, ASCII was published as ASA X3.4-1963,[6][14] leaving 28 code positions without any assigned meaning, reserved for future standardization, and one unassigned control code.[3]: 66, 245 There was some debate at the time whether there should be more control characters rather than the lowercase alphabet.[3]: 435 The indecision did not last long: during May 1963 the CCITT Working Party on the New Telegraph Alphabet proposed to assign lowercase characters tosticks[a][15] 6 and 7,[16] andInternational Organization for Standardization TC 97 SC 2 voted during October to incorporate the change into its draft standard.[17] The X3.2.4 task group voted its approval for the change to ASCII at its May 1963 meeting.[18] Locating the lowercase letters insticks[a][15] 6 and 7 caused the characters to differ in bit pattern from the upper case by a single bit, which simplifiedcase-insensitive character matching and the construction of keyboards and printers.
The X3 committee made other changes, including other new characters (thebrace andvertical bar characters),[19] renaming some control characters (SOM became start of header (SOH)) and moving or removing others (RU was removed).[3]: 247–248 ASCII was subsequently updated as USAS X3.4-1967,[7][20] then USAS X3.4-1968,[21] ANSI X3.4-1977, and finally, ANSI X3.4-1986.[9][22]
In the X3.15 standard, the X3 committee also addressed how ASCII should be transmitted (least significant bit first)[3]: 249–253 [29] and recorded on perforated tape. They proposed a9-track standard for magnetic tape and attempted to deal with somepunched card formats.
ITA2 was in turn based onBaudot code, the 5-bit telegraph code Émile Baudot invented in 1870 and patented in 1874.[31]
The committee debated the possibility of ashift function (like inITA2), which would allow more than 64 codes to be represented by asix-bit code. In a shifted code, some character codes determine choices between options for the following character codes. It allows compact encoding, but is less reliable fordata transmission, as an error in transmitting the shift code typically makes a long part of the transmission unreadable. The standards committee decided against shifting, and so ASCII required at least a seven-bit code.[3]: 215 §13.6, 236 §4
The committee considered an eight-bit code, since eight bits (octets) would allow two four-bit patterns to efficiently encode two digits withbinary-coded decimal. However, it would require all data transmission to send eight bits when seven could suffice. The committee voted to use a seven-bit code to minimize costs associated with data transmission. Since perforated tape at the time could record eight bits in one position, it also allowed for aparity bit forerror checking if desired.[3]: 217 §c, 236 §5 Eight-bit machines (with octets as the native data type) that did not use parity checking typically set the eighth bit to 0.[32]
The code itself was patterned so that most control codes were together and all graphic codes were together, for ease of identification. The first two so-calledASCII sticks[a][15] (32 positions) were reserved for control characters.[3]: 220, 236 8, 9) The"space" character had to come before graphics to makesorting easier, so it became position 20hex;[3]: 237 §10 for the same reason, many special signs commonly used as separators were placed before digits. The committee decided it was important to support uppercase64-character alphabets, and chose to pattern ASCII so it could be reduced easily to a usable 64-character set of graphic codes,[3]: 228, 237 §14 as was done in theDEC SIXBIT code (1963).Lowercase letters were therefore not interleaved withuppercase. To keep options available for lowercase letters and other graphics, the special and numeric codes were arranged before the letters, and the letterA was placed in position 41hex to match the draft of the corresponding British standard.[3]: 238 §18 The digits 0–9 are prefixed with 011, but the remaining4 bits correspond to their respective values in binary, making conversion withbinary-coded decimal straightforward (for example, 5 in encoded to 0110101, where 5 is0101 in binary).
