Tuesday, December 30, 2008

ASCII

SOURCE:WIKIPEDIA

American Standard Code for Information Interchange (ASCII), pronounced /ˈæski/[1] is a character encoding based on the English alphabet. ASCII codes represent text in computers, communications equipment, and other devices that work with text. Most modern character encodings — which support many more characters than did the original — have a historical basis in ASCII.

Historically, ASCII developed from telegraphic codes and its first commercial use was as a seven-bit teleprinter code promoted by Bell data services. Work on ASCII formally began October 6, 1960 with the first meeting of the ASA X3.2 subcommittee. The first edition of the standard was published in 1963,[2][3] a major revision in 1967,[4] and the most recent update in 1986[5]. Compared to earlier telegraph codes, the proposed Bell code and ASCII were both ordered for more convenient sorting (i.e., alphabetization) of lists, and added features for devices other than teleprinters. Some ASCII features, including the “ESCape sequence”,[6] were due to Robert Bemer.

ASCII includes definitions for 128 characters: 33 are non-printing, mostly obsolete control characters that affect how text is processed; 94 are printable characters (excluding the space). The ASCII character encoding[7] — or a compatible extension — is used on nearly all common computers, especially personal computers and workstations.

The American Standard Code for Information Interchange (ASCII) was developed under the auspices of a committee of the American Standards Association, called the X3 committee, by its X3.2 (later X3L2) subcommittee, and later by that subcommittee’s X3.2.4 working group. The ASA became the United States of America Standards Institute or USASI[8] and ultimately the American National Standards Institute.

The X3.2 subcommittee designed ASCII based on earlier teleprinter encoding systems. Like other character encodings, ASCII specifies a correspondence between digital bit patterns and character symbols (i.e. graphemes and control characters). This allows digital devices to communicate with each other and to process, store, and communicate character-oriented information such as written language. The encodings in use before ASCII included 26 alphabetic characters, 10 numerical digits, and from 11 to 25 special graphic symbols. To include control characters compatible with the Comité Consultatif International Téléphonique et Télégraphique standard, Fieldata and early EBCDIC, more than 64 codes were required.

The committee debated the possibility of a shift key function (like the Baudot code), which would allow more than 64 codes to be represented by six bits. In a shifted code, some character codes determine choices between options for the following character codes. This allows compact encoding, but is less reliable for data transmission; an error in transmitting the shift code typically makes a long part of the transmission unreadable. The standards committee decided against shifting, and so ASCII required at least a seven-bit code.[9]

The committee considered an eight-bit code, since eight bits would allow two four-bit patterns to efficiently encode two digits with binary coded decimal. However this would require all data transmission to send eight bits when seven could suffice. The committee voted to use a seven-bit code to minimize costs associated with data transmission. Since perforated tape at the time could record eight bits in one position, this also allowed for a parity bit for error checking if desired.[10] Machines with octets as the native data type that did not use parity checking typically set the eighth bit to 0.[11]

The code itself was structured so that most control codes were together, and all graphic codes were together. The first two columns (32 positions) were reserved for control characters.[12] The “space” character had to come before graphics to make sorting algorithms easy, so it became position 32.[13] The committee decided it was important to support the upper case 64-character alphabets, and chose to structure ASCII so it could easily be reduced to a usable 64-character set of graphic codes.[14]Lower case letters were therefore not interleaved with upper case. To keep options for lower case letters and other graphics open, the special and numeric codes were placed before the letters, and the letter ‘A’ was placed in position 65 to match the draft of the corresponding British standard.[15] The digits 0–9 were placed so they correspond to values in binary prefixed with 0011, making conversion with binary-coded decimal straightforward.

