ASCIIcharacter set and a character encoding based on the Roman alphabet as used in modern English and other Western European languages. It is most commonly used by computers and other communication equipment to represent text and by control devices that work with text.
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2 See also
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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 encoding, 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".
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) 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.
ASCII does not specify any way to include information about the conceptual structure or appearance of a piece of text. That requires other standards, such as those specifying markup languages. conceptual structure can be included using XML and appearance can be specified by using HTML for relatively simple things, SGML for more complex things, or PostScript, Display PostScript, or Tex for advanced layout and font control.
ASCII was first published as a standard in 1963 by the American Standards Association (ASA), which later became ANSI. There are many variations of ASCII, but its present, most widely-used form is ANSI X3.4-1986, also standardized as ECMA-6, ISO/IEC 646:1991 International Reference Version, and ITU-T Recommendation T.50 (09/92). It is embedded in its probable replacement, Unicode, as the 'lowest' 127 characters. ASCII is considered by some the most successful software standard ever promulgated.
Historically, ASCII developed from telegraphic codes and its first commercial use was as a 7-bit teleprinter code promoted by Bell data services. The Bell System had been planning to use a 6-bit code derived from Fieldata that added punctuation and lower-case letter to the earlier 5-bit Baudot teleprinter code but was persuaded to instead join the ASA subcommittee that was developing ASCII. Baudot helped in the automation of sending and receiving of telegraphic messages, and took many features from Morse code; it was however, a constant length code unlike Morse code. Compared to earlier telegraph codes, the proposed Bell code and ASCII were both reordered for more convenient sorting (ie, alphabetization) of lists, and added features for devices other than teleprinters. Some ASCII features, including the 'ESCape sequence', were due to Robert Bemer.
ASCII control characters
The first thirty-two codes (numbers 0-31 decimal) in ASCII are reserved for control characters: codes that were not originally intended to carry information, but rather to control devices (such as printerss) that make use of ASCII. For example, character 10 represents the "line feed" function (which causes a printer to advance its paper), and character 27 represents the "escape" key found on the top left of common keyboards.
Code 127 (all seven bits on) is another special character known as "delete" or "rubout". Though its function is similar to that of other control characters, this pattern was used so that it could be used to 'erase' a section of paper tape, a popular storage medium until the 80's, by punching all possible holes at a particular character position.
Many of the ASCII control codes are to mark data packets, or to control a data transmission protocol (i.e., ENQuiry (effectively, "any stations out there?"), ACKnowledge, Negative AcKnowledge, Start Of Header, Start Of Text, End Of Text, etc). ESCape and SUBstitute permit a communications protocol to, for instance, mark binary data so that if it contains codes with the same pattern as a protocol character, the code will be processed as data.
The separator characters (Record Separator, etc.) were intended for use with magnetic tape systems.
XON and XOFF are common interpretations of two of the Device Control characters and are generally used to throttle data flow to a slow device, such as a printer, from a fast device, such as a computer so data does not overrun and be lost.
Early users of ASCII adopted some of the control codes to represent 'meta-information' such as end-of-line, start/end of a data element, and so on. These assignments often conflict, so part of the effort in converting data from one format to another is making the correct meta-information transformations. For example, the character(s) representing end-of-line ('new line') in text data files/streams vary from operating system to operating system. When moving files from one system to another, these characters must be recognized as end-of-line markers and converted appropriately.
