What is ASCII Wikipedia

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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).

ASCII (/ˈæskiː/ (listen) ASS-kee),^[1]:6abbreviated from American Standard Code for Information Interchange, is a character encoding standard for electronic communication. ASCII codes represent text in computers, telecommunications equipment, and other devices. Most modern character-encoding schemes are based on ASCII, although they support many additional characters.

ASCII

ASCII (1967 or later)
MIME / IANA	us-ascii
Alias(es)	ASCII
Language(s)	English
Classification	ISO 646 series
Extensions	Unicode ISO/IEC 8859(series) KOI-8 OEM (series) Windows-125x(series) Others
Preceded by	ITA 2, FIELDATA
Succeeded by	ISO 8859, Unicode
Other related encoding(s)	PETSCII
v t e

ASCII is the traditional name for the encoding system; the Internet Assigned Numbers Authority (IANA) prefers the updated name US-ASCII, which clarifies that this system was developed in the US and based on the typographical symbols predominantly in use there.^[2]

ASCII is one of the IEEE milestones.

ASCII chart from an earlier-than 1972 printer manual (b1 is the least significant bit.)

OverviewEdit

ASCII was developed from telegraph code. Its first commercial use was as a seven-bitteleprinter 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) (now the American National Standards Institute or ANSI) X3.2 subcommittee. The first edition of the standard was published in 1963,^[3][4] underwent a major revision during 1967,^[5][6] and experienced its most recent update during 1986.^[7] 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.

Originally based on the English alphabet, ASCII encodes 128 specified characters into seven-bit integers as shown by the ASCII chart above.^[8] Ninety-five of the encoded characters are printable: these include the digits 0 to 9, lowercase letters a to z, uppercase letters A to Z, and punctuation symbols. In addition, the original ASCII specification included 33 non-printing control codes which originated with Teletype machines; most of these are now obsolete,^[9]although a few are still commonly used, such as the carriage return, line feed and tab codes.

For example, lowercase i would be represented in the ASCII encoding by binary1101001 = hexadecimal 69 (i is the ninth letter) = decimal 105.

HistoryEdit

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 (now INCITS). The ASA became the United States of America Standards Institute (USASI)^[1]:211and ultimately the American National Standards Institute (ANSI).

With the other special characters and control codes filled in, ASCII was published as ASA X3.4-1963,^[4][10] leaving 28 code positions without any assigned meaning, reserved for future standardization, and one unassigned control code.^{[1]:66, 245} There was some debate at the time whether there should be more control characters rather than the lowercase alphabet.^[1]:435 The indecision did not last long: during May 1963 the CCITT Working Party on the New Telegraph Alphabet proposed to assign lowercase characters to sticks^[a][11] 6 and 7,^[12] and International Organization for Standardization TC 97 SC 2 voted during October to incorporate the change into its draft standard.^[13] The X3.2.4 task group voted its approval for the change to ASCII at its May 1963 meeting.^[14] Locating the lowercase letters in sticks^[a][11] 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),^[15] renaming some control characters (SOM became start of header (SOH)) and moving or removing others (RU was removed).^{[1]:247–248} ASCII was subsequently updated as USAS X3.4-1967,^[5][16] then USAS X3.4-1968, ANSI X3.4-1977, and finally, ANSI X3.4-1986.^[7][17]

Revisions of the ASCII standard:

• ASA X3.4-1963^{[1][4][16][17]}
• ASA X3.4-1965 (approved, but not published, nevertheless used by IBM 2260 & 2265 Display Stations and IBM 2848Display Control)^{[1]:423, 425–428, 435–439[18][16][17]}
• USAS X3.4-1967^[1][5][17]
• USAS X3.4-1968^[1][17]
• ANSI X3.4-1977^[17]
• ANSI X3.4-1986^[7][17]
• ANSI X3.4-1986 (R1992)
• ANSI X3.4-1986 (R1997)
• ANSI INCITS 4-1986 (R2002)^[19]
• ANSI INCITS 4-1986 (R2007)^[20]
• ANSI INCITS 4-1986 (R2012)

In the X3.15 standard, the X3 committee also addressed how ASCII should be transmitted (least significant bit first),^{[1]:249–253[21]} and how it should be recorded on perforated tape. They proposed a 9-track standard for magnetic tape, and attempted to deal with some punched card formats.

Design considerationsEdit

Bit widthEdit

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 of 1924,^[22][23] FIELDATA(1956^{[citation needed]}), and early EBCDIC (1963), more than 64 codes were required for ASCII.