Many of the non-alphanumeric characters were positioned to correspond to their shifted position on typewriters. Thus #, $ and % were placed to correspond to 3, 4, and 5 in the adjacent column. The parentheses could not correspond to 9 and 0, however, because the place corresponding to 0 was taken by the space character. Since many European typewriters placed the parentheses with 8 and 9, these corresponding positions were chosen for the parentheses. The @ symbol was not used in continental Europe and the committee expected it would be replaced by an accented À in France, so the @ was placed in position 64 next to the letter A.[16]

The control codes felt essential for data transmission were the start of message (SOM), end of address (EOA), end of message (EOM), end of transmission (EOT), “who are you?” (WRU), “are you?” (RU), a reserved device control (DC0), synchronous idle (SYNC), and acknowledge (ACK). These were positioned to maximize the Hamming distance between their bit patterns.[17]

With the other special characters and control codes filled in, ASCII was published as ASA X3.4-1963, leaving 28 code positions without assigned meaning, reserved for future standardization.[18] This version did not specify codes for lower case characters because there was some debate there should be more control characters instead.[19] In late 1963 the International Organization for Standardization voted to assign lower case characters to columns 6 and 7. The X3 committee incorporated this decision, locating the lowercase letters so they differ in bit pattern from the upper case by a single bit. This simplified case-insensitive character matching. The X3 committee made other changes, including other new characters (the curly bracket characters), renaming some control characters (SOM became start of header (SOH)) and moving or removing others (RU was removed).[20] ASCII was subsequently updated as USASI X3.4-1967, then USASI X3.4-1968, ANSI X3.4-1977, and finally, ANSI X3.4-1986.

The X3 committee also addressed how ASCII should be transmitted (low bit first), and how it should be recorded on perforated tape. They proposed a 9-track standard for magnetic tape, and attempted to deal with some forms of punched card formats.

ASCII itself first entered commercial use in 1963 as a seven-bit teleprinter code for American Telephone & Telegraph’s TWX (Teletype Wide-area eXchange) network. TWX originally used the earlier five-bit Baudot code, which was also used by the competing Telex teleprinter system. Bob Bemer introduced features such as the escape sequence.[2] His British colleague Hugh McGregor Ross helped to popularize this work — according to Bemer, “so much so that the code that was to become ASCII was first called the Bemer-Ross Code in Europe”.[21]

On March 11, 1968, U.S. President Lyndon B. Johnson mandated that all computers purchased by the United States federal government support ASCII, stating:

I have also approved recommendations of the Secretary of Commerce regarding standards for recording the Standard Code for Information Interchange on magnetic tapes and paper tapes when they are used in computer operations. All computers and related equipment configurations brought into the Federal Government inventory on and after July 1, 1969, must have the capability to use the Standard Code for Information Interchange and the formats prescribed by the magnetic tape and paper tape standards when these media are used.[22]

Other international standards bodies have ratified character encodings such as ISO/IEC 646 that are identical or nearly identical to ASCII, with extensions for characters outside the English alphabet and symbols used outside the United States, such as the symbol for the United Kingdom’s pound sterling (£). Almost every country needed an adapted version of ASCII since ASCII only suited the needs of the USA and a few other countries. For example, Canada had its own version that supported French. Other adapted encodings include ISCII (India), VISCII (Vietnam), and YUSCII (Yugoslavia). Although these encodings are sometimes referred to as ASCII, true ASCII is strictly defined only by ANSI standard.

ASCII has been incorporated into the Unicode character set as the first 128 symbols, so the ASCII characters have the same numeric codes in both sets. This allows UTF-8 to be backward compatible with ASCII, a significant advantage.

Asteroid 3568 ASCII is named after the character encoding.

ASCII reserves the first 32 codes (numbers 0–31 decimal) for control characters: codes originally intended not to carry printable information, but rather to control devices (such as printers) that make use of ASCII, or to provide meta-information about data streams such as those stored on magnetic tape. For example, character 10 represents the “line feed” function (which causes a printer to advance its paper), and character 8 represents “backspace”. Control characters that do not include carriage return, line feed or white space are called non-whitespace control characters.[23] Except for the control characters that prescribe elementary line-oriented formatting, ASCII does not define any mechanism for describing the structure or appearance of text within a document. Other schemes, such as markup languages, address page and document layout and formatting.

The original ASCII standard used only short descriptive phrases for each control character. The ambiguity this left was sometimes intentional (where a character would be used slightly differently on a terminal link than on a data stream) and sometimes more accidental (such as what “delete” means).