Many of the non-alphanumeric characters were positioned to correspond to their shifted position on typewriters; an important subtlety is that these were based onmechanical typewriters, notelectric typewriters.[33] Mechanical typewriters followed thede facto standard set by theRemington No. 2 (1878), the first typewriter with a shift key, and the shifted values of23456789- were"#$%_&'() – early typewriters omitted0 and1, usingO (capital lettero) andl (lowercase letterL) instead, but1! and0) pairs became standard once 0 and 1 became common. Thus, in ASCII!"#$% were placed in the second stick,[a][15] positions 1–5, corresponding to the digits 1–5 in the adjacent stick.[a][15] The parentheses could not correspond to9 and0, however, because the place corresponding to0 was taken by the space character. This was accommodated by removing_ (underscore) from6 and shifting the remaining characters, which corresponded to many European typewriters that placed the parentheses with8 and9. This discrepancy from typewriters led tobit-paired keyboards, notably theTeletype Model 33, which used the left-shifted layout corresponding to ASCII, differently from traditional mechanical typewriters.
Electric typewriters, notably theIBM Selectric (1961), used a somewhat different layout that has becomede facto standard on computers – following theIBM PC (1981), especiallyModel M (1984) – and thus shift values for symbols on modern keyboards do not correspond as closely to the ASCII table as earlier keyboards did. The/? pair also dates to the No. 2, and the,< .> pairs were used on some keyboards (others, including the No. 2, did not shift, (comma) or. (full stop) so they could be used in uppercase without unshifting). However, ASCII split the;: pair (dating to No. 2), and rearranged mathematical symbols (varied conventions, commonly-* =+) to:* ;+ -=.
Some then-common typewriter characters were not included, notably½ ¼ ¢, while^ ` ~ were included as diacritics for international use, and< > for mathematical use, together with the simple line characters\ | (in addition to common/). The@ symbol was not used in continental Europe and the committee expected it would be replaced by an accentedÀ in the French variation, so the@ was placed in position 40hex, right before the letter A.[3]: 243
The control codes felt essential for data transmission were the start of message (SOM), end of address (EOA),end of message (EOM), end of transmission (EOT), "who are you?" (WRU), "are you?" (RU), a reserved device control (DC0), synchronous idle (SYNC), and acknowledge (ACK). These were positioned to maximize theHamming distance between their bit patterns.[3]: 243–245
ASCII-code order is also calledASCIIbetical order.[34]Collation of data is sometimes done in this order rather than "standard" alphabetical order (collating sequence). The main deviations in ASCII order are:
All uppercase come before lowercase letters; for example, "Z" precedes "a"
Digits and many punctuation marks come before letters
An intermediate order converts uppercase letters to lowercase before comparing ASCII values.
ASCII reserves the first 32code points (numbers 0–31 decimal) and the last one (number 127 decimal) forcontrol characters. These are codes intended to controlperipheral devices (such asprinters), or to providemeta-information about data streams, such as those stored on magnetic tape. Despite their name, these code points do not represent printable characters (i.e. they are not characters at all, but signals). For debugging purposes, "placeholder" symbols (such as those given inISO 2047 and its predecessors) are assigned to them.
For example, character 0x0A represents the "line feed" function (which causes a printer to advance its paper), and character 8 represents "backspace".RFC2822 refers to control characters that do not include carriage return, line feed orwhite space as non-whitespace control characters.[35] Except for the control characters that prescribe elementary line-oriented formatting, ASCII does not define any mechanism for describing the structure or appearance of text within a document. Other schemes, such asmarkup languages, address page and document layout and formatting.
The original ASCII standard used only short descriptive phrases for each control character. The ambiguity this caused was sometimes intentional, for example where a character would be used slightly differently on a terminal link than on adata stream, and sometimes accidental, for example the standard is unclear about the meaning of "delete".
