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ASCII

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There are 95 printable ASCII characters, numbered 32 to 126.
There are 95 printable ASCII characters, numbered 32 to 126.

ASCII (American Standard Code for Information Interchange), generally pronounced [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 have a historical basis in ASCII.

ASCII was first published as a standard in 1967 and was last updated in 1986. It currently defines codes for 33 non-printing, mostly obsolete control characters that affect how text is processed, plus the following 95 printable characters (starting with the space character):

!"#$%&'()*+,-./0123456789:;<=>?

@ABCDEFGHIJKLMNOPQRSTUVWXYZ[]^_
`abcdefghijklmnopqrstuvwxyz~

Contents

Overview

Like other character representation computer codes, ASCII specifies a correspondence between digital bit patterns and the symbols/glyphs of a written language, thus allowing digital devices to communicate with each other and to process, store, and communicate character-oriented information. The ASCII character encodingInternational Organization for Standardization (December 1, 1975). "[The set of control characters for ISO 646]". Internet Assigned Numbers Authority Registry. Alternate U.S. version: [link]. Accessed August 7, 2005. — or a compatible extension (see below) — is used on nearly all common computers, especially personal computers and workstations. The preferred MIME name for this encoding is "US-ASCII".Internet Assigned Numbers Authority (January 28, 2005). "[Character Sets]". Accessed August 7, 2005.

ASCII is, strictly, a seven-bit code, meaning that it uses the bit patterns representable with seven binary digits (a range of 0 to 127 decimal) to represent character information. At the time ASCII was introduced, many computers dealt with eight-bit groups (bytes or, more specifically, octets) as the smallest unit of information; the eighth bit was commonly used as a parity bit for error checking on communication lines or other device-specific functions. Machines which did not use parity typically set the eighth bit to zero, though some systems such as Prime machines running PRIMOS set the eighth bit of ASCII characters to one.

ASCII only defines a relationship between specific characters and bit sequences; aside from reserving a few control codes for line-oriented formatting, it does not define any mechanism for describing the structure or appearance of text within a document. Such concepts are within the realm of other systems such as the markup languages.

History

ASCII developed from telegraphic codes and first entered commercial use as a seven-bit teleprinter code promoted by Bell data services. The Bell System had previously planned to use a six-bit code, derived from Fieldata, that added punctuation and lower-case letters to the earlier five-bit Baudot teleprinter code, but was persuaded instead to join the ASA subcommittee that had started to develop ASCII. Baudot helped in the automation of sending and receiving telegraphic messages, and took many features from Morse code; however, unlike Morse code, Baudot used constant-length codes. Compared to 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, as Bemer said, "so much so that the code that was to become ASCII was first called the Bemer-Ross Code in Europe".

The American Standards Association (ASA, later to become ANSI) first published ASCII as a standard in 1963. ASCII-1963 lacked the lowercase letters, and had an up-arrow (↑) instead of the caret (^) and a left-arrow (←) instead of the underscore (_). The 1967 version added the lowercase letters, changed the names of a few control characters and moved the two controls ACK and ESC from the lowercase letters area into the control codes area.

ASCII was subsequently updated and published as ANSI X3.4-1968, ANSI X3.4-1977, and finally, ANSI X3.4-1986.

Other international standards bodies have ratified character encodings that are identical or nearly identical to ASCII. These encodings are sometimes referred to as ASCII, even though ASCII is strictly defined only by the ASA/ANSI standards:

ASCII has also become embedded in its probable replacement, Unicode, as the 'lowest' 128 characters. Some observers consider ASCII the most "successful" software standard ever promulgated.

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".

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, aka XON), 19 (Control-S, DC3, aka XOFF), and 127 (DELete) became de-facto standards. Its noncompliant use of code 15 (Control-O, Shift In) as "left arrow", usually 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 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. Unix systems, for example, interpreted "Delete" to mean "remove the character before the cursor". Most other systems used "Backspace" for that meaning and used "Delete" 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. For example, ESC was used as the opposite kind of escape in the very influential TECO editor, where normal characters were data to be entered and one used ESC to introduce a command. In modern use, ESC is most often used as an "out of band" character used to terminate an operation.

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. But many early video terminals did not allow a linefeed without an implied carriage return, so the carriage return became redundant. Hence plain text files on Unix systems only contain linefeeds. But many printers did not implement linefeeds, and automatically advanced a line when performing a carriage return. So older Macintosh systems, among others, store only carriage returns in plain text files. CP/M (the predecessor of MS-DOS) attempted to be more "compliant" with the ambiguous ASCII standard, which is why it puts both carriage return and linefeed in plain text files - and uses control-Z (SUB) to mark the end of the actual text in the file. Control-C (ETX, End of TeXt) might have made more sense, but was already in wide use as a program abort signal.

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, Control-N and Control-O are basically never used to invoke an alternate character set for a short run of characters, etc. 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, "is there anybody out there?").