Probably the most influential single device on the interpretation of these characters was the ASR-33 Teletype series, which was a printing terminal with an available paper tape reader/punch option. Paper tape was a very popular medium for long-term program storage up through the 1980s, lower cost and in some ways less fragile than magnetic tape. In particular, the Teletype 33 machine assignments for codes 17 (Control-Q, DC1, also known as XON), 19 (Control-S, DC3, also known as XOFF), and 127 (DELete) became de-facto standards. Because the keytop for the O key also showed a left-arrow symbol (from ASCII-1963, which had this character instead of underscore), a noncompliant use of code 15 (Control-O, Shift In) interpreted as “delete previous character” was also adopted by many early timesharing systems but eventually faded out.

The use of Control-S (XOFF, an abbreviation for “transmit off”) as a handshaking signal warning a sender to stop transmission because of impending overflow, and Control-Q (XON, “transmit on”) to resume sending, persists to this day in many systems as a manual output control technique. On some systems Control-S retains its meaning but Control-Q is replaced by a second Control-S to resume output.

Code 127 is officially named “delete” but the Teletype label was “rubout”. Since the original standard gave no detailed interpretation for most control codes, interpretations of this code varied. The original Teletype meaning, and the intent of the standard, was to make it an ignored character, the same as NUL (all zeroes). This was specifically useful for paper tape, because punching the all-ones bit pattern on top of an existing mark would obliterate it. Tapes designed to be “hand edited” could even be produced with spaces of extra NULs (blank tape) so that a block of characters could be “rubbed out” and then replacements put into the empty space.

As video terminals began to replace printing ones, the value of the “rubout” character was lost. DEC systems, for example, interpreted “Delete” to mean “remove the character before the cursor,” and this interpretation also became common in Unix systems. Most other systems used “Backspace” for that meaning and used “Delete” as it was used on paper tape, to mean “remove the character after the cursor”. That latter interpretation is the most common today.

Many more of the control codes have taken on meanings quite different from their original ones. The “escape” character (code 27), for example, was originally intended to allow sending other control characters as literals instead of invoking their meaning. This is the same meaning of “escape” encountered in URL encodings, C language strings, and other systems where certain characters have a reserved meaning. Over time this meaning has been coopted and has eventually drifted. In modern use, an ESC sent to the terminal usually indicates the start of a command sequence, usually in the form of an ANSI escape code. An ESC sent from the terminal is most often used as an “out of band” character used to terminate an operation, as in the TECO and vi text editors.

The inherent ambiguity of many control characters, combined with their historical usage, created problems when transferring “plain text” files between systems. The clearest example of this is the newline problem on various operating systems. On printing terminals there is no question that you terminate a line of text with both “Carriage Return” and “Linefeed”. The first returns the printing carriage to the beginning of the line and the second advances to the next line without moving the carriage. However, requiring two characters to mark the end of a line introduced unnecessary complexity and questions as to how to interpret each character when encountered alone. To simplify matters, plain text files on Unix and Amiga systems use line feeds alone to separate lines. Similarly, older Macintosh systems, among others, use only carriage returns in plain text files. Various DEC operating systems used both characters to mark the end of a line, perhaps for compatibility with teletypes, and this de facto standard was copied in the CP/M operating system and then in MS-DOS and eventually Microsoft Windows. Transmission of text over the Internet, for protocols as E-mail and the World Wide Web, uses both characters.

The DEC operating systems, along with CP/M, tracked file length only in units of disk blocks and used Control-Z (SUB) to mark the end of the actual text in the file (also done for CP/M compatibility in some cases in MS-DOS, though MS-DOS has always recorded exact file-lengths). Text strings ending with the null character are known as ASCIZ or C strings.