Probably the most influential single device affecting the interpretation of these characters was theTeletype Model 33 ASR, which was a printing terminal with an availablepaper tape reader/punch option. Paper tape was a very popular medium for long-term program storage until the 1980s, less costly and in some ways less fragile than magnetic tape. In particular, the Teletype Model 33 machine assignments for codes 17 (control-Q, DC1, also known as XON), 19 (control-S, DC3, also known as XOFF), and 127 (delete) becamede facto standards. The Model 33 was also notable for taking the description of control-G (code 7, BEL, meaning audibly alert the operator) literally, as the unit contained an actual bell which it rang when it received a BEL character. Because the keytop for the O key also showed a left-arrow symbol (from ASCII-1963, which had this character instead ofunderscore), a noncompliant use of code 15 (control-O, shift in) interpreted as "delete previous character" was also adopted by many early timesharing systems but eventually became neglected.
When a Teletype 33 ASR equipped with the automatic paper tape reader received a control-S (XOFF, an abbreviation for transmit off), it caused the tape reader to stop; receiving control-Q (XON, transmit on) caused the tape reader to resume. This so-calledflow control technique became adopted by several early computer operating systems as a "handshaking" signal warning a sender to stop transmission because of impendingbuffer overflow; it persists to this day in many systems as a manual output control technique. On some systems, control-S retains its meaning, but control-Q is replaced by a second control-S to resume output.
The 33 ASR also could be configured to employ control-R (DC2) and control-T (DC4) to start and stop the tape punch; on some units equipped with this function, the corresponding control character lettering on the keycap above the letter was TAPE andTAPE respectively.[36]
The Teletype could not move its typehead backwards, so it did not have a key on its keyboard to send a BS (backspace). Instead, there was a key markedRUB OUT that sent code 127 (DEL). The purpose of this key was to erase mistakes in a manually-input paper tape: the operator had to push a button on the tape punch to back it up, then type the rubout, which punched all holes and replaced the mistake with a character that was intended to be ignored.[37] Teletypes were commonly used with the less-expensive computers fromDigital Equipment Corporation (DEC); these systems had to use what keys were available, and thus the DEL character was assigned to erase the previous character.[38][39] Because of this, DEC video terminals (by default) sent the DEL character for the key marked "Backspace" while the separate key marked "Delete" sent anescape sequence; many other competing terminals sent a BS character for the backspace key.
The early Unix tty drivers, unlike some modern implementations, allowed only one character to be set to erase the previous character in canonical input processing (where a very simple line editor is available); this could be set to BSor DEL, but not both, resulting in recurring situations of ambiguity where users had to decide depending on what terminal they were using (shells that allow line editing, such asksh,bash, andzsh, understand both). The assumption that no key sent a BS character allowed Ctrl+H to be used for other purposes, such as the "help" prefix command inGNU Emacs.[40]
Many more of the control characters have been assigned meanings quite different from their original ones. The "escape" character (ESC, code 27), for example, was intended originally to allow sending of other control characters as literals instead of invoking their meaning, an "escape sequence". This is the same meaning of "escape" encountered in URL encodings,C language strings, and other systems where certain characters have a reserved meaning. Over time this interpretation has been co-opted and has eventually been changed.
In modern usage, an ESC sentto the terminal usually indicates the start of a command sequence, which can be used to address the cursor, scroll a region, set/query various terminal properties, and more. They are usually in the form of a so-called "ANSI escape code" (often starting with a "Control Sequence Introducer", "CSI", "ESC [") from ECMA-48 (1972) and its successors. Some escape sequences do not have introducers, like the "Reset to Initial State", "RIS" command "ESC c".[41]
In contrast, an ESC readfrom the terminal is most often used as anout-of-band character used to terminate an operation or special mode, as in theTECO andvitext editors. Ingraphical user interface (GUI) andwindowing systems, ESC generally causes an application to abort its current operation or toexit (terminate) altogether.
The inherent ambiguity of many control characters, combined with their historical usage, created problems when transferring "plain text" files between systems. The best example of this is thenewline problem on variousoperating systems. Teletype machines required that a line of text be terminated with both "carriage return" (which moves the printhead to the beginning of the line) and "line feed" (which advances the paper one line without moving the printhead). The name "carriage return" comes from the fact that on a manualtypewriter the carriage holding the paper moves while the typebars that strike the ribbon remain stationary. The entire carriage had to be pushed (returned) to the right in order to position the paper for the next line.
