The ASCII reference article from the English Wikipedia on 24-Apr-2004
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ASCII

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ASCII (American Standard Code for Information Interchange, generally pronounced ass-key) is a character 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.

Table of contents
1 Overview
2 See also
3 External links

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 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.

BinaryDecimalHexAbbreviationPrintable
Representation
Name/Meaning
0000 0000000NULNull character
0000 0001101SOHStart of Header
0000 0010202STXStart of Text
0000 0011303ETXEnd of Text
0000 0100404EOTEnd of Transmission
0000 0101505ENQEnquiry
0000 0110606ACKAcknowledgement
0000 0111707BELBell
0000 1000808BSBackspace
0000 1001909HTHorizontal Tab
0000 1010100ALFLine feed
0000 1011110BVTVertical Tab
0000 1100120CFFForm Feed
0000 1101130DCRCarriage return
0000 1110140ESOShift Out
0000 1111150FSIShift In
0001 00001610DLEData Link Escape
0001 00011711DC1Device Control 1 -- oft. XON
0001 00101812DC2Device Control 2
0001 00111913DC3Device Control 3 -- oft. XOFF
0001 01002014DC4Device Control 4
0001 01012115NAKNegative Acknowledgement
0001 01102216SYNSynchronous Idle
0001 01112317ETBEnd of Trans. Block
0001 10002418CANCancel
0001 10012519EMEnd of Medium
0001 1010261ASUBSubstitute
0001 1011271BESCEscape
0001 1100281CFSFile Separator
0001 1101291DGSGroup Separator
0001 1110301ERSRecord Separator
0001 1111311FUSUnit Separator
0111 11111277FDELDelete

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).

BinaryDecimalHexGraphic
0010 00003220(blank) (␠)
0010 00013321Exclamation mark
0010 00103422"
0010 00113523#
0010 01003624$
0010 01013725%
0010 01103826&
0010 01113927'
0010 10004028(
0010 10014129)
0010 1010422A*
0010 1011432B+
0010 1100442CComma
0010 1101452D-
0010 1110462EFull stop
0010 1111472F/
0011 000048300
0011 000149311
0011 001050322
0011 001151333
0011 010052344
0011 010153355
0011 011054366
0011 011155377
0011 100056388
0011 100157399
0011 1010583AColon
0011 1011593BSemicolon
0011 1100603C<
0011 1101613D=
0011 1110623E>
0011 1111633FQuestion mark
 
BinaryDecimalHexGraphic
0100 00006440@
0100 00016541A
0100 00106642B
0100 00116743C
0100 01006844D
0100 01016945E
0100 01107046F
0100 01117147G
0100 10007248H
0100 10017349I
0100 1010744AJ
0100 1011754BK
0100 1100764CL
0100 1101774DM
0100 1110784EN
0100 1111794FO
0101 00008050P
0101 00018151Q
0101 00108252R
0101 00118353S
0101 01008454T
0101 01018555U
0101 01108656V
0101 01118757W
0101 10008858X
0101 10018959Y
0101 1010905AZ
0101 1011915B[
0101 1100925C\\
0101 1101935D]
0101 1110945E^
0101 1111955F_
 
BinaryDecimalHexGraphic
0110 00009660`
0110 00019761a
0110 00109862b
0110 00119963c
0110 010010064d
0110 010110165e
0110 011010266f
0110 011110367g
0110 100010468h
0110 100110569i
0110 10101066Aj
0110 10111076Bk
0110 11001086Cl
0110 11011096Dm
0110 11101106En
0110 11111116Fo
0111 000011270p
0111 000111371q
0111 001011472r
0111 001111573s
0111 010011674t
0111 010111775u
0111 011011876v
0111 011111977w
0111 100012078x
0111 100112179y
0111 10101227Az
0111 10111237B{
0111 11001247C|
0111 11011257D}
0111 11101267E~

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.

See also

External links


ASCII is also a name of one of the oldest and most prestigious computer magazines published in Japan. See ASCII (magazine)