ASCII

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ASCII is an acronym for American Standard Code for Information Interchange. Introduced as ANSI Standard X3.4 in 1968, it is a character set and a character encoding based on the Roman alphabet as used in modern English, used by computers and other communication equipment to represent text and to control devices that work with text. Like other codes (such as IBM's EBCDIC), it specifies a correspondence between integers that can be represented digitally and the symbols of a written language, allowing digital devices to communicate with each other and to process and store 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) to represent information. Even at that 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.

The non-printable control characters:

 Decimal  Hex   Value
 -------  ----  -----
   000     00    NUL    (Null char.)
   001     01    SOH    (Start of Header)
   002     02    STX    (Start of Text)
   003     03    ETX    (End of Text)
   004     04    EOT    (End of Transmission)
   005     05    ENQ    (Enquiry)
   006     06    ACK    (Acknowledgment)
   007     07    BEL    (Bell)
   008     08     BS    (Backspace)
   009     09     HT    (Horizontal Tab)
   010     0A     LF    (Line Feed)
   011     0B     VT    (Vertical Tab)
   012     0C     FF    (Form Feed)
   013     0D     CR    (Carriage Return)
   014     0E     SO    (Shift Out)
   015     0F     SI    (Shift In)
   016     10    DLE    (Data Link Escape)
   017     11    DC1    (XON) (Device Control 1)
   018     12    DC2          (Device Control 2)
   019     13    DC3    (XOFF)(Device Control 3)
   020     14    DC4          (Device Control 4)
   021     15    NAK    (Negative Acknowledgement)
   022     16    SYN    (Synchronous Idle)
   023     17    ETB    (End of Trans. Block)
   024     18    CAN    (Cancel)
   025     19     EM    (End of Medium)
   026     1A    SUB    (Substitute)
   027     1B    ESC    (Escape)
   028     1C     FS    (File Separator)
   029     1D     GS    (Group Separator)
   030     1E     RS    (Request to Send)(Record Separator)
   031     1F     US    (Unit Separator)

Printable characters:

 Decimal  Hex   Value    Decimal  Hex   Value    Decimal  Hex   Value
 -------  ----  -----    -------  ----  -----    -------  ----  -----
   032     20  (Space)     064     40     @        096     60     ` 
   033     21     !        065     41     A        097     61     a 
   034     22     "        066     42     B        098     62     b 
   035     23     #        067     43     C        099     63     c 
   036     24     $        068     44     D        100     64     d 
   037     25     %        069     45     E        101     65     e 
   038     26     &        070     46     F        102     66     f 
   039     27     '        071     47     G        103     67     g 
   040     28     (        072     48     H        104     68     h 
   041     29     )        073     49     I        105     69     i 
   042     2A     *        074     4A     J        106     6A     j 
   043     2B     +        075     4B     K        107     6B     k 
   044     2C     ,        076     4C     L        108     6C     l 
   045     2D     -        077     4D     M        109     6D     m 
   046     2E     .        078     4E     N        110     6E     n 
   047     2F     /        079     4F     O        111     6F     o 
   048     30     0        080     50     P        112     70     p 
   049     31     1        081     51     Q        113     71     q 
   050     32     2        082     52     R        114     72     r 
   051     33     3        083     53     S        115     73     s 
   052     34     4        084     54     T        116     74     t 
   053     35     5        085     55     U        117     75     u 
   054     36     6        086     56     V        118     76     v 
   055     37     7        087     57     W        119     77     w 
   056     38     8        088     58     X        120     78     x 
   057     39     9        089     59     Y        121     79     y 
   058     3A     :        090     5A     Z        122     7A     z 
   059     3B     ;        091     5B     [        123     7B     { 
   060     3C     <        092     5C     \        124     7C     | 
   061     3D     =        093     5D     ]        125     7D     } 
   062     3E     >        094     5E     ^        126     7E     ~ 
   063     3F     ?        095     5F     _        127     7F    DEL

The first thirty-two codes (numbers 0--31) 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. 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.

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 miscelaneous symbols (see Table 1). 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 the time, by punching out all its holes.

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. For example, the ASCII code 93 (the right square bracket, "]") is used in the German variant ISO 646-DE for the uppercase letter U with umlaut (Ü), and in the Danish variant ISO 646-DK for the uppercase letter A with ring (Å).

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.

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

The portmanteau word "ASCIIbetical" evolved to describe data that is sorted in ASCII collated order rather than standard alphabetical order (which requires some human judgment, and varies with language). (See [1].)

See also Extended ASCII, Unicode.