ASCII

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Not to be confused with MS Windows-1252 or other types of Extended ASCII.
This article is about the character encoding. For other uses, see ASCII (disambiguation).
A chart of ASCII from a 1972 printer manual

The American Standard Code for Information Interchange (ASCII /ˈæski/ ASS-kee)[1] is a character-encoding scheme originally based on the English alphabet that encodes 128 specified characters - the numbers 0-9, the letters a-z and A-Z, some basic punctuation symbols, some control codes that originated with Teletype machines, and a blank space - into the 7-bit binary integers.[2]

ASCII codes represent text in computers, communications equipment, and other devices that use text. Most modern character-encoding schemes are based on ASCII, though they support many additional characters.

ASCII developed from telegraphic codes. Its first commercial use was as a 7-bit teleprinter code promoted by Bell data services. Work on the ASCII standard began on October 6, 1960, with the first meeting of the American Standards Association's (ASA) X3.2 subcommittee. The first edition of the standard was published during 1963,[3][4] a major revision during 1967,[5] and the most recent update during 1986.[6] 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.

ASCII includes definitions for 128 characters: 33 are non-printing control characters (many now obsolete)[7] that affect how text and space are processed[8] and 95 printable characters, including the space (which is considered an invisible graphic[9][10]).

The IANA prefers the name US-ASCII.[11] ASCII was the most commonly used character encoding on the World Wide Web until December 2007, when it was surpassed by UTF-8,[12][13][14] which includes ASCII as a subset.

History[edit]

The American Standard Code for Information Interchange (ASCII) was developed under the auspices of a committee of the American Standards Association, called the X3 committee, by its X3.2 (later X3L2) subcommittee, and later by that subcommittee's X3.2.4 working group. The ASA became the United States of America Standards Institute or USASI[15] and ultimately the American National Standards Institute.

The X3.2 subcommittee designed ASCII based on the earlier teleprinter encoding systems. Like other character encodings, ASCII specifies a correspondence between digital bit patterns and character symbols (i.e. graphemes and control characters). This allows digital devices to communicate with each other and to process, store, and communicate character-oriented information such as written language. Before ASCII was developed, the encodings in use included 26 alphabetic characters, 10 numerical digits, and from 11 to 25 special graphic symbols. To include all these, and control characters compatible with the Comité Consultatif International Téléphonique et Télégraphique (CCITT) International Telegraph Alphabet No. 2 (ITA2) standard, Fieldata, and early EBCDIC, more than 64 codes were required for ASCII.

The committee debated the possibility of a shift function (like the Baudot code), which would allow more than 64 codes to be represented by six bits. In a shifted code, some character codes determine choices between options for the following character codes. It allows compact encoding, but is less reliable for data transmission; 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.[16]

The committee considered an eight-bit code, since eight bits (octets) would allow two four-bit patterns to efficiently encode two digits with binary-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 a parity bit for error checking if desired.[17] Eight-bit machines (with octets as the native data type) that did not use parity checking typically set the eighth bit to 0.[18]

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 columns (32 positions) were reserved for control characters.[19] The "space" character had to come before graphics to make sorting easier, so it became position 20hex;[20] for the same reason, many special signs commonly used as separators were placed before digits. The committee decided it was important to support upper case 64-character alphabets, and chose to pattern ASCII so it could be reduced easily to a usable 64-character set of graphic codes.[21] Lower case letters were therefore not interleaved with upper case. To keep options available for lower case letters and other graphics, the special and numeric codes were arranged before the letters, and the letter "A" was placed in position 41hex to match the draft of the corresponding British standard.[22] The digits 0–9 were arranged so they correspond to values in binary prefixed with 011, making conversion with binary-coded decimal straightforward.