Binary Oct Dec Hex Abbr PR[1] CS[2] Description
0000 0000 000 0 00 NUL ^@ Null character
0000 0001 001 1 01 SOH ^A Start of Header
0000 0010 002 2 02 STX ^B Start of Text
0000 0011 003 3 03 ETX ^C End of Text
0000 0100 004 4 04 EOT ^D End of Transmission
0000 0101 005 5 05 ENQ ^E Enquiry
0000 0110 006 6 06 ACK ^F Acknowledgment
0000 0111 007 7 07 BEL ^G Bell
0000 1000 010 8 08 BS ^H Backspace[3][7]
0000 1001 011 9 09 HT ^I Horizontal Tab
0000 1010 012 10 0A LF ^J Line feed
0000 1011 013 11 0B VT ^K Vertical Tab
0000 1100 014 12 0C FF ^L Form feed
0000 1101 015 13 0D CR ^M Carriage return[6]
0000 1110 016 14 0E SO ^N Shift Out
0000 1111 017 15 0F SI ^O Shift In
0001 0000 020 16 10 DLE ^P Data Link Escape
0001 0001 021 17 11 DC1 ^Q Device Control 1 (oft. XON)
0001 0010 022 18 12 DC2 ^R Device Control 2
0001 0011 023 19 13 DC3 ^S Device Control 3 (oft. XOFF)
0001 0100 024 20 14 DC4 ^T Device Control 4
0001 0101 025 21 15 NAK ^U Negative Acknowledgement
0001 0110 026 22 16 SYN ^V Synchronous Idle
0001 0111 027 23 17 ETB ^W End of Trans. Block
0001 1000 030 24 18 CAN ^X Cancel
0001 1001 031 25 19 EM ^Y End of Medium
0001 1010 032 26 1A SUB ^Z Substitute
0001 1011 033 27 1B ESC ^[ Escape[5]
0001 1100 034 28 1C FS ^ File Separator
0001 1101 035 29 1D GS ^] Group Separator
0001 1110 036 30 1E RS ^^ Record Separator
0001 1111 037 31 1F US ^_ Unit Separator
0111 1111 127 127 7F DEL ^? Delete[4][7]

  1. Printable Representation, the Unicode characters 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. Control key Sequence, 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.
  3. The Backspace character can also be entered by pressing the "Backspace", "Bksp", or ← key on some systems.
  4. 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.
  5. The Escape character can also be entered by pressing the "Escape" or "Esc" key on some systems.
  6. The Carriage Return character can also be entered by pressing the "Return", "Ret", "Enter", or ↵ key on most systems.
  7. The ambiguity surrounding the Backspace key comes from systems that translated the DEL control character into a BS (backspace) before transmitting it. Some software was unable to process the character and would display "^H" instead. "^H" persists in messages today as a deliberate humorous device, e.g. "there's a sucker^H^H^H^H^H^Hpotential customer born every minute". A less common variant of this involves the use of "^W", which in some text editors means "delete previous word". The example sentence would therefore also work as "there's a sucker^W potential customer born every minute".

ASCII printable characters

Code 32, the "space" character, denotes the space between words, as produced by the large space-bar of a keyboard. Codes 33 to 126, known as the printable characters, represent letters, digits, punctuation marks, and a few miscellaneous symbols.

Seven-bit ASCII provided seven "national" characters and, if the combined hardware and software permit, can use overstrikes to simulate some additional international characters: in such a scenario a backspace can precede a grave accent (which the American and British standards, but only those standards, also call "opening single quotation mark"), a tilde, or a breath mark (inverted vel).

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Structural features

Aliases for ASCII

RFC 1345 (published in June 1992) and the [IANA registry of character sets] (ongoing), both recognize the following case-insensitive aliases for ASCII as suitable for use on the Internet:

Of these, only the aliases "US-ASCII" and "ASCII" have achieved widespread use. One often finds them 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 prolog of some XML documents.

Variants of ASCII

As computer technology spread throughout the world, different standards bodies and corporations developed many variations of ASCII in order 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 mis-apply that term to cover all variants, including those that do not preserve ASCII's character-map in the 7-bit range.

The PETSCII Code used by Commodore International for their 8-bit systems is probably unique among post-1970 codes in being based on ASCII-1963 instead of the far more common ASCII-1967.

ISO 646 (1972), the first attempt to remedy the pro-English-language bias, created compatibility problems, since it remained a 7-bit character-set. It made no additional codes available, so it reassigned some in language-specific variants. It thus became impossible to know what character a code represented without knowing which variant to work with, and text-processing systems could generally cope with only one variant anyway.

Eventually, improved technology brought out-of-band means to represent the information formerly encoded in the eighth bit of each byte, freeing this bit to add another 128 additional character-codes for new assignments. For example, IBM developed 8-bit code pages, such as 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.

Eight-bit standards such as ISO/IEC 8859 (derived from the DEC-MCS) and Mac OS Roman developed as true extensions of ASCII, leaving the original character-mapping intact and just adding additional values above the 7-bit range. This enabled the representation of a broader range of languages, but these standards continued to suffer from incompatibilities and limitations. Still, ISO-8859-1, its variant Windows-1252 (often mislabeled as ISO-8859-1 even by Microsoft software) and original 7-bit ASCII remain the most common character encodings in use today.

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 basically uses 7-bit codes, Unicode and the UCS use relatively abstract "code points": non-negative integer numbers that map, using different encoding forms and schemes, to sequences of one or more 8-bit bytes. To permit backward compatibility, Unicode and the UCS assign the first 128 code points to the same characters as ASCII. One can therefore think of ASCII as a 7-bit encoding scheme for a very small subset of Unicode and of the UCS. The popular UTF-8 encoding-form prescribes the use of one to four 8-bit code values for each code point character, and equates exactly to ASCII for the code values below 128. Other encoding forms such as UTF-16 resemble ASCII in how they represent the first 128 characters of Unicode, but tend to use 16 or 32 bits per character, so they require conversion for compatibility.

The blend word ASCIIbetical has evolved to describe the collation of data in ASCII-code order rather than "standard" alphabetical order.Jargon File. [ASCIIbetical]. Accessed December 17, 2005.

The abbreviation ASCIIZ or ASCIZ refers to a null-terminated ASCII string.

Trivia

Asteroid 3568 ASCII is named after the character encoding.

See also

ASCII extensions

(where all ASCII printable characters are identical to ASCII)

ASCII variants

(where some ASCII printable characters have been replaced)

References

For specific points

General

External links

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