Binary

Oct

Dec

Hex

Abbr

PR[a]

CS[b]

CEC[c]

Description

000 0000

000

0

00

NUL

^@

\0

Null character

000 0001

001

1

01

SOH

^A


Start of Header

000 0010

002

2

02

STX

^B


Start of Text

000 0011

003

3

03

ETX

^C


End of Text

000 0100

004

4

04

EOT

^D


End of Transmission

000 0101

005

5

05

ENQ

^E


Enquiry

000 0110

006

6

06

ACK

^F


Acknowledgment

000 0111

007

7

07

BEL

^G

\a

Bell

000 1000

010

8

08

BS

^H

\b

Backspace[d][i]

000 1001

011

9

09

HT

^I

\t

Horizontal Tab

000 1010

012

10

0A

LF

^J

\n

Line feed

000 1011

013

11

0B

VT

^K

\v

Vertical Tab

000 1100

014

12

0C

FF

^L

\f

Form feed

000 1101

015

13

0D

CR

^M

\r

Carriage return[h]

000 1110

016

14

0E

SO

^N


Shift Out

000 1111

017

15

0F

SI

^O


Shift In

001 0000

020

16

10

DLE

^P


Data Link Escape

001 0001

021

17

11

DC1

^Q


Device Control 1 (oft. XON)

001 0010

022

18

12

DC2

^R


Device Control 2

001 0011

023

19

13

DC3

^S


Device Control 3 (oft. XOFF)

001 0100

024

20

14

DC4

^T


Device Control 4

001 0101

025

21

15

NAK

^U


Negative Acknowledgement

001 0110

026

22

16

SYN

^V


Synchronous Idle

001 0111

027

23

17

ETB

^W


End of Trans. Block

001 1000

030

24

18

CAN

^X


Cancel

001 1001

031

25

19

EM

^Y


End of Medium

001 1010

032

26

1A

SUB

^Z


Substitute

001 1011

033

27

1B

ESC

^[

\e[f]

Escape[g]

001 1100

034

28

1C

FS

^\


File Separator

001 1101

035

29

1D

GS

^]


Group Separator

001 1110

036

30

1E

RS

^^


Record Separator

001 1111

037

31

1F

US

^_


Unit Separator


111 1111

177

127

7F

DEL

^?


Delete[e][i]

  • ^[a] Printable Representation, the Unicode characters from the area U+2400 to U+2421 reserved for representing control characters when it is necessary to print or display them rather than have them perform their intended function. Some browsers may not display these properly.
  • ^[b] Control key Sequence/caret notation, the traditional key sequences for inputting control characters. The caret (^) represents the “Control” or “Ctrl” key that must be held down while pressing the second key in the sequence. The caret-key representation is also used by some software to represent control characters.
  • ^[c] Character Escape Codes in C programming language and many other languages influenced by it, such as Java and Perl (though not all implementations necessarily support all escape codes).
  • ^[d] The Backspace character can also be entered by pressing the “Backspace”, “Bksp”, or ← key on some systems.
  • ^[e] The Delete character can also be entered by pressing the “Delete” or “Del” key. It can also be entered by pressing the “Backspace”, “Bksp”, or ← key on some systems.
  • ^[f] The ‘\e’ escape sequence is not part of ISO C and many other language specifications. However, it is understood by several compilers.
  • ^[g] The Escape character can also be entered by pressing the “Escape” or “Esc” key on some systems.
  • ^[h] The Carriage Return character can also be entered by pressing the “Return”, “Ret”, “Enter”, or ↵ key on most systems.
  • [i]ab The ambiguity surrounding Backspace comes from mismatches between the intent of the human or software transmitting the Backspace and the interpretation by the software receiving it. If the transmitter expects Backspace to erase the previous character and the receiver expects Delete to be used to erase the previous character, many receivers will echo the Backspace as “^H”, just as they would echo any other uninterpreted control character. (A similar mismatch in the other direction may yield Delete displayed as “^?”.)

ASCII printable characters

Code 32, the “space” character, denotes the space between words, as produced by the space-bar of a keyboard. The “space” character is considered an invisible graphic rather than a control character.[24] Codes 33 to 126, known as the printable characters, 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: in such a scenario a backspace can precede a grave accent (which the American and British standards, but only those standards, also call “opening single quotation mark”), a backtick, or a breath mark (inverted vel).

Binary

Oct

Dec

Hex

Glyph

010 0000

040

32

20

010 0001

041

33

21

!

010 0010

042

34

22

010 0011

043

35

23

#

010 0100

044

36

24

$

010 0101

045

37

25

%

010 0110

046

38

26

&

010 0111

047

39

27

010 1000

050

40

28

(

010 1001

051

41

29

)

010 1010

052

42

2A

*

010 1011

053

43

2B

+

010 1100

054

44

2C

,

010 1101

055

45

2D

-

010 1110

056

46

2E

.

