Wikipedia: ASCII

Showing revision 31

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 workstation?s). 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.

  32      40 (    48 0    56 8    64 @    72 H    80 P    88 X    96 `   104 h   112 p   120 x
  33 !    41 )    49 1    57 9    65 A    73 I    81 Q    89 Y    97 a   105 i   113 q   121 y
  34 "    42 *    50 2    58 :    66 B    74 J    82 R    90 Z    98 b   106 j   114 r   122 z
  35 #    43 +    51 3    59 ;    67 C    75 K    83 S    91 [    99 c   107 k   115 s   123 {
  36 $    44 ,    52 4    60 <    68 D    76 L    84 T    92 \   100 d   108 l   116 t   124 |
  37 %    45 -    53 5    61 =    69 E    77 M    85 U    93 ]   101 e   109 m   117 u   125 }
  38 &    46 .    54 6    62 >    70 F    78 N    86 V    94 ^   102 f   110 n   118 v   126 ~
  39 '    47 /    55 7    63 ?    71 G    79 O    87 W    95 _   103 g   111 o   119 w

: Table 1: The printable ASCII characters

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.

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].)