Many of the non-alphanumeric characters were positioned to correspond to their shifted position on typewriters. Thus #, $ and % were placed to correspond to 3, 4, and 5 in the adjacent column. The parentheses could not correspond to 9 and 0, however, because the place corresponding to 0 was taken by the space character. Since many European typewriters placed the parentheses with 8 and 9, those corresponding positions were chosen for the parentheses. The @ symbol was not used in continental Europe and the committee expected it would be replaced by an accented À in the French variation, so the @ was placed in position 40hex, right before the letter A.[23]

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 the Hamming distance between their bit patterns.[24]

With the other special characters and control codes filled in, ASCII was published as ASA X3.4-1963, leaving 28 code positions without any assigned meaning, reserved for future standardization, and one unassigned control code.[25] There was some debate at the time whether there should be more control characters rather than the lower case alphabet.[26] The indecision did not last long: during May 1963 the CCITT Working Party on the New Telegraph Alphabet proposed to assign lower case characters to columns 6 and 7,[27] and International Organization for Standardization TC 97 SC 2 voted during October to incorporate the change into its draft standard.[28] The X3.2.4 task group voted its approval for the change to ASCII at its May 1963 meeting.[29] Locating the lowercase letters in columns 6 and 7 caused the characters to differ in bit pattern from the upper case by a single bit, which simplified case-insensitive character matching and the construction of keyboards and printers.

The X3 committee made other changes, including other new characters (the brace and vertical bar characters),[30] renaming some control characters (SOM became start of header (SOH)) and moving or removing others (RU was removed).[31] ASCII was subsequently updated as USASI X3.4-1967, then USASI X3.4-1968, ANSI X3.4-1977, and finally, ANSI X3.4-1986 (the first two are occasionally retronamed ANSI X3.4-1967, and ANSI X3.4-1968).

The X3 committee also addressed how ASCII should be transmitted (least significant bit first), and how it should be recorded on perforated tape. They proposed a 9-track standard for magnetic tape, and attempted to deal with some forms of punched card formats.

ASCII itself was first used commercially during 1963 as a seven-bit teleprinter code for American Telephone & Telegraph's TWX (TeletypeWriter eXchange) network. TWX originally used the earlier five-bit Baudot code, which was also used by the competing Telex teleprinter system. Bob Bemer introduced features such as the escape sequence.[3] His British colleague Hugh McGregor Ross helped to popularize this work—according to Bemer, "so much so that the code that was to become ASCII was first called the Bemer-Ross Code in Europe".[32] Because of his extensive work on ASCII, Bemer has been called "the father of ASCII."[33]

On March 11, 1968, U.S. President Lyndon B. Johnson mandated that all computers purchased by the United States federal government support ASCII, stating:[34]

I have also approved recommendations of the Secretary of Commerce 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.

Other international standards bodies have ratified character encodings such as ISO/IEC 646 that are identical or nearly identical to ASCII, with extensions for characters outside the English alphabet and symbols used outside the United States, such as the symbol for the United Kingdom's pound sterling (£). Almost every country needed an adapted version of ASCII, since ASCII suited the needs of only the USA and a few other countries. For example, Canada had its own version that supported French characters. Other adapted encodings include ISCII (India), VISCII (Vietnam), and YUSCII (Yugoslavia). Although these encodings are sometimes referred to as ASCII, true ASCII is defined strictly only by the ANSI standard.

ASCII was incorporated into the Unicode character set as the first 128 symbols, so the 7-bit ASCII characters have the same numeric codes in both sets. This allows UTF-8 to be backward 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.[35]

ASCII control characters[edit]

Main article: Control character

ASCII reserves the first 32 codes (numbers 0–31 decimal) for control characters: codes originally intended not to represent printable information, but rather to control devices (such as printers) that make use of ASCII, or to provide meta-information about data streams such as those stored on magnetic tape.

For example, character 10 represents the "line feed" function (which causes a printer to advance its paper), and character 8 represents "backspace". RFC 2822 refers to control characters that do not include carriage return, line feed or white space as non-whitespace control characters.[36] 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 as markup 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 a data stream, and sometimes accidental, for example with the meaning of "delete".

Probably the most influential single device on the interpretation of these characters was the Teletype Model 33 ASR, which was a printing terminal with an available paper 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) became de facto standards. The Model 33 was also notable for taking the description of Control-G (BEL, meaning audibly alert the operator) literally; 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 of underscore), 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 technique became adopted by several early computer operating systems as a "handshaking" signal warning a sender to stop transmission because of impending 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 and TAPE respectively.[37]

Code 127 is officially named "delete" but the Teletype label was "rubout". Since the original standard did not give detailed interpretation for most control codes, interpretations of this code varied. The original Teletype meaning, and the intent of the standard, was to make it an ignored character, the same as NUL (all zeroes). This was useful specifically for paper tape, because punching the all-ones bit pattern on top of an existing mark would obliterate it.[38] Tapes designed to be "hand edited" could even be produced with spaces of extra NULs (blank tape) so that a block of characters could be "rubbed out" and then replacements put into the empty space.