010 1111

057

47

2F

/

011 0000

060

48

30

0

011 0001

061

49

31

1

011 0010

062

50

32

2

011 0011

063

51

33

3

011 0100

064

52

34

4

011 0101

065

53

35

5

011 0110

066

54

36

6

011 0111

067

55

37

7

011 1000

070

56

38

8

011 1001

071

57

39

9

011 1010

072

58

3A

:

011 1011

073

59

3B

;

011 1100

074

60

3C


011 1101

075

61

3D

=

011 1110

076

62

3E

>

011 1111

077

63

3F

?

Binary

Oct

Dec

Hex

Glyph

100 0000

100

64

40

@

100 0001

101

65

41

A

100 0010

102

66

42

B

100 0011

103

67

43

C

100 0100

104

68

44

D

100 0101

105

69

45

E

100 0110

106

70

46

F

100 0111

107

71

47

G

100 1000

110

72

48

H

100 1001

111

73

49

I

100 1010

112

74

4A

J

100 1011

113

75

4B

K

100 1100

114

76

4C

L

100 1101

115

77

4D

M

100 1110

116

78

4E

N

100 1111

117

79

4F

O

101 0000

120

80

50

P

101 0001

121

81

51

Q

101 0010

122

82

52

R

101 0011

123

83

53

S

101 0100

124

84

54

T

101 0101

125

85

55

U

101 0110

126

86

56

V

101 0111

127

87

57

W

101 1000

130

88

58

X

101 1001

131

89

59

Y

101 1010

132

90

5A

Z

101 1011

133

91

5B

[

101 1100

134

92

5C

\

101 1101

135

93

5D

]

101 1110

136

94

5E

^

101 1111

137

95

5F

_

Binary

Oct

Dec

Hex

Glyph

110 0000

140

96

60

`

110 0001

141

97

61

a

110 0010

142

98

62

b

110 0011

143

99

63

c

110 0100

144

100

64

d

110 0101

145

101

65

e

110 0110

146

102

66

f

110 0111

147

103

67

g

110 1000

150

104

68

h

110 1001

151

105

69

i

110 1010

152

106

6A

j

110 1011

153

107

6B

k

110 1100

154

108

6C

l

110 1101

155

109

6D

m

110 1110

156

110

6E

n

110 1111

157

111

6F

o

111 0000

160

112

70

p

111 0001

161

113

71

q

111 0010

162

114

72

r

111 0011

163

115

73

s

111 0100

164

116

74

t

111 0101

165

117

75

u

111 0110

166

118

76

v

111 0111

167

119

77

w

111 1000

170

120

78

x

111 1001

171

121

79

y

111 1010

172

122

7A

z

111 1011

173

123

7B

{

111 1100

174

124

7C

|

111 1101

175

125

7D

}

111 1110

176

126

7E

~

Aliases

A June 1992 RFC[25] and the IANA registry of character sets[26] recognize the following case-insensitive aliases for ASCII as suitable for use on the Internet:

  • ANSI_X3.4-1968 (canonical name)
  • iso-ir-6
  • ANSI_X3.4-1986
  • ISO_646.irv:1991
  • ASCII (with ASCII-7 and ASCII-8 variants)
  • ISO646-US
  • US-ASCII (preferred MIME name[26])
  • us
  • IBM367
  • cp367
  • csASCII

Of these, only the aliases “US-ASCII” and “ASCII” have achieved widespread use. One often finds them in the optional “charset” parameter in the Content-Type header of some MIME messages, in the equivalent “meta” element of some HTML documents, and in the encoding declaration part of the prologue of some XML documents.

Variants

As computer technology spread throughout the world, different standards bodies and corporations developed many variations of ASCII in order to facilitate the expression of non-English languages that used Roman-based alphabets. One could class some of these variations as “ASCII extensions“, although some misuse that term to cover all variants, including those that do not preserve ASCII’s character-map in the 7-bit range.

The PETSCII Code used by Commodore International for their 8-bit systems is probably unique among post-1970 codes in being based on ASCII-1963 instead of the far more common ASCII-1967, such as found on the ZX Spectrum computer. Atari and Galaksija computers also used ASCII variants.