DEC operating systems (OS/8,RT-11,RSX-11,RSTS,TOPS-10, etc.) used both characters to mark the end of a line so that the console device (originally Teletype machines) would work. By the time so-called "glass TTYs" (later called CRTs or "dumb terminals") came along, the convention was so well established thatbackward compatibility necessitated continuing to follow it. WhenGary Kildall createdCP/M, he was inspired by some of the command line interface conventions used in DEC's RT-11 operating system.
Until the introduction of PC DOS in 1981,IBM had no influence in this because their 1970s operating systems used EBCDIC encoding instead of ASCII, and they were oriented toward punch-card input and line printer output on which the concept of "carriage return" was meaningless. IBM's PC DOS (also marketed asMS-DOS by Microsoft) inherited the convention by virtue of being loosely based on CP/M,[42] andWindows in turn inherited it from MS-DOS.
Requiring two characters to mark the end of a line introduces unnecessary complexity and ambiguity as to how to interpret each character when encountered by itself. To simplify matters,plain text data streams, including files, onMultics used line feed (LF) alone as a line terminator.[43]: 357 The tty driver would handle the LF to CRLF conversion on output so files can be directly printed to terminal, and NL (newline) is often used to refer to CRLF inUNIX documents.Unix andUnix-like systems, andAmiga systems, adopted this convention from Multics. On the other hand, the originalMacintosh OS,Apple DOS, andProDOS used carriage return (CR) alone as a line terminator; however, since Apple later replaced these obsolete operating systems with their Unix-basedmacOS (formerly named OS X) operating system, they now use line feed (LF) as well. The Radio ShackTRS-80 also used a lone CR to terminate lines.
Computers attached to theARPANET included machines running operating systems such as TOPS-10 andTENEX using CR-LF line endings; machines running operating systems such as Multics using LF line endings; and machines running operating systems such asOS/360 that represented lines as a character count followed by the characters of the line and which used EBCDIC rather than ASCII encoding. TheTelnet protocol defined an ASCII "Network Virtual Terminal" (NVT), so that connections between hosts with different line-ending conventions and character sets could be supported by transmitting a standard text format over the network. Telnet used ASCII along with CR-LF line endings, and software using other conventions would translate between the local conventions and the NVT.[44] TheFile Transfer Protocol adopted the Telnet protocol, including use of the Network Virtual Terminal, for use when transmitting commands and transferring data in the default ASCII mode.[45][46] This adds complexity to implementations of those protocols, and to other network protocols, such as those used for E-mail and the World Wide Web, on systems not using the NVT's CR-LF line-ending convention.[47][48]
The PDP-6 monitor,[38] and its PDP-10 successor TOPS-10,[39] used control-Z (SUB) as an end-of-file indication for input from a terminal. Some operating systems such as CP/M tracked file length only in units of disk blocks, and used control-Z to mark the end of the actual text in the file.[49] For these reasons, EOF, orend-of-file, was used colloquially and conventionally as athree-letter acronym for control-Z instead of SUBstitute. The end-of-text character (ETX), also known ascontrol-C, was inappropriate for a variety of reasons, while using control-Z as the control character to end a file is analogous to the letter Z's position at the end of the alphabet, and serves as a very convenientmnemonic aid. A historically common and still prevalent convention uses the ETX character convention to interrupt and halt a program via an input data stream, usually from a keyboard.
The Unix terminal driver uses the end-of-transmission character (EOT), also known as control-D, to indicate the end of a data stream.
