| ASCII | |
| Alias: | ISO-IR-006,[1] ANSI_X3.4-1968, ANSI_X3.4-1986, ISO_646.irv:1991, ISO646-US, us, IBM367, cp367 |
| Mime: | us-ascii |
| Extensions: |
|
| Prev: | ITA 2, FIELDATA |
| Next: | ISO/IEC 8859, ISO/IEC 10646 (Unicode) |
| Classification: | ISO/IEC 646 series |
ASCII,[2] an acronym for 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. ASCII has just 128 code points, of which only 95 are, 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, including Unicode which has over a million code points, but the first 128 of these are the same as ASCII.
The Internet Assigned Numbers Authority (IANA) prefers the name US-ASCII for this character encoding.[3]
ASCII is one of the IEEE milestones.[4]
ASCII was developed in part from telegraph code. Its first commercial use was in the Teletype Model 33 and the Teletype Model 35 as a seven-bit teleprinter code promoted by Bell data services. Work on the ASCII standard began in May 1961, 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,[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.
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 specified characters 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 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 s; most of these are now obsolete,[13] 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 binary 1101001 = hexadecimal 69 (i is the ninth letter) = decimal 105.
Despite being an American standard, ASCII does not have a code point for the cent (¢). It also does not support English terms with diacritical marks such as résumé and jalapeño, or proper nouns with diacritical marks such as Beyoncé.
HistoryWith the other special characters and control codes filled in, ASCII was published as ASA X3.4-1963,[6] [15] leaving 28 code positions without any assigned meaning, reserved for future standardization, and one unassigned control code.[2] There was some debate at the time whether there should be more control characters rather than the lowercase alphabet.[2] 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 6 and 7,[16] and International 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 in sticks 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),[19] renaming some control characters (SOM became start of header (SOH)) and moving or removing others (RU was removed).[2] 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)[2] and recorded on perforated tape. They proposed a 9-track standard for magnetic tape and attempted to deal with some punched card formats.
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 1932,[26] [27] FIELDATA (1956), and early EBCDIC (1963), more than 64 codes were required for ASCII.
ITA2 was in turn based on Baudot code, the 5-bit telegraph code Émile Baudot invented in 1870 and patented in 1874.
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.[2]
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.[2] Eight-bit machines (with octets as the native data type) that did not use parity checking typically set the eighth bit to 0.[28]
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 (32 positions) were reserved for control characters.[2] The "space" character had to come before graphics to make sorting easier, so it became position 20hex;[2] 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,[2] 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 41hex to match the draft of the corresponding British standard.[2] 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 decimal straightforward (for example, 5 in encoded to 0110101, where 5 is 0101 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 on mechanical typewriters, not electric typewriters.[29] Mechanical typewriters followed the de facto standard set by the Remington No. 2 (1878), the first typewriter with a shift key, and the shifted values of 23456789- were "#$%_&' early typewriters omitted 0 and 1, using O (capital letter o) and l (lowercase letter L) instead, but 1! and 0) pairs became standard once 0 and 1 became common. Thus, in ASCII !"#$% were placed in the second stick, positions 1–5, corresponding to the digits 1–5 in the adjacent stick. 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, differently from traditional mechanical typewriters.
Electric typewriters, notably the IBM Selectric (1961), used a somewhat different layout that has become de facto 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 then-common typewriter characters were not included, notably ½ 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 accented À in the French variation, so the @ was placed in position 40hex, right before the letter A.[2]
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.[2]
Character orderAn intermediate order converts uppercase letters to lowercase before comparing ASCII values.
Character setSee main article: Control character. ASCII reserves the first 32 code points (numbers 0–31 decimal) and the last one (number 127 decimal) for control characters. These are codes intended to control peripheral devices (such as printers), or to provide meta-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 in ISO 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". refers to control characters that do not include carriage return, line feed or white space as non-whitespace control characters.[31] 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 the standard is unclear about the meaning of "delete".
Probably the most influential single device affecting 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 (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 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 so-called flow control technique became adopted by several early computer operating systems as a "handshaking" signal warning a sender to stop transmission because of impending buffer 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.[32]
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 marked 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.[33] Teletypes were commonly used with the less-expensive computers from Digital Equipment Corporation (DEC); these systems had to use what keys were available, and thus the DEL character was assigned to erase the previous character.[34] [35] Because of this, DEC video terminals (by default) sent the DEL character for the key marked "Backspace" while the separate key marked "Delete" sent an escape 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 BS or 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 as ksh, bash, and zsh, 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 in GNU Emacs.[36]
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 sent to 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", "") from ECMA-48 (1972) and its successors. Some escape sequences do not have introducers, like the VT100 full reset command "".[37]
In contrast, an ESC read from the terminal is most often used as an out-of-band character used to terminate an operation or special mode, 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 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 that backward compatibility necessitated continuing to follow it. When Gary Kildall created CP/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 as MS-DOS by Microsoft) inherited the convention by virtue of being loosely based on CP/M,[38] and Windows 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, on Multics used line feed (LF) alone as a line terminator.[39] 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 in UNIX documents. Unix and Unix-like systems, and Amiga systems, adopted this convention from Multics. On the other hand, the original Macintosh OS, Apple DOS, and ProDOS used carriage return (CR) alone as a line terminator; however, since Apple later replaced these obsolete operating systems with their Unix-based macOS (formerly named OS X) operating system, they now use line feed (LF) as well. The Radio Shack TRS-80 also used a lone CR to terminate lines.