Some software assigned special meanings to ASCII characters sent to the software from the terminal. Operating systems from Digital Equipment Corporation, for example, interpreted DEL as an input character as meaning "remove previously-typed input character",[39][40] and this interpretation also became common in Unix systems. Most other systems used BS for that meaning and used DEL to mean "remove the character at the cursor".[citation needed] That latter interpretation is the most common now.[citation needed]

Many more of the control codes have been given meanings quite different from their original ones. The "escape" character (ESC, code 27), for example, was intended originally to allow sending other control characters as literals instead of invoking their meaning. 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 meaning has been co-opted and has eventually been changed. In modern use, an ESC sent to the terminal usually indicates the start of a command sequence, usually in the form of a so-called "ANSI escape code" (or, more properly, a "Control Sequence Introducer") beginning with ESC followed by a "[" (left-bracket) character. An ESC sent from the terminal is most often used as an out-of-band character used to terminate an operation, as in the TECO and vi text editors. In graphical user interface (GUI) and windowing systems, ESC generally causes an application to abort its current operation or to exit (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 the newline problem on various operating 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 manual typewriter the carriage holding the paper moved while the position where the typebars struck the ribbon remained stationary. The entire carriage had to be pushed (returned) to the right in order to position the left margin of 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 terminals) came along, the convention was so well established that backward compatibility necessitated continuing the convention. When Gary Kildall cloned RT-11 to create CP/M he followed established DEC convention. Until the introduction of PC DOS in 1981, IBM had no hand in this because their 1970s operating systems used EBCDIC 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 as MS-DOS by Microsoft) inherited the convention by virtue of being a clone of CP/M, and Windows inherited it from MS-DOS.

Unfortunately, requiring two characters to mark the end of a line introduces unnecessary complexity and questions as to how to interpret each character when encountered alone. To simplify matters plain text data streams, including files, on Multics[41] used line feed (LF) alone as a line terminator. Unix and Unix-like systems, and Amiga systems, adopted this convention from Multics. The original Macintosh OS, Apple DOS, and ProDOS, on the other hand, used carriage return (CR) alone as a line terminator; however, since Apple replaced it with the Unix-based OS X operating system, they now use line feed (LF) as well.

Computers attached to the ARPANET included machines running operating systems such as TOPS-10 and TENEX using CR-LF line endings, machines running operating systems such as Multics using LF line endings, and machines running operating systems such as OS/360 that represented lines as a character count followed by the characters of the line and that used EBCDIC rather than ASCII. The Telnet 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; it used ASCII, along with CR-LF line endings, and software using other conventions would translate between the local conventions and the NVT.[42] The File 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.[43][44] 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.[45][46]

Older operating systems such as TOPS-10, along with CP/M, tracked file length only in units of disk blocks and used Control-Z (SUB) to mark the end of the actual text in the file. For this reason, EOF, or end-of-file, was used colloquially and conventionally as a three-letter acronym (TLA) for Control-Z instead of SUBstitute. For a variety of reasons, the end-of-text code, ETX aka Control-C, was inappropriate and using Z as the control code to end a file is analogous to it ending the alphabet, a very convenient mnemonic aid. An historic common, and still prevalent, convention uses the ETX aka Control-C code convention to interrupt and halt a program via an input data stream, usually from a keyboard.

In C library and Unix conventions, the null character is used to terminate text strings; such null-terminated strings can be known in abbreviation as ASCIZ or ASCIIZ, where here Z stands for "zero".