At the time of adoption, the codes 20hex to 7Ehex would cause the printing of a visible character (a glyph), and thus were designated "printable characters". These codes represent letters, digits,punctuation marks, and a few miscellaneous symbols. There are 95 printable characters in total.[n]
The empty space between words, as produced by the space bar of a keyboard, is character code 20hex. Since the space character is visible in printed text it considered a "printable character", even though it is unique in having no visible glyph. It is listed in the printable character table, as per the ASCII standard, instead of in the control character table.[3]: 223 [10]
Code 7Fhex corresponds to the non-printable "delete" (DEL) control character and is listed in the control character table.
Earlier versions of ASCII used the up arrow instead of thecaret (5Ehex) and the left arrow instead of the underscore (5Fhex).[6][50]
ASCII was first used commercially during 1963 as a seven-bit teleprinter code forAmerican Telephone & Telegraph's TWX (TeletypeWriter eXchange) network. TWX originally used the earlier five-bitITA2, which was also used by the competingTelex teleprinter system.Bob Bemer introduced features such as theescape sequence.[5] His British colleagueHugh McGregor Ross helped to popularize this work – according to Bemer, "so much so that the code that was to become ASCII was first called theBemer–Ross Code in Europe".[51] Because of his extensive work on ASCII, Bemer has been called "the father of ASCII".[52]
I have also approved recommendations of theSecretary of Commerce [Luther H. Hodges] regarding standards for recording the Standard Code for Information Interchange on magnetic tapes and paper tapes when they are used in computer operations.All computers and related equipment configurations brought into the Federal Government inventory on and after July 1, 1969, must have the capability to use the Standard Code for Information Interchange and the formats prescribed by the magnetic tape and paper tape standards when these media are used.
As computer technology spread throughout the world, differentstandards bodies and corporations developed many variations of ASCII to facilitate the expression of non-English languages that used Roman-based alphabets. One could class some of these variations as "ASCII extensions", although some misuse that term to represent all variants, including those that do not preserve ASCII's character-map in the 7-bit range. Furthermore, the ASCII extensions have also been mislabelled as ASCII.
From early in its development,[59] ASCII was intended to be just one of several national variants of an international character code standard.
Other international standards bodies have ratified character encodings such asISO 646 (1967) that are identical or nearly identical to ASCII, with extensions for characters outside the Englishalphabet and symbols used outside the United States, such as the symbol for the United Kingdom'spound sterling (£); e.g. withcode page 1104. Almost every country needed an adapted version of ASCII, since ASCII suited the needs of only the US and a few other countries. For example, Canada had its own version that supported French characters.
Many other countries developed variants of ASCII to include non-English letters (e.g.é,ñ,ß,Ł), currency symbols (e.g.£,¥), etc. See alsoYUSCII (Yugoslavia).
It would share most characters in common, but assign other locally useful characters to severalcode points reserved for "national use". However, the four years that elapsed between the publication of ASCII-1963 and ISO's first acceptance of an international recommendation during 1967[60] caused ASCII's choices for the national use characters to seem to bede facto standards for the world, causing confusion and incompatibility once other countries did begin to make their own assignments to these code points.
ISO/IEC 646, like ASCII, is a 7-bit character set. It does not make any additional codes available, so the same code points encoded different characters in different countries. Escape codes were defined to indicate which national variant applied to a piece of text, but they were rarely used, so it was often impossible to know what variant to work with and, therefore, which character a code represented, and in general, text-processing systems could cope with only one variant anyway.
Because the bracket and brace characters of ASCII were assigned to "national use" code points that were used for accented letters in other national variants of ISO/IEC 646, a German, French, or Swedish, etc. programmer using their national variant of ISO/IEC 646, rather than ASCII, had to write, and thus read, something such as
ä aÄiÜ = 'Ön'; ü
instead of
{ a[i] = '\n'; }
C trigraphs were created to solve this problem forANSI C, although their late introduction and inconsistent implementation in compilers limited their use. Many programmers kept their computers on ASCII, so plain-text in Swedish, German etc. (for example, in e-mail orUsenet) contained "{, }" and similar variants in the middle of words, something those programmers got used to. For example, a Swedish programmer mailing another programmer asking if they should go for lunch, could get "N{ jag har sm|rg}sar" as the answer, which should be "Nä jag har smörgåsar" meaning "No I've got sandwiches".
