American Standard Code for Information Interchange (ASCII), generally pronounced ask-ee [ˈæski], is a character encoding based on the English alphabet. ASCII codes represent text in computers, communications equipment, and other devices that work with text. Most modern character encodings — which support many more characters than did the original — have a historical basis in ASCII.
Work on ASCII began in 1960. The first edition of the standard was published in 1963, a major revision in 1967, and the most recent update in 1986. It currently defines codes for 128 characters: 33 are non-printing, mostly obsolete control characters that affect how text is processed, and 95 are printable characters.
“ 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.
”
—Lyndon B. Johnson, Memorandum Approving the Adoption by the Federal Government of a Standard Code for Information Interchange, March 11th, 1968 at Gerhard Peters (ed.), The American Presidency Project,
Overview
Like other character representation computer codes, ASCII specifies a correspondence between digital bit patterns and the glyphs (i.e., symbols) of a written language. This allows digital devices to communicate with each other and to process, store, and communicate character-oriented information. The ASCII character encoding—or a compatible extension is used on nearly all common computers, especially personal computers and workstations. The preferred MIME name for this encoding is "US-ASCII".
Except for a few of the ASCII control characters that prescribe some 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.
ASCII is, strictly, a seven-bit code, meaning it uses patterns of seven binary digits (a range of 0 to 127 decimal) to represent each character. When ASCII was introduced, many computers used eight-bit bytes (groups of bits), called octets, as the smallest unit of information. In seven-bit ASCII encoding, the eighth bit was commonly used as a parity bit for error checking on communication lines or for other device-specific functions. Machines that did not use parity checking typically set the eighth bit to 0.
The American National Standards Institute (then called the United States of America Standards Institute or USASI) developed ASCII based on earlier teleprinter encoding systems. Circa 1956, Ivan Idelson, at Ferranti in the UK, had proposed the Cluff-Foster-Idelson coding of characters on 7 track paper tape to a British Standards committee. This eventually becomes ASCII. ASCII itself first entered commercial use in 1963 as a seven-bit teleprinter code for American Telephone & Telegraph's TWX (Teletype Wide-area eXchange) network. TWX originally used the earlier five-bit Baudot code, which was also used by the competing Telex teleprinter system. The Bell System had planned to upgrade to a six-bit code derived from the Fieldata project, which added punctuation and lower-case letters to the Baudot code, but was persuaded instead to join the American Standards Association (part of ANSI) subcommittee that had started to develop ASCII.[citation needed] Compared with earlier telegraph codes, the proposed Bell code and ASCII both underwent re-ordering for more convenient sorting (especially alphabetization) of lists, and added features for devices other than teleprinters. Bob Bemer introduced features such as the escape sequence. 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".
ASCII was subsequently updated and published as USASI X3.4-1968, ANSI X3.4-1977, and finally, ANSI X3.4-1986.
Other international standards bodies have ratified character encodings such as ISO 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 adopted version of ASCII since ASCII only suited the needs of the USA and a few other countries. For example, Canada had its own version that supported French. Although these encodings are sometimes referred to as ASCII, true ASCII is strictly defined only by ANSI standard.
ASCII has been incorporated into the Unicode character set as the first 128 symbols, so the ASCII characters have the same numeric codes in both sets. This allows UTF-8 to be backward compatible with ASCII, a significant advantage.
ASCII control characters
ASCII reserves the first 32 codes (numbers 0–31 decimal) for control characters: codes originally intended not to carry 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".
Binary Oct Dec Hex Abbr PR[a] CS[b] CEC[c] Description
000 0000 000 0 00 NUL ␀ ^@ \0 Null character
000 0001 001 1 01 SOH ␁ ^A Start of Header
000 0010 002 2 02 STX ␂ ^B Start of Text
000 0011 003 3 03 ETX ␃ ^C End of Text
000 0100 004 4 04 EOT ␄ ^D End of Transmission
000 0101 005 5 05 ENQ ␅ ^E Enquiry
000 0110 006 6 06 ACK ␆ ^F Acknowledgment
000 0111 007 7 07 BEL ␇ ^G \a Bell
000 1000 010 8 08 BS ␈ ^H \b Backspace[d][i]
000 1001 011 9 09 HT ␉ ^I \t Horizontal Tab
000 1010 012 10 0A LF ␊ ^J \n Line feed
000 1011 013 11 0B VT ␋ ^K \v Vertical Tab
000 1100 014 12 0C FF ␌ ^L \f Form feed
000 1101 015 13 0D CR ␍ ^M \r Carriage return[h]
000 1110 016 14 0E SO ␎ ^N Shift Out
000 1111 017 15 0F SI ␏ ^O Shift In
001 0000 020 16 10 DLE ␐ ^P Data Link Escape
001 0001 021 17 11 DC1 ␑ ^Q Device Control 1 (oft. XON)
001 0010 022 18 12 DC2 ␒ ^R Device Control 2
001 0011 023 19 13 DC3 ␓ ^S Device Control 3 (oft. XOFF)
001 0100 024 20 14 DC4 ␔ ^T Device Control 4
001 0101 025 21 15 NAK ␕ ^U Negative Acknowledgement
001 0110 026 22 16 SYN ␖ ^V Synchronous Idle
001 0111 027 23 17 ETB ␗ ^W End of Trans. Block
001 1000 030 24 18 CAN ␘ ^X Cancel
001 1001 031 25 19 EM ␙ ^Y End of Medium
001 1010 032 26 1A SUB ␚ ^Z Substitute
001 1011 033 27 1B ESC ␛ ^[ \e[f] Escape[g]
001 1100 034 28 1C FS ␜ ^\ File Separator
001 1101 035 29 1D GS ␝ ^] Group Separator
001 1110 036 30 1E RS ␞ ^^ Record Separator
001 1111 037 31 1F US ␟ ^_ Unit Separator
111 1111 177 127 7F DEL ␡ ^? Delete[e][i]
^[a] Printable Representation, 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.