ASCII control code chart[edit]

BinaryOctDecHexAbbr[a][b][c]Name
000 0000000000NUL^@\0Null character
000 0001001101SOH^AStart of Header
000 0010002202STX^BStart of Text
000 0011003303ETX^CEnd of Text
000 0100004404EOT^DEnd of Transmission
000 0101005505ENQ^EEnquiry
000 0110006606ACK^FAcknowledgment
000 0111007707BEL^G\aBell
000 1000010808BS^H\bBackspace[d][e]
000 1001011909HT^I\tHorizontal Tab[f]
000 1010012100ALF^J\nLine feed
000 1011013110BVT^K\vVertical Tab
000 1100014120CFF^L\fForm feed
000 1101015130DCR^M\rCarriage return[g]
000 1110016140ESO^NShift Out
000 1111017150FSI^OShift In
001 00000201610DLE^PData Link Escape
001 00010211711DC1^QDevice Control 1 (oft. XON)
001 00100221812DC2^RDevice Control 2
001 00110231913DC3^SDevice Control 3 (oft. XOFF)
001 01000242014DC4^TDevice Control 4
001 01010252115NAK^UNegative Acknowledgment
001 01100262216SYN^VSynchronous idle
001 01110272317ETB^WEnd of Transmission Block
001 10000302418CAN^XCancel
001 10010312519EM^YEnd of Medium
001 1010032261ASUB^ZSubstitute
001 1011033271BESC^[\e[h]Escape[i]
001 1100034281CFS^\File Separator
001 1101035291DGS^]Group Separator
001 1110036301ERS^^[j]Record Separator
001 1111037311FUS^_Unit Separator
111 11111771277FDEL^?Delete[k][e]
  1. ^ The Unicode characters from the 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.
  2. ^ Caret notation often used to represent control characters on a terminal. On most text terminals, holding down the Ctrl 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 and 2.
  3. ^ Character Escape Codes in C programming language and many other languages influenced by it, such as Java and Perl (though not all implementations necessarily support all escape codes).
  4. ^ The Backspace character can also be entered by pressing the ← Backspace key on some systems.
  5. ^ a b The ambiguity of Backspace 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 any Delete key produce something other than the Delete character.
  6. ^ The Tab character can also be entered by pressing the Tab key on most systems.
  7. ^ The Carriage Return character can also be entered by pressing the Enter or Return key on most systems.
  8. ^ The '\e' escape sequence is not part of ISO C and many other language specifications. However, it is understood by several compilers, including GCC.
  9. ^ The Escape character can also be entered by pressing the Esc key on some systems.
  10. ^ ^^ means Ctrl+^ (pressing the "Ctrl" and caret keys).
  11. ^ The Delete character can sometimes be entered by pressing the ← Backspace key on some systems.

Other representations might be used by specialist equipment, for example ISO 2047 graphics or hexadecimal numbers.

ASCII printable characters[edit]

Codes 20hex to 7Ehex, known as the printable characters, represent letters, digits, punctuation marks, and a few miscellaneous symbols. There are 95 printable characters in total.[a]

Code 20hex, the space character, denotes the space between words, as produced by the space-bar of a keyboard. Since the space character is considered an invisible graphic (rather than a control character)[10][9] and thus would not normally be visible, it is represented here by Unicode character U+2420 "␠"; Unicode characters U+2422 "␢" and U+2423 "␣" are also available for use when a visible representation of a space is necessary.

Code 7Fhex corresponds to the non-printable "Delete" (DEL) control character and is therefore omitted from this chart; it is covered in the previous section's chart. Earlier versions of ASCII used the up-arrow instead of the caret (5Ehex) and the left-arrow instead of the underscore (5Fhex).[47]

ASCII printable code chart[edit]

