ASCII

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File:Ascii full.png
There are 95 printable ASCII characters, numbered 32 to 126.

ASCII (American Standard Code for Information Interchange), generally pronounced [ˈæski], (ASK-ee) is a character set and a character encoding based on the Roman alphabet as used in modern English (see English alphabet). ASCII codes represent text in computers, in other communications equipment, and in control devices that work with text. Most recent character encoding has an ASCII-like base.

ASCII defines the following printable characters, presented here in numerical order of their ASCII value:

(spacebar)
 !"#$%&'()*+,-./
0123456789:;<=>?
@ABCDEFGHIJKLMNO
PQRSTUVWXYZ[\]^_
`abcdefghijklmno
pqrstuvwxyz{|}~(del)

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[1] — 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".[2]

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 or, more specifically, octets) as the smallest unit of information; the eighth bit was commonly used as a parity bit for error checking on communication lines or other device-specific functions. Machines which did not use parity typically set the eighth bit to zero, though some systems such as Prime machines running PRIMOS set the eighth bit of ASCII characters to one.

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, TeX, or XSL-FO for advanced layout and font control.

History

Historically, ASCII developed from telegraphic codes and first entered commercial use as a 7-bit teleprinter code promoted by Bell data services. The Bell System had previously planned to use a 6-bit code (derived from Fieldata) that added punctuation and lower-case letters to the earlier 5-bit Baudot teleprinter-code, but became persuaded instead to join the ASA subcommittee that had started to develop ASCII. Baudot helped in the automation of sending and receiving of telegraphic messages, and took many features from Morse code; however, unlike Morse code, Baudot used constant-length codes. Compared to earlier telegraph codes, the proposed Bell code and ASCII both underwent re-ordering for more convenient sorting (especially alphabetization) of lists, and added features for devices other than teleprinters. Bob Bemer introduced features such as the 'escape sequence'.

The American Standards Association (ASA, later to become ANSI) first published ASCII as a standard in 1963. ASCII-1963 lacked the lowercase letters, and had an up-arrow (↑) instead of the caret (^) and a left-arrow (←) instead of the underscore (_). The 1967 version added the lowercase letters, changed the names of a few control characters and moved the two controls ACK and ESC from the lowercase letters area into the control codes area.

Many variations of ASCII exist, but its present, most widely-used form uses the ANSI X3.4-1986 definition, also standardized as ECMA-6, ISO/IEC 646:1991 International Reference Version, ITU-T Recommendation T.50 (09/92), and Request for Comments RFC 20. ASCII has become embedded in its probable replacement, Unicode, as the 'lowest' 128 characters. Some observers consider ASCII the most "successful" software standard ever promulgated.

ASCII control characters

ASCII reserves the first 32 codes (numbers 0–31 decimal) for control characters: codes originally intended not to carry character 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 often found in the top left corner of common keyboards.

Code 127 (all seven bits on), another special character, equates to "delete" or "rubout". Though its function resembles that of other control characters, the designers of ASCII used this pattern so that it could "erase" a section of paper tape (a popular storage medium until the 1980s) by punching all possible holes at a particular character position, thus effectively replacing any previous information.

Many of the ASCII control codes serve (or served) to mark data packets, or to control a data transmission protocol (e.g. 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 recipient will process the code as data.

The designers of ASCII intended the separator characters ("Record Separator", etc.) for use with magnetic tape systems.

Two of the device control characters, commonly interpreted as XON and XOFF, generally function as flow control characters 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 get lost.

Early users of ASCII adopted some of the control codes to represent "meta information" such as end-of-line, start/end of a data element, and so on. These assignments often conflict, so part of the effort in converting data from one format to another involves making the correct meta information transformations. For example, the character(s) representing end-of-line ("newline") in text data files/streams vary from operating system to operating system. When moving files from one system to another, the conversion process must recognize these characters as end-of-line markers and handle them appropriately.

Today, ASCII users use the control characters less and less—with the exception of "carriage return" and/or "line feed". Modern markup languages, modern communication protocols, the move from text-based to graphical devices, and the demise of teleprinters, punch-cards, and paper tapes have rendered most of the control characters obsolete.

