ASCII

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 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 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. In fact, users of ASCII quickly adopted some of the control codes to represent 'meta-information' such as end_of_line, or start or end of data element, and so on. These assingments often conflict and part of the effort in converting data from one format to another is making the correct meta-information transformations. An example is Unix and MS-DOS. Unix uses the LF (line feed) control character to mean end_of_line in a text file. MS-DOS uses the CR/LF (carriage return/line feed) combination to mean the same thing. Files being moved from one system to the other must detect and substitute accordingly.

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.

Binary	Decimal	Hex	Abbreviation	Printable Representation	Name/Meaning
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 0101	5	05	ENQ	␅	Enquiry
0000 0110	6	06	ACK	␆	Acknowledgement
0000 0111	7	07	BEL	␇	Bell
0000 1000	8	08	BS	␈	Backspace
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 1000	24	18	CAN	␘	Cancel
0001 1001	25	19	EM	␙	End of Medium
0001 1010	26	1A	SUB	␚	Substitute
0001 1011	27	1B	ESC	␛	Escape
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
0111 1111	127	7F	DEL	␡	Delete

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 brower, (ie, 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.

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 standard, but only the American standard, also calls "opening single quotation mark"), a tilde, or a breath mark (inverted vel).

Binary Decimal Hex Graphic 0010 0000 32 20 (blank) (␠) 0010 0001 33 21 ! 0010 0010 34 22 " 0010 0011 35 23 # 0010 0100 36 24 $ 0010 0101 37 25 % 0010 0110 38 26 & 0010 0111 39 27 ' 0010 1000 40 28 ( 0010 1001 41 29 ) 0010 1010 42 2A * 0010 1011 43 2B + 0010 1100 44 2C , 0010 1101 45 2D - 0010 1110 46 2E . 0010 1111 47 2F / 0011 0000 48 30 0 0011 0001 49 31 1 0011 0010 50 32 2 0011 0011 51 33 3 0011 0100 52 34 4 0011 0101 53 35 5 0011 0110 54 36 6 0011 0111 55 37 7 0011 1000 56 38 8 0011 1001 57 39 9 0011 1010 58 3A : 0011 1011 59 3B ; 0011 1100 60 3C < 0011 1101 61 3D = 0011 1110 62 3E > 0011 1111 63 3F ?

] Binary Decimal Hex Graphic 0100 0000 64 40 @ 0100 0001 65 41 A 0100 0010 66 42 B 0100 0011 67 43 C 0100 0100 68 44 D 0100 0101 69 45 E 0100 0110 70 46 F 0100 0111 71 47 G 0100 1000 72 48 H 0100 1001 73 49 I 0100 1010 74 4A J 0100 1011 75 4B K 0100 1100 76 4C L 0100 1101 77 4D M 0100 1110 78 4E N 0100 1111 79 4F O 0101 0000 80 50 P 0101 0001 81 51 Q 0101 0010 82 52 R 0101 0011 83 53 S 0101 0100 84 54 T 0101 0101 85 55 U 0101 0110 86 56 V 0101 0111 87 57 W 0101 1000 88 58 X 0101 1001 89 59 Y 0101 1010 90 5A Z 0101 1011 91 5B [ 0101 1100 92 5C \ 0101 1101 93 5D ] 0101 1110 94 5E ^ 0101 1111 95 5F _

Binary Decimal Hex Graphic 0110 0000 96 60 ` 0110 0001 97 61 a 0110 0010 98 62 b 0110 0011 99 63 c 0110 0100 100 64 d 0110 0101 101 65 e 0110 0110 102 66 f 0110 0111 103 67 g 0110 1000 104 68 h 0110 1001 105 69 i 0110 1010 106 6A j 0110 1011 107 6B k 0110 1100 108 6C l 0110 1101 109 6D m 0110 1110 110 6E n 0110 1111 111 6F o 0111 0000 112 70 p 0111 0001 113 71 q 0111 0010 114 72 r 0111 0011 115 73 s 0111 0100 116 74 t 0111 0101 117 75 u 0111 0110 118 76 v 0111 0111 119 77 w 0111 1000 120 78 x 0111 1001 121 79 y 0111 1010 122 7A z 0111 1011 123 7B { 0111 1100 124 7C | 0111 1101 125 7D } 0111 1110 126 7E ~

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 so-called 128 character US-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. Unicode, being a 16-bit code, has a much wider choice of glyphs, and is rapidly supplanting ISO 8859 and ASCII in many places. But it requires software and hardware to work 16-bits at a time and has considerable potential to waste memory for applications that don't need the additional 'code points'. Unicode is backward compatible: the first 127 code points are the same as in ASCII, and the first 256 code points of Unicode are the same as ISO 8859-1. The UTF-8 variant of Unicode, allows use of 8 bit character representations at the cost of longer (24 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.

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. Some of these names are more whimsical than others. (See especially the end of the list.)

ASCIIZ or ASCIZ is an adjective used to refer to a null-terminated ASCII string.

See also

• ASCII art
• Binary and text files
• Extended ASCII
• Unicode
• UTF-8
• EBCDIC

External links

• ASCII: American Standard Code for Information Infiltration by Tom Jennings
• Jargon File: ASCIIbetical
• Bob Bemer's home page some historical notes about ASCII from one of its designers

Referenced By