ITA2 were in turn based on the 5-bit telegraph code Émile Baudot invented in 1870 and patented in 1874.^[23]

The committee debated the possibility of a shift function (like in ITA2), which would allow more than 64 codes to be represented by a six-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 for data 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.^{[1]:215, 236 § 4}

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.^{[1]:217, 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.^[24] In some printers, the high bit was used to enable Italics printing^{[citation needed]}.

Internal organizationEdit

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-called ASCII sticks^[a][11] (32 positions) were reserved for control characters.^{[1]:220, 236 § 8,9)} The "space" character had to come before graphics to make sorting easier, so it became position 20_hex;^{[1]: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 uppercase 64-character alphabets, and chose to pattern ASCII so it could be reduced easily to a usable 64-character set of graphic codes,^{[1]:228, 237 § 14} as was done in the DEC SIXBIT code (1963). Lowercase letters were therefore not interleaved with uppercase. To keep options available for lowercase letters and other graphics, the special and numeric codes were arranged before the letters, and the letter A was placed in position 41_hex to match the draft of the corresponding British standard.^{[1]:238 § 18} The digits 0–9 are prefixed with 011, but the remaining 4 bits correspond to their respective values in binary, making conversion with binary-coded decimalstraightforward.

Many of the non-alphanumeric characters were positioned to correspond to their shifted position on typewriters; an important subtlety is that these were based on mechanicaltypewriters, not electric typewriters.^[25]Mechanical typewriters followed the standard set by the Remington No. 2 (1878), the first typewriter with a shift key, and the shifted values of 23456789- were "#$%_&'() – early typewriters omitted 0and 1, using O (capital letter o) and l(lowercase letter L) instead, but 1! and 0) pairs became standard once 0 and 1 became common. Thus, in ASCII !"#$%were placed in the second stick,^[a][11]positions 1–5, corresponding to the digits 1–5 in the adjacent stick.^[a][11] The parentheses could not correspond to 9 and 0, however, because the place corresponding to 0 was taken by the space character. This was accommodated by removing _(underscore) from 6 and shifting the remaining characters, which corresponded to many European typewriters that placed the parentheses with 8 and 9. This discrepancy from typewriters led to bit-paired keyboards, notably the Teletype Model 33, which used the left-shifted layout corresponding to ASCII, not to traditional mechanical typewriters. Electric typewriters, notably the IBM Selectric (1961), used a somewhat different layout that has become standard on computers – following the IBM PC (1981), especially Model 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 common characters were not included, notably ½¼¢, 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 accented À in the French variation, so the @ was placed in position 40_hex, right before the letter A.^[1]: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 the Hamming distance between their bit patterns.^{[1]:243–245}

Character orderEdit

ASCII-code order is also called ASCIIbeticalorder.^[26] 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.

Character groups

UseEdit

ASCII 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 ITA2, which was also used by the competing Telexteleprinter 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".^[43] Because of his extensive work on ASCII, Bemer has been called "the father of ASCII".^[44]

On March 11, 1968, U.S. President Lyndon B. Johnson mandated that all computers purchased by the United States Federal Government support ASCII, stating:^[45][46][47]

I have also approved recommendations of the Secretary 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.

ASCII was the most common character encoding on the World Wide Web until December 2007, when UTF-8 encoding surpassed it; UTF-8 is backward compatible with ASCII.^[48][49][50]

Variants and derivations

See also

NotesEdit

1. ^ ^{a b c d e} The 128 characters of the 7-bit ASCII character set are divided into eight 16-character groups called sticks 0–7, associated with the three most-significant bits.^[11] Depending on the horizontal or vertical representation of the character map, sticks correspond with either table rows or columns.
2. ^ 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.
3. ^ Caret notation is 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.
4. ^ Character escape sequences in C programming language and many other languages influenced by it, such as Javaand Perl (though not all implementations necessarily support all escape sequences).
5. ^ The Backspace character can also be entered by pressing the ← Backspace key on some systems.
6. ^ ^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.
7. ^ The Tab character can also be entered by pressing the Tab ↹ key on most systems.
8. ^ The Carriage Return character can also be entered by pressing the ↵ Enter or Return key on most systems.
9. ^ The \e escape sequence is not part of ISO C and many other language specifications. However, it is understood by several compilers, including GCC.
10. ^ The Escape character can also be entered by pressing the Esc key on some systems.
11. ^ ^^ means Ctrl+^ (pressing the "Ctrl" and caret keys).
12. ^ The Delete character can sometimes be entered by pressing the ← Backspacekey on some systems.
13. ^ Printed out, the characters are:

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

References