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 which used EBCDIC rather than ASCII encoding. 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. Telnet used ASCII along with CR-LF line endings, and software using other conventions would translate between the local conventions and the NVT.[40] 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.[41] [42] 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.[43] [44]
The PDP-6 monitor,[34] and its PDP-10 successor TOPS-10,[35] 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.[45] For these reasons, EOF, or end-of-file, was used colloquially and conventionally as a three-letter acronym for control-Z instead of SUBstitute. The end-of-text character (ETX), also known as control-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 convenient mnemonic 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.
In the C programming language, and in 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".
| Abbreviation | Name (1967) | ||||||||||
|---|---|---|---|---|---|---|---|---|---|---|---|
| 1963 | 1965 | 1967 | |||||||||
| 000 0000 | 000 | 0 | 00 | NULL | NUL | ␀ | Null | ||||
| 000 0001 | 001 | 1 | 01 | SOM | SOH | ␁ | Start of Heading | ||||
| 000 0010 | 002 | 2 | 02 | EOA | STX | ␂ | Start of Text | ||||
| 000 0011 | 003 | 3 | 03 | EOM | ETX | ␃ | End of Text | ||||
| 000 0100 | 004 | 4 | 04< | -- Please note that the character below, U+2404, is correct. If you are seeing it as "ENQ", please upgrade your fonts as the one you have is buggy. --> | EOT | ␄ | End of Transmission | ||||
| 000 0101 | 005 | 5 | 05< | -- Please note that the character below, U+2405, is correct. If you are seeing it as "EOT", please upgrade your fonts as the one you have is buggy. --> | WRU | ENQ | ␅ | Enquiry | |||
| 000 0110 | 006 | 6 | 06 | RU | ACK | ␆ | Acknowledgement | ||||
| 000 0111 | 007 | 7 | 07 | BELL | BEL | ␇ | Bell | ||||
| 000 1000 | 010 | 8 | 08 | FE0 | BS | ␈ | Backspace | ||||
| 000 1001 | 011 | 9 | 09 | HT/SK | HT | ␉ | Horizontal Tab | ||||
| 000 1010 | 012 | 10 | 0A | LF | ␊ | Line Feed | |||||
| 000 1011 | 013 | 11 | 0B | VTAB | VT | ␋ | Vertical Tab | ||||
| 000 1100 | 014 | 12 | 0C | FF | ␌ | Form Feed | |||||
| 000 1101 | 015 | 13 | 0D | CR | ␍ | Carriage Return | |||||
| 000 1110 | 016 | 14 | 0E | SO | ␎ | Shift Out | |||||
| 000 1111 | 017 | 15 | 0F | SI | ␏ | Shift In | |||||
| 001 0000 | 020 | 16 | 10 | DC0 | DLE | ␐ | Data Link Escape | ||||
| 001 0001 | 021 | 17 | 11 | DC1 | ␑ | Device Control 1 (often XON) | |||||
| 001 0010 | 022 | 18 | 12 | DC2 | ␒ | Device Control 2 | |||||
| 001 0011 | 023 | 19 | 13 | DC3 | ␓ | Device Control 3 (often XOFF) | |||||
| 001 0100 | 024 | 20 | 14 | DC4 | ␔ | Device Control 4 | |||||
| 001 0101 | 025 | 21 | 15 | ERR | NAK | ␕ | Negative Acknowledgement | ||||
| 001 0110 | 026 | 22 | 16 | SYNC | SYN | ␖ | Synchronous Idle | ||||
| 001 0111 | 027 | 23 | 17 | LEM | ETB | ␗ | End of Transmission Block | ||||
| 001 1000 | 030 | 24 | 18 | S0 | CAN | ␘ | Cancel | ||||
| 001 1001 | 031 | 25 | 19 | S1 | EM | ␙ | End of Medium | ||||
| 001 1010 | 032 | 26 | 1A | S2 | SS | SUB | ␚ | Substitute | |||
| 001 1011 | 033 | 27 | 1B | S3 | ESC | ␛ | Other representations might be used by specialist equipment, for example ISO 2047 graphics or hexadecimal numbers. Printable charactersCodes 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. | ||||