|0000 0000||0||00||NUL||␀||Null character|
|0000 0001||1||01||SOH||␁||Start of Header|
|0000 0010||2||02||STX||␂||Start of Text|
|0000 0011||3||03||ETX||␃||End of Text|
|0000 0100||4||04||EOT||␄||End of Transmission|
|0000 1001||9||09||HT||␉||Horizontal Tab|
|0000 1010||10||0A||LF||␊||Line feed|
|0000 1011||11||0B||VT||␋||Vertical Tab|
|0000 1100||12||0C||FF||␌||Form Feed|
|0000 1101||13||0D||CR||␍||Carriage return|
|0000 1110||14||0E||SO||␎||Shift Out|
|0000 1111||15||0F||SI||␏||Shift In|
|0001 0000||16||10||DLE||␐||Data Link Escape|
|0001 0001||17||11||DC1||␑||Device Control 1 -- oft. XON|
|0001 0010||18||12||DC2||␒||Device Control 2|
|0001 0011||19||13||DC3||␓||Device Control 3 -- oft. XOFF|
|0001 0100||20||14||DC4||␔||Device Control 4|
|0001 0101||21||15||NAK||␕||Negative Acknowledgement|
|0001 0110||22||16||SYN||␖||Synchronous Idle|
|0001 0111||23||17||ETB||␗||End of Trans. Block|
|0001 1001||25||19||EM||␙||End of Medium|
|0001 1100||28||1C||FS||␜||File Separator|
|0001 1101||29||1D||GS||␝||Group Separator|
|0001 1110||30||1E||RS||␞||Record Separator|
|0001 1111||31||1F||US||␟||Unit Separator|
In the table above, the fifth column contains glyphs reserved for representing control codes in a data stream, ie, when they must be printed or displayed rather than (or in addition to) causing action; your browser, (ie, your HTML user agent) may require the installation of additional fonts in order to display them.
ASCII printable characters
Code 32 is the "space" character, denoting the space between words, which is produced by the large space bar of a keyboard. Codes 33 to 126 are called the printable characters, which 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: a BackSpace can be followed with the grave accent (which the American and British standards, but only the American and British standards, also call "opening single quotation mark"), a tilde, or a breath mark (inverted vel).
Note how uppercase characters can be converted to lowercase by adding 32 to their ASCII value; in binary, this can be accomplished simply by setting the sixth-least significant bit to 1.
Variants of ASCII
As computer technology spread throughout the world, many variations of ASCII, somewhat inappropriately referred to as ASCII extensions, were developed by corporations and standards bodies in order to facilitate the expression of non-English languages that still used Roman-based alphabets. ISO 646 (1972) was the first attempt to remedy the English bias, although it created compatibility problems, since it was still a seven-bit character set. No additional codes were available, so some were re-assigned in language-specific variants. It was thus impossible to know what character was represented by a code without knowing what variant was in use, and text processing systems were generally able to 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. Eight-bit standards such as ISO/IEC 8859 were true extensions of ASCII, leaving the original character mapping intact and just adding additional values above the 7-bit range. This enabled a broader range of languages to be represented, but these standards were still plagued with incompatibilities and limitations. Still, ISO/IEC 8859-1 and original 7-bit ASCII are the most common character encodings in use today.
Unicode, being originally a 16-bit code and now effectively a 21-bit code, has a much wider array of characters, and its various encoding forms are rapidly supplanting ISO/IEC 8859 and ASCII in many environments. However, Unicode typically requires software and hardware to work 16 or 32 bits at a time, and has considerable potential to waste memory for applications that only need to work with a limited range of characters. Unicode is backward compatible: the first 127 code points map to the same characters as in ASCII, and the first 256 code points of Unicode map to the same characters as in ISO/IEC 8859-1. The UTF-8 encoding form of Unicode allows use of 8 bit character representations at the cost of longer (16, 24, or 32 bit) representations of less commonly used representations. It is widely used in the Western world since the Latin characters are often sufficient for those languages. Other languages, especially Chinese, Japanese, and Korean, may prefer UTF-16 since this encoding uses 16 bits for their characters where UTF-8 would require 24.
The portmanteau word ASCIIbetical has evolved to describe the collation of data in ASCII code order rather than 'standard' alphabetical order (which requires some tricky computation, and varies with language).
ASCII contains many characters which were not commonly used, or at least spoken of, outside of the computing context; the "popularization" of these characters required that names be agreed upon for them. See the pronunciation guide in the external links, below.
ASCIIZ or ASCIZ is an adjective used to refer to a null-terminated ASCII string.
ASCII is also a name of one of the oldest and most prestigious computer magazines published in Japan. See ASCII (magazine)