BinaryOctDecHexGlyph
010 00000403220(space)
010 00010413321!
010 00100423422"
010 00110433523#
010 01000443624$
010 01010453725%
010 01100463826&
010 01110473927'
010 10000504028(
010 10010514129)
010 1010052422A*
010 1011053432B+
010 1100054442C,
010 1101055452D-
010 1110056462E.
010 1111057472F/
011 000006048300
011 000106149311
011 001006250322
011 001106351333
011 010006452344
011 010106553355
011 011006654366
011 011106755377
011 100007056388
011 100107157399
011 1010072583A:
011 1011073593B;
011 1100074603C<
011 1101075613D=
011 1110076623E>
011 1111077633F?
BinaryOctDecHexGlyph
100 00001006440@
100 00011016541A
100 00101026642B
100 00111036743C
100 01001046844D
100 01011056945E
100 01101067046F
100 01111077147G
100 10001107248H
100 10011117349I
100 1010112744AJ
100 1011113754BK
100 1100114764CL
100 1101115774DM
100 1110116784EN
100 1111117794FO
101 00001208050P
101 00011218151Q
101 00101228252R
101 00111238353S
101 01001248454T
101 01011258555U
101 01101268656V
101 01111278757W
101 10001308858X
101 10011318959Y
101 1010132905AZ
101 1011133915B[
101 1100134925C\
101 1101135935D]
101 1110136945E^
101 1111137955F_
BinaryOctDecHexGlyph
110 00001409660`
110 00011419761a
110 00101429862b
110 00111439963c
110 010014410064d
110 010114510165e
110 011014610266f
110 011114710367g
110 100015010468h
110 100115110569i
110 10101521066Aj
110 10111531076Bk
110 11001541086Cl
110 11011551096Dm
110 11101561106En
110 11111571116Fo
111 000016011270p
111 000116111371q
111 001016211472r
111 001116311573s
111 010016411674t
111 010116511775u
111 011016611876v
111 011116711977w
111 100017012078x
111 100117112179y
111 10101721227Az
111 10111731237B{
111 11001741247C|
111 11011751257D}
111 11101761267E~

Aliases[edit]

A June 1992 RFC[48] and the Internet Assigned Numbers Authority registry of character sets[11] recognize the following case-insensitive aliases for ASCII as suitable for use on the Internet:

  • ANSI_X3.4-1968 (canonical name)
  • iso-ir-6
  • ANSI_X3.4-1986
  • ISO_646.irv:1991
  • ASCII
  • ISO646-US
  • US-ASCII (preferred MIME name)[11]
  • us
  • IBM367
  • cp367
  • csASCII

Of these, the IANA encourages use of the name "US-ASCII" for Internet uses of ASCII (even if it is a redundant acronym, but the US is needed because of abuse of the ASCII term). One often finds this in the optional "charset" parameter in the Content-Type header of some MIME messages, in the equivalent "meta" element of some HTML documents, and in the encoding declaration part of the prologue of some XML documents.

Variants[edit]

As computer technology spread throughout the world, different standards 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.

Many other countries developed variants of ASCII to include non-English letters (e.g. é, ñ, ß, Ł), currency symbols (e.g. £, ¥), etc.

The PETSCII code Commodore International used for their 8-bit systems is probably unique among post-1970 codes in being based on ASCII-1963, instead of the more common ASCII-1967, such as found on the ZX Spectrum computer. Atari 8-bit computers and Galaksija computers also used ASCII variants.

7-bit[edit]

From early in its development,[49] ASCII was intended to be just one of several national variants of an international character code standard, ultimately published as ISO/IEC 646 (1972), which would share most characters in common but assign other locally useful characters to several code 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[50] caused ASCII's choices for the national use characters to seem to be de 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, was a 7-bit character set. It did 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 for ANSI C, although their late introduction and inconsistent implementation in compilers limited their use. Many programmers kept their computers on US-ASCII, so plain-text in Swedish, German etc. (for example, in e-mail or Usenet) contained "{, }" and similar variants in the middle of words, something those programmers got used to.

8-bit[edit]

Eventually, as 8-, 16- and 32-bit (and later 64-bit) computers began to replace 18- and 36-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.

Most early home computer systems developed their own 8-bit character sets containing line-drawing and game glyphs, and often filled in some or all of the control characters from 0-31 with more graphics. Kaypro CP/M computers used the "upper" 128 characters for the Greek alphabet. The IBM PC defined code page 437, which replaced the control-characters with graphic symbols such as smiley faces, and mapped additional graphic characters to the upper 128 positions. Operating systems such as DOS supported these code-pages, and manufacturers of IBM PCs supported them in hardware. Digital Equipment Corporation developed the Multinational Character Set (DEC-MCS) for use in the popular VT220 terminal, this was one of the first extensions designed more for international languages than for block graphics. The Macintosh defined Mac OS Roman and Postscript also defined a set, both of these contained both international letters and typographic punctuation marks instead of graphics, more like modern character sets. The ISO/IEC 8859 standard (derived from the DEC-MCS) finally provided a standard that most systems copied (at least as accurately as they copied ASCII, but with many substitutions). A popular further extension designed by Microsoft, Windows-1252 (often mislabeled as ISO-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 encodings until 2008 when UTF-8 became more common.[13]

