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ASCII

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ASCII (American Standard Code for Information Interchange) is a character set and a character encoding based on the Roman alphabet as used in modern English. It is most commonly used by computers and other communication equipment to represent text and by control devices that work with text.

Like other codes, ASCII specifies a correspondence between integers that can be represented digitally and the symbols of a written language, thus allowing digital devices to communicate with each other and to process and store 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 a seven-bit code, meaning that it uses the integers representable with seven binary digits (a range of 0 to 127 decimal) to represent information. Even at the time that ASCII was introduced, most computers dealt with eight-bit bytes as the smallest unit of information; the eighth bit was commonly used for error checking on communication lines or other device-specific functions.

ASCII does not specify any way to represent information about the structure or appearance of a piece of text. That requires other standards markup language.

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 X3.4-1967, also standardized as ECMA-6, ISO/IEC 646:1991 International Reference Version, and ITU-T Recommendation T.50 (09/92). It is embedded in page zero of its probable replacement, unicode. ASCII is generally considered the most successful software standard ever promulgated.

Historically, ASCII developed from telegraphic codes. It started as a commercial 7-bit teleprinter code promoted by Bell data services. ASA reordered the code for sorting (alphabetization) of lists, and added features for devices other than teleprinters. Bell's code added punctuation and lower-case letter to the earlier 5-bit Baudot[?] teleprinter code. Baudot automated sending and receiving of telegraphic messages and took many features from Morse code.

ASCII Control Characters

The first thirty-two codes (numbers 0-31 decimal) in ASCII are reserved for control characters: codes that may not themselves represent information, but that are used to control devices (such as printers) 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, it was placed at this position so that it could be used to erase a section of paper tape, a popular storage medium at one time, by punching out all its holes. Code 0 (all bits off) is ignored by many computer systems.

Many of the codes are to mark data packets, and control a data transmission protocol (i.e. enquiry (any stations out there?), acknowledge, negative acknowledge, start of header, start of text, end of text). Escape and substitute permit a protocol to mark binary data so that if it contains codes with the same values as protocol characters, the codes will be processed as data.

The separator characters (record separator, etc.) were designed for use with magnetic tape systems.

XON and XOFF are often sent from a slow device, such as a printer, to start and stop a flow of data so no data is lost.

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 0110606ACKAcknowledgment
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 00011711DC1XON Device Control 1
0001 00101812DC2Device Control 2
0001 00111913DC3XOFF Device Control 3
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 graphic characters that are reserved for representing the position of control codes in a data stream; your HTML user agent may require the installation of additional fonts in order to display them.

See new line.

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.

ASCII provides some internationalization for French and Spanish (both spoken in the U.S.) by providing a backspace with the grave, accent (miscalled a "single quote"), tilde, and breath mark (inverted vel).

BinaryDecimalHexGraphic
0010 00003220(blank) (␠)
0010 00013321!
0010 00103422"
0010 00113523#
0010 01003624$
0010 01013725%
0010 01103826&
0010 01113927'
0010 10004028(
0010 10014129)
0010 1010422A*
0010 1011432B+
0010 1100442C,
0010 1101452D-
0010 1110462E.
0010 1111472F/
0011 000048300
0011 000149311
0011 001050322
0011 001151333
0011 010052344
0011 010153355
0011 011054366
0011 011155377
0011 100056388
0011 100157399
0011 1010583A:
0011 1011593B;
0011 1100603C<
0011 1101613D=
0011 1110623E>
0011 1111633F?
 
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

The international spread of computer technology led to many variations and extensions to the ASCII character set, since ASCII does not include accented letters and other symbols necessary to write most languages besides English that use Roman-based alphabets. International standard ISO 646 (1972) was the first attempt to remedy this problem, although it regrettably created compatibility problems as well. ISO 646 was still a seven-bit character set, and since no additional codes were available, some were re-assigned in language-specific variants. See ISO 646 for details.

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 8859 enabled a broader range of languages to be represented, but were still plagued with incompatibilities and limitations. Still, ISO 8859-1 and original 7-bit ASCII are the most common character encodings in use today, though Unicode (with a much larger code set) is quickly becoming standard in many places. These newer codes are backward-compatible: that is, the first 127 code points of each code are the same as ASCII, and the first 256 code points of Unicode are the same as ISO 8859-1.

The portmanteau word "ASCIIbetical" has evolved to describe the collation of data in ASCII code order rather than genuine alphabetical order (which requires some tricky computation, and varies with language). (See the Jargon File (http://www.catb.org/~esr/jargon/html/entry/ASCIIbetical-order).)

See also: Extended ASCII, Unicode, ASCII art.


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



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