In Japan and Korea, still as of the 2020s,[update] a variation of ASCII is used, in which thebackslash (5C hex) is rendered as ¥ (aYen sign, in Japan) or ₩ (aWon sign, in Korea). This means that, for example, the file path C:\Users\Smith is shown as C:¥Users¥Smith (in Japan) or C:₩Users₩Smith (in Korea).
In Europe,teletext character sets, which are variants of ASCII, are used for broadcast TV subtitles, defined byWorld System Teletext and broadcast using the DVB-TXT standard for embedding teletext into DVB transmissions.[61] In the case that the subtitles were initially authored for teletext and converted, the derived subtitle formats are constrained to the same character sets.
Eventually, as 8-,16-, and32-bit (and later64-bit) computers began to replace12-,18-, and36-bit computers as the norm, it became common to use an 8-bit byte to store each character in memory, providing an opportunity for extended, 8-bit relatives of ASCII. In most cases these developed as true extensions of ASCII, leaving the original character-mapping intact, but adding additional character definitions after the first 128 (i.e., 7-bit) characters. ASCII itself remained a seven-bit code: the term "extended ASCII" has no official status.
For some countries, 8-bit extensions of ASCII were developed that included support for characters used in local languages; for example,ISCII for India andVISCII for Vietnam.KayproCP/M computers used the "upper" 128 characters for the Greek alphabet.[citation needed]
Even for markets where it was not necessary to add many characters to support additional languages, manufacturers of early home computer systems often developed their own 8-bit extensions of ASCII to include additional characters, such asbox-drawing characters,semigraphics, andvideo game sprites. Often, these additions also replaced control characters (index 0 to 31, as well as index 127) with even more platform-specific extensions. In other cases, the extra bit was used for some other purpose, such as togglinginverse video; this approach was used byATASCII, an extension of ASCII developed byAtari.
Most ASCII extensions are based on ASCII-1967 (the current standard), but some extensions are instead based on the earlier ASCII-1963. For example,PETSCII, which was developed byCommodore International for their8-bit systems, is based on ASCII-1963. Likewise, manySharp MZ character sets are based on ASCII-1963.
TheISO/IEC 8859 standard (derived from the DEC-MCS) provided a standard that most systems copied (or at least were based on, when not copied exactly). A popular further extension designed by Microsoft,Windows-1252 (often mislabeled asISO-8859-1), added the typographic punctuation marks needed for traditional text printing. ISO-8859-1, Windows-1252, and the original 7-bit ASCII were the most common character encoding methods on theWorld Wide Web until 2008, whenUTF-8 overtook them.[57]
ISO/IEC 4873 introduced 32 additional control codes defined in the 80–9Fhexadecimal range, as part of extending the 7-bit ASCII encoding to become an 8-bit system.[63]
Unicode and the ISO/IEC 10646Universal Character Set (UCS) have a much wider array of characters and their various encoding forms have begun to supplant ISO/IEC 8859 and ASCII rapidly in many environments. While ASCII is limited to 128 characters, Unicode and the UCS support more characters by separating the concepts of unique identification (usingnatural numbers calledcode points) and encoding (to 8-, 16-, or 32-bit binary formats, calledUTF-8,UTF-16, andUTF-32, respectively).
ASCII was incorporated into the Unicode (1991) character set as the first 128 symbols, so the 7-bit ASCII characters have the same numeric codes in both sets. This allowsUTF-8 to bebackward compatible with 7-bit ASCII, as a UTF-8 file containing only ASCII characters is identical to an ASCII file containing the same sequence of characters. Even more importantly,forward compatibility is ensured as software that recognizes only 7-bit ASCII characters as special and does not alter bytes with the highest bit set (as is often done to support 8-bit ASCII extensions such as ISO-8859-1) will preserve UTF-8 data unchanged.[64]
^abcdeThe 128 characters of the 7-bit ASCII character set are divided into eight 16-character groups calledsticks 0–7, associated with the threemost-significant bits.[15] Depending on the horizontal or vertical representation of the character map,sticks can correspond with either table rows or columns.