Unicode[edit]

Unicode and the ISO/IEC 10646 Universal 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 (using natural numbers called code points) and encoding (to 8-, 16- or 32-bit binary formats, called UTF-8, UTF-16 and UTF-32).

To allow backward compatibility, the 128 ASCII and 256 ISO-8859-1 (Latin 1) characters are assigned Unicode/UCS code points that are the same as their codes in the earlier standards. Therefore, ASCII can be considered a 7-bit encoding scheme for a very small subset of Unicode/UCS, and ASCII (when prefixed with 0 as the eighth bit) is valid UTF-8.

Order[edit]

ASCII-code order is also called ASCIIbetical order.[51] Collation of data is sometimes done in this order rather than "standard" alphabetical order (collating sequence). The main deviations in ASCII order are:

An intermediate order—readily implemented—converts uppercase letters to lowercase before comparing ASCII values. Naïve number sorting can be averted by zero-filling all numbers (e.g. "02" will sort before "10" as expected), although this is an external fix and has nothing to do with the ordering itself.

See also[edit]

Notes[edit]

  1. ^ Printed out, the characters are:
      !"#$%&'()*+,-./0123456789:;<=>?@ABCDEFGHIJKLMNOPQRSTUVWXYZ[\]^_`abcdefghijklmnopqrstuvwxyz{|}~ 

References[edit]