^[b] Control key Sequence/caret notation, the traditional key sequences for inputting control characters. The caret (^) represents the "Control" or "Ctrl" key that must be held down while pressing the second key in the sequence. The caret-key representation is also used by some software to represent control characters.
^[c] Character Escape Codes in C programming language and many other languages influenced by it, such as Java and Perl.
^[d] The Backspace character can also be entered by pressing the "Backspace", "Bksp", or ← key on some systems.
^[e] The Delete character can also be entered by pressing the "Delete" or "Del" key. It can also be entered by pressing the "Backspace", "Bksp", or ← key on some systems.
^[f] The '\e' escape sequence is not part of ISO C and many other language specifications. However, it is understood by several compilers.
^[g] The Escape character can also be entered by pressing the "Escape" or "Esc" key on some systems.
^[h] The Carriage Return character can also be entered by pressing the "Return", "Ret", "Enter", or ↵ key on most systems.
[i]a b The ambiguity surrounding Backspace comes from mismatches between the intent of the human or software transmitting the Backspace and the interpretation by the software receiving it. If the transmitter expects Backspace to erase the previous character and the receiver expects Delete to be used to erase the previous character, many receivers will echo the Backspace as "^H", just as they would echo any other uninterpreted control character. (A similar mismatch in the other direction may yield Delete displayed as "^?".)
The original ASCII standard used only short descriptive phrases for each control character. The ambiguity this left was sometimes intentional (where a character would be used slightly differently on a terminal link than on a data stream) and sometimes more accidental (such as what "delete" means).
Probably the most influential single device on the interpretation of these characters was the Teletype corporation model 33 series, 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 up through the 1980s, lower cost and in some ways less fragile than magnetic tape. In particular, the Teletype 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. 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 faded out.
The use of Control-S (XOFF, an abbreviation for "transmit off") as a handshaking signal warning a sender to stop transmission because of impending overflow, and Control-Q (XON, "transmit on") to resume sending, 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.
Code 127 is officially named "delete" but the Teletype label was "rubout". Since the original standard gave no 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 specifically useful for paper tape, because punching the all-ones bit pattern on top of an existing mark would obliterate it. 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.
As video terminals began to replace printing ones, the value of the "rubout" character was lost. DEC systems, for example, interpreted "Delete" to mean "remove the character before the cursor," and this interpretation also became common in Unix systems. Most other systems used "Backspace" for that meaning and used "Delete" as it was used on paper tape, to mean "remove the character after the cursor". That latter interpretation is the most common today.
Many more of the control codes have taken on meanings quite different from their original ones. The "escape" character (code 27), for example, was originally intended 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 coopted and has eventually drifted. In modern use, an ESC sent to the terminal usually indicates the start of a command sequence, usually in the form of an ANSI escape code. 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.
The inherent ambiguity of many control characters, combined with their historical usage, has also created problems when transferring "plain text" files between systems. The clearest example of this is the newline problem on various operating systems. On printing terminals there is no question that you terminate a line of text with both "Carriage Return" and "Linefeed". The first returns the printing carriage to the beginning of the line and the second advances to the next line without moving the carriage. However, requiring two characters to mark the end of a line introduced unnecessary complexity and questions as to how to interpret each character when encountered alone. To simplify matters, plain text files on Unix systems use line feeds alone to separate lines. Similarly, older Macintosh systems, among others, use only carriage returns in plain text files. Various DEC operating systems used both characters to mark the end of a line, perhaps for compatibility with teletypes, and this de facto standard was copied in the CP/M operating system and then in MS-DOS and eventually Microsoft Windows. Transmission of text over the Internet, for protocols as Email and the World Wide Web, uses both characters. The DEC operating systems, 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 (also done for CP/M compatibility in some cases in MS-DOS, though MS-DOS has always recorded exact file-lengths). Control-C (ETX, End of TeXt) might have made more sense, but was already in wide use as a program abort signal. UNIX's use of Control-D (EOT, End of Transmission) appears on its face similar, but is used only from the terminal and never stored in a file.
While the codes mentioned above have retained some semblance of their original meanings, many of the codes originally intended for stream delimiters or for link control on a terminal have lost all meaning except their relation to a letter. Control-A is almost never used to mean "start of header" except on an ANSI magnetic tape. When connecting a terminal to a system, or asking the system to recognize that a logged-out terminal wants to log in, modern systems are much more likely to want a carriage return or an ESCape than Control-E (ENQuire, meaning "is there anybody out there?").
The abbreviation ASCIIZ or ASCIZ refers to a null-terminated ASCII string (also known as a C string).