^The Unicode characters from the "Control Pictures" area U+2400 to U+2421 reserved for representing control characters when it is necessary to print or display them rather than have them perform their intended function. Some browsers may not display these properly.
^Caret notation is often used to represent control characters on a terminal. On most text terminals, holding down theCtrl key while typing the second character will type the control character. Sometimes the shift key is not needed, for instance^@ may be typable with just Ctrl+2 or Ctrl+Space.
^Character escape sequences in C programming language and many other languages influenced by it, such asJava andPerl (though not all implementations necessarily support all escape sequences).
^ Entering any Single-Byte character is supported by escaping its octal value. However, because of the role of NULL inC-strings, this case see particular use.
^TheBackspace character can also be entered by pressing the← Backspace key on some systems.
^abThe ambiguity ofBackspace is due to early terminals designed assuming the main use of the keyboard would be to manually punch paper tape while not connected to a computer. To delete the previous character, one had to back up the paper tape punch, which for mechanical and simplicity reasons was a button on the punch itself and not the keyboard, then type the rubout character. They therefore placed a key producing rubout at the location used on typewriters for backspace. When systems used these terminals and provided command-line editing, they had to use the "rubout" code to perform a backspace, and often did not interpret the backspace character (they might echo "^H" for backspace). Other terminals not designed for paper tape made the key at this location produce Backspace, and systems designed for these used that character to back up. Since the delete code often produced a backspace effect, this also forced terminal manufacturers to make anyDelete key produce something other than the Delete character.
^TheTab character can also be entered by pressing theTab ↹ key on most systems.
^TheCarriage Return character can also be entered by pressing the↵ Enter orReturn key on most systems.
^The\eescape sequence is not part ofISO C and many other language specifications. However, it is understood by several compilers, includingGCC.
^TheEscape character can also be entered by pressing theEsc key on some systems.
^^^ meansCtrl+^ (pressing the "Ctrl" andcaret keys).
^TheDelete character can sometimes be entered by pressing the← Backspace key on some systems.
^abcdAmerican National Standard for Information Systems — Coded Character Sets — 7-Bit American National Standard Code for Information Interchange (7-Bit ASCII), ANSI X3.4-1986 (Technical report).American National Standards Institute (ANSI). 1986-03-26.
^Korpela, Jukka K. (2014-03-14) [2006-06-07].Unicode Explained – Internationalize Documents, Programs, and Web Sites (2nd release of 1st ed.).O'Reilly Media, Inc. p. 118.ISBN978-0-596-10121-3.
^ANSI INCITS 4-1986 (R2007): American National Standard for Information Systems – Coded Character Sets – 7-Bit American National Standard Code for Information Interchange (7-Bit ASCII), 2007 [1986]
^Bit Sequencing of the American National Standard Code for Information Interchange in Serial-by-Bit Data Transmission,American National Standards Institute (ANSI), 1966, X3.15-1966
^Haynes, Jim (2015-01-13)."First-Hand: Chad is Our Most Important Product: An Engineer's Memory of Teletype Corporation". Engineering and Technology History Wiki (ETHW). Retrieved2023-02-14.There was the change from 1961 ASCII to 1968 ASCII. Some computer languages used characters in 1961 ASCII such as up arrow and left arrow. These characters disappeared from 1968 ASCII. We worked with Fred Mocking, who by now was in Sales atTeletype, on a type cylinder that would compromise the changing characters so that the meanings of 1961 ASCII were not totally lost. The underscore character was made rather wedge-shaped so it could also serve as a left arrow.
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