Footnotes
  1. ^ "Pronunciation for ASCII". Merriam Webster (audio). Retrieved 2008-04-14. 
  2. ^ R. Shirley (August 2007). "RFC 4949". Retrieved 2013-12-23. 
  3. ^ a b Brandel, Mary (July 6, 1999). "1963: The Debut of ASCII". CNN. Retrieved 2008-04-14. 
  4. ^ "American Standard Code for Information Interchange, ASA X3.4-1963". American Standards Association. June 17, 1963. Retrieved 2014-05-23. 
  5. ^ USA Standard Code for Information Interchange, USAS X3.4-1967. United States of America Standards Institute. July 7, 1967. 
  6. ^ American National Standard for Information Systems — Coded Character Sets — 7-Bit American National Standard Code for Information Interchange (7-Bit ASCII), ANSI X3.4-1986. American National Standards Institute. March 26, 1986. 
  7. ^ Maini, Anil Kumar (2007). Digital Electronics: Principles, Devices and Applications. John Wiley and Sons. p. 28. ISBN 978-0-470-03214-5. "In addition, it defines codes for 33 nonprinting, mostly obsolete control characters that affect how the text is processed." 
  8. ^ International Organization for Standardization (December 1, 1975). "The set of control characters for ISO 646". Internet Assigned Numbers Authority Registry. Alternate U.S. version: [1]. Accessed 2008-04-14.
  9. ^ a b "RFC 20: ASCII format for Network Interchange", ANSI X3.4-1968, October 16, 1969.
  10. ^ a b Mackenzie 1980, p. 223.
  11. ^ a b c Internet Assigned Numbers Authority (May 14, 2007). "Character Sets". Accessed 2008-04-14.
  12. ^ Dubost, Karl (May 6, 2008). "UTF-8 Growth On The Web". W3C Blog. World Wide Web Consortium. Retrieved 2010-08-15. 
  13. ^ a b Davis, Mark (May 5, 2008). "Moving to Unicode 5.1". Official Google Blog. Google. Retrieved 2010-08-15. 
  14. ^ Davis, Mark (Jan 28, 2010). "Unicode nearing 50% of the web". Official Google Blog. Google. Retrieved 2010-08-15. 
  15. ^ Mackenzie 1980, p. 211.
  16. ^ Mackenzie 1980, p. 215, Decision 4.
  17. ^ Mackenzie 1980, p. 217, Decision 5.
  18. ^ Stanley A. Sawyer, Steven George Krantz (1995). A TeX Primer for Scientists. CRC Press, LLC. p. 13. ISBN 978-0-8493-7159-2. 
  19. ^ Mackenzie 1980, p. 220, Decisions 8,9.
  20. ^ Mackenzie 1980, p. 237, Decision 10.
  21. ^ Mackenzie 1980, p. 228, Decision 14.
  22. ^ Mackenzie 1980, p. 238, Decision 18.
  23. ^ Mackenzie 1980, p. 243.
  24. ^ Mackenzie 1980, pp. 243-245.
  25. ^ Mackenzie 1980, pp. 66, 245.
  26. ^ Mackenzie 1980, p. 435.
  27. ^ Brief Report: Meeting of CCITT Working Party on the New Telegraph Alphabet, May 13–15, 1963.
  28. ^ Report of ISO/TC/97/SC 2 – Meeting of October 29–31, 1963.
  29. ^ Report on Task Group X3.2.4, June 11, 1963, Pentagon Building, Washington, DC.
  30. ^ Report of Meeting No. 8, Task Group X3.2.4, December 17 and 18, 1963
  31. ^ Mackenzie 1980, p. 247–248.
  32. ^ Bob Bemer (n.d.). Bemer meets Europe. Trailing-edge.com. Accessed 2008-04-14. Employed at IBM at that time
  33. ^ "Biography of Robert William Bemer". 
  34. ^ Lyndon B. Johnson (March 11, 1968). Memorandum Approving the Adoption by the Federal Government of a Standard Code for Information Interchange. The American Presidency Project. Accessed 2008-04-14.
  35. ^ "utf-8(7) - Linux manual page". Man7.org. 2014-02-26. Retrieved 2014-04-21. 
  36. ^ RFC 2822 (April 2001). "NO-WS-CTL".
  37. ^ McConnell, Robert; Haynes, James; Warren, RIchard. "Understanding ASCII Codes". Retrieved 11 May 2014. 
  38. ^ "Re: editor and word processor history (was: Re: RTF for emacs)". 
  39. ^ "PDP-6 Multiprogramming System Manual". Digital Equipment Corporation. 1965. p. 43. 
  40. ^ "PDP-10 Reference Handbook, Book 3, Communicating with the Monitor". Digital Equipment Corporation. 1969. p. 5-5. 
  41. ^ Ossanna, J. F.; Saltzer, J. H. (November 17–19, 1970). "Technical and human engineering problems in connecting terminals to a time-sharing system". Proceedings of the November 17–19, 1970, Fall Joint Computer Conference. p. 357: AFIPS Press. pp. 355–362. "Using a "new-line" function (combined carriage-return and line-feed) is simpler for both man and machine than requiring both functions for starting a new line; the American National Standard X3.4-19687 permits the line-feed code to carry the new-line meaning." 
  42. ^ T. O'Sullivan (May 19, 1971). TELNET Protocol. IETF. pp. 4-5. RFC 158. https://tools.ietf.org/html/rfc158. Retrieved 2013-01-28.
  43. ^ Nancy J. Neigus (Aug 12, 1973). File Transfer Protocol. IETF. RFC 542. https://tools.ietf.org/html/rfc542. Retrieved 2013-01-28.
  44. ^ Jon Postel (June 1980). File Transfer Protocol. IETF. RFC 765. https://tools.ietf.org/html/rfc765. Retrieved 2013-01-28.
  45. ^ EOL translation plan for Mercurial
  46. ^ Daniel J. Bernstein. "Bare LFs in SMTP". Retrieved 2013-01-28. 
  47. ^ ASA X3.4-1963.
  48. ^ RFC 1345 (June 1992).
  49. ^ "Specific Criteria," attachment to memo from R. W. Reach, "X3-2 Meeting – September 14 and 15," September 18, 1961
  50. ^ R. Maréchal, ISO/TC 97 – Computers and Information Processing: Acceptance of Draft ISO Recommendation No. 1052, December 22, 1967
  51. ^ "ASCIIbetical definition". PC Magazine. Retrieved 2008-04-14. 
Bibliography

Further reading[edit]

External links[edit]