Binary Oct Dec Hex Abbr PRTemplate:Ref 1 CSTemplate:Ref 2 Description
0000 0000 000 0 00 NUL ^@ Null character
0000 0001 001 1 01 SOH ^A Start of Header
0000 0010 002 2 02 STX ^B Start of Text
0000 0011 003 3 03 ETX ^C End of Text
0000 0100 004 4 04 EOT ^D End of Transmission
0000 0101 005 5 05 ENQ ^E Enquiry
0000 0110 006 6 06 ACK ^F Acknowledgement
0000 0111 007 7 07 BEL ^G Bell
0000 1000 010 8 08 BS ^H BackspaceTemplate:Ref 3Template:Ref 7
0000 1001 011 9 09 HT ^I Horizontal Tab
0000 1010 012 10 0A LF ^J Line feed
0000 1011 013 11 0B VT ^K Vertical Tab
0000 1100 014 12 0C FF ^L Form feed
0000 1101 015 13 0D CR ^M Carriage returnTemplate:Ref 6
0000 1110 016 14 0E SO ^N Shift Out
0000 1111 017 15 0F SI ^O Shift In
0001 0000 020 16 10 DLE ^P Data Link Escape
0001 0001 021 17 11 DC1 ^Q Device Control 1 (oft. XON)
0001 0010 022 18 12 DC2 ^R Device Control 2
0001 0011 023 19 13 DC3 ^S Device Control 3 (oft. XOFF)
0001 0100 024 20 14 DC4 ^T Device Control 4
0001 0101 025 21 15 NAK ^U Negative Acknowledgement
0001 0110 026 22 16 SYN ^V Synchronous Idle
0001 0111 027 23 17 ETB ^W End of Trans. Block
0001 1000 030 24 18 CAN ^X Cancel
0001 1001 031 25 19 EM ^Y End of Medium
0001 1010 032 26 1A SUB ^Z Substitute
0001 1011 033 27 1B ESC ^[ EscapeTemplate:Ref 5
0001 1100 034 28 1C FS ^\ File Separator
0001 1101 035 29 1D GS ^] Group Separator
0001 1110 036 30 1E RS ^^ Record Separator
0001 1111 037 31 1F US ^_ Unit Separator
0111 1111 177 127 7F DEL ^? DeleteTemplate:Ref 4Template:Ref 7
  1. Printable Representation, the Unicode glyphs reserved for representing control characters when it is necessary to print or display them rather than have them perform their intended function.
  2. Control key Sequence, 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.
  3. The Backspace character can also be entered by pressing the "Backspace", "Bksp", or ← key on some systems.
  4. 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.
  5. The Escape character can also be entered by pressing the "Escape" or "Esc" key on some systems.
  6. The Carriage Return character can also be entered by pressing the "Return", "Ret", "Enter", or ↵ key on most systems.
  7. The ambiguity surrounding the Backspace key comes from systems that translated the DEL control character into a BS (backspace) before transmitting it. Some software was unable to process the character and would display "^H" instead. "^H" persists in messages today as a deliberate humorous device, e.g. "there's a sucker^H^H^H^H^H^H potential customer born every minute". A less common variant of this involves the use of "^W", which in some text editors means "delete previous word". The example sentence would therefore also work as "there's a sucker^W potential customer born every minute".

ASCII printable characters

Code 32, the "space" character, denotes the space between words, as produced by the large space-bar of a keyboard. 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 tilde, or a breath mark (inverted vel).

Binary Dec Hex Glyph
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 ?
 
Bin Dec Hex Glyph
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 _
 
Bin Dec Hex Glyph
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 ~

Processors can convert uppercase characters to lowercase by adding 32 to their ASCII value; in binary, devices can accomplish this simply by setting the sixth-least significant bit to 1.

Aliases for ASCII

RFC 1345 (published in June 1992) and the IANA registry of character sets (ongoing), both recognize the following case-insensitive aliases for ASCII as suitable for use on the Internet:

  • ANSI_X3.4-1968 (canonical name)
  • ANSI_X3.4-1986
  • ASCII
  • US-ASCII (preferred MIME name)
  • us
  • ISO646-US
  • ISO_646.irv:1991
  • iso-ir-6
  • 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 prolog of some XML documents.

Variants of ASCII

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 mis-apply that term to cover all variants, including those that do not preserve ASCII's character-map in the 7-bit range.

ISO 646 (1972), the first attempt to remedy the pro-English-language bias, created compatibility problems, since it remained a 7-bit character-set. It made no additional codes available, so it reassigned some in language-specific variants. It thus became impossible to know what character a code represented without knowing which variant to work with, and text-processing systems could generally cope with only one variant anyway.

Eventually, 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. 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 bytes. Operating systems such as DOS supported these code-pages, and manufacturers of IBM PCs supported them in hardware.

Eight-bit standards such as ISO/IEC 8859 and MacRoman developed as true extensions of ASCII, leaving the original character-mapping intact and just adding additional values above the 7-bit range. This enabled the representation of a broader range of languages, but these standards continued to suffer from incompatibilities and limitations. Still, ISO/IEC 8859-1 and original 7-bit ASCII remain the most common character encodings in use today.

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 basically uses 7-bit codes, Unicode and the UCS use relatively abstract "code points": non-negative integer numbers that map, using different encoding forms and schemes, to sequences of one or more 8-bit bytes. To permit backward compatibility, Unicode and the UCS assign the first 128 code points to the same characters as ASCII. One can therefore think of ASCII as a 7-bit encoding scheme for a very small subset of Unicode and of the UCS. The popular UTF-8 encoding-form prescribes the use of one to four 8-bit code values for each code point character, and equates exactly to ASCII for the code values below 128. Other encoding forms such as UTF-16 resemble ASCII in how they represent the first 128 characters of Unicode, but tend to use 16 or 32 bits per character, so they require conversion for compatibility.

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 not in common use (or at least not commonly spoken of) outside of the computing context; the "popularization" of these characters encouraged users to agree on standard names for them. See the pronunciation guide in the external links, below.

The abbreviation ASCIIZ or ASCIZ refers to a null-terminated ASCII string.

See also

Related topics

Computer (family)-specific ASCII variants

ASCII in space

External links

Notes and references

  1. ^  International Organization for Standardization (December 1, 1975). "The set of control characters for ISO 646". Internet Assigned Numbers Authority Registry. Alternate USA version: [3]. Accessed August 7, 2005.
  2. ^  Internet Assigned Numbers Authority (January 28, 2005). "Character Sets". Accessed August 7, 2005.

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