Incompatibility vs interoperability

From early in its development,[27] ASCII was intended to be just one of several national variants of an international character code standard, ultimately published as ISO/IEC 646 (1972), which would share most characters in common but assign other locally-useful characters to several code points reserved for “national use.” However, the four years that elapsed between the publication of ASCII-1963 and ISO’s first acceptance of an international recommendation in 1967[28] caused ASCII’s choices for the national use characters to appear to be de facto standards for the world, leading to confusion and incompatibility once other countries did begin to make their own assignments to these code points.

ISO/IEC 646, like ASCII, was a 7-bit character set. It made no additional codes available, so the same code points encoded different characters in different countries. Escape codes were defined to indicate which national variant applied to a piece of text, but these were rarely used, so it was often impossible to know what variant to work with and therefore which character a code represented, and text-processing systems could generally cope with only one variant anyway.

Because the bracket and brace characters of ASCII were assigned to “national use” code points that were used for accented letters in other national variants of ISO/IEC 646, a German, French, or Swedish, etc., programmer had to get used to reading and writing
ä aÄiÜ=’Ön’; ü
or, using
trigraphs,
??
instead of
{ a[i]=’\n’; }

Eventually, as 8-, 16-, and 32-bit computers began to replace 18- and 36-bit computers as the norm, it became common to use an 8-bit byte to store each character in memory, providing an opportunity for extended, 8-bit, relatives of ASCII, with the 128 additional characters providing room to avoid most of the ambiguity that had been necessary in 7-bit codes.

For example, IBM developed 8-bit code pages, such as code page 437, which replaced the control-characters with graphic symbols such as smiley faces, and mapped additional graphic characters to the upper 128 positions. Operating systems such as DOS supported these code-pages, and manufacturers of IBM PCs supported them in hardware. Digital Equipment Corporation developed the Multinational Character Set (DEC-MCS) for use in the popular VT220terminal.

Eight-bit standards such as ISO/IEC 8859 (derived from the DEC-MCS) and Mac OS Roman developed as true extensions of ASCII, leaving the original character-mapping intact, but adding additional character definitions after the first 128 (i.e., 7-bit) characters. This enabled representation of characters used in a broader range of languages. Because there were several competing 8-bit code standards, they continued to suffer from incompatibilities and limitations. Still, ISO-8859-1 (Latin 1), its variant Windows-1252 (often mislabeled as ISO-8859-1), and the original 7-bit ASCII remain the most common character encodings in use today.

Unicode

Unicode and the ISO/IEC 10646 Universal Character Set (UCS) have a much wider array of characters, and their various encoding forms have begun to supplant ISO/IEC 8859 and ASCII rapidly in many environments. While ASCII is limited to 128 characters, Unicode and the UCS support more characters by separating the concepts of unique identification (using natural numbers called code points) and encoding (to 8-, 16- or 32-bit binary formats, called UTF-8, UTF-16 and UTF-32).

To permit backward compatibility, the 128 ASCII and 256 ISO-8859-1 (Latin 1) characters are assigned Unicode/UCS code points that are the same as their codes in the earlier standards. Therefore, ASCII can be considered a 7-bit encoding scheme for a very small subset of Unicode/UCS, and, conversely, the UTF-8 encoding forms are binary-compatible with ASCII for code points below 128, meaning every properly encoded ASCII file is also a valid UTF-8 file. The other encoding forms resemble ASCII in how they represent the first 128 characters of Unicode, but use 16 or 32 bits per character, so they require conversion for compatibility.

Order

Collation of data is sometimes done in ASCII-code order rather than “standard” alphabetical order. The main deviations are:

  • capitals come before lowercase letters, i.e. “Z” before “a”
  • characters in extended character sets such as “é” come after “z”

The slang expression ASCIIbetical is sometimes used for this order.[29] In programming, alphanumeric sorting means to sort by numeric value, without regard of any character set. An alphanumerically sorted array of bytes will appear ASCIIbetically when viewed in an ASCII-compatible character set.

A refined version of this order converts uppercase letters to lowercase before comparing ASCII values.



























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