ASCII

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ASCII (American Standard Code for Information Interchange), generally pronounced [ˈæski], 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 — have a historical basis in ASCII.

ASCII was created in 1963,^[1] was first published as a standard in 1967 and was last updated in 1986. It currently defines codes for 128 characters: 33 are non-printing, mostly obsolete control characters that affect how text is processed, and 95 are printable characters.

Contents 1 Overview 2 ASCII control characters 2.1 Non-whitespace control characters 3 ASCII printable characters 4 Structural features 5 Aliases 6 Variants 6.1 Incompatibility vs interoperability 6.2 Unicode fix 7 Trivia 8 See also 8.1 ASCII extensions 8.2 ASCII variants 9 Further reading 10 References 11 External links

[edit] 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^[2] — 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".^[3]

ASCII is, strictly, a seven-bit code, meaning 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,^[4] though some systems such as Prime machines running PRIMOS set the eighth bit of ASCII characters to one.

ASCII only defines a relationship between specific characters and bit sequences; aside from reserving a few control codes for line-oriented formatting, it does not define any mechanism for describing the structure or appearance of text within a document. Such concepts are within the realm of other systems such as the markup languages.

ASCII developed from telegraphic codes and first entered commercial use as a seven-bit teleprinter code promoted by data services in 1963. The Bell System had previously planned to use a six-bit code, derived from fieldata, that added punctuation and lower-case letters to the earlier five-bit Baudot teleprinter code, but was persuaded instead to join the American National Standards Institute subcommittee that had started to develop ASCII. Baudot helped in the automation of sending and receiving 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. His British colleague Hugh McGregor Ross helped to popularize this work, as Bemer said, "so much so that the code that was to become ASCII was first called the Bemer-Ross Code in Europe".

ASCII was subsequently updated and published as ANSI X3.4-1968, ANSI X3.4-1977, and finally, ANSI X3.4-1986.

Other international standards bodies have ratified character encodings that are identical or nearly identical to ASCII. These encodings are sometimes referred to as ASCII, even though ASCII is strictly defined only by the ASA/ANSI standards:

ASCII has also become embedded in its probable replacement, Unicode, as the lowest 128 characters. In terms of mere adoption, ASCII is one of the most successful software standards ever.^{[citation needed]}

[edit] ASCII control characters

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".

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]

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 Teletype corporation model 33 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. Its noncompliant use of code 15 (Control-O, Shift In) as "left arrow", usually 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 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. Unix systems, for example, interpreted "Delete" to mean "remove the character before the cursor". Most other systems used "Backspace" for that meaning and used "Delete" 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, has also 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 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. 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). Control-C (ETX, End of TeXt) might have made more sense, but was already in wide use as a program abort signal. UNIX's use of Control-D (EOT, End of Transmission) appears on its face similar, but is used only from the terminal and never stored in a file.

While the codes mentioned above have retained some semblance of their original meanings, many of the codes originally intended for stream delimiters or for link control on a terminal have lost all meaning except their relation to a letter. Control-A is almost never used to mean "start of header" except on an ANSI magnetic tape. When connecting a terminal to a system, or asking the system to recognize that a logged-out terminal wants to log in, modern systems are much more likely to want a carriage return or an ESCape than Control-E (ENQuire, meaning "is there anybody out there?").

[edit] Non-whitespace control characters

RFC 2822 refers to NO-WS-CTL, non-whitespace control characters. These are control characters that do not include carriage return, line feed, and white space characters (see here), i.e.: decimal 1-8, 11-12, 14-31, and 127.

[edit] 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 backtick, or a breath mark (inverted vel).

Binary Oct Dec Hex Glyph 010 0000 040 32 20 SP 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 ~

[edit] Structural features

• The digits 0-9 are represented with their values in binary prefixed with 0011 (this means that converting BCD to ASCII is simply a matter of taking each BCD nibble separately and prefixing 0011 to it).
• Lowercase and uppercase letters only differ in bit pattern by a single bit simplifying case conversion to a range test (to avoid converting characters that are not letters) and a single bitwise operation. Fast case conversion is important because it is often used in case-ignoring search algorithms.
• In contrast with EBCDIC, the lowercase and uppercase letters each occupy 26 consecutive positions.

[edit] Aliases

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 (with ASCII-7 and ASCII-8 variants)
• 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.

[edit] 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 mis-apply 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.

[edit] Incompatibility vs interoperability

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 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 VT220 terminal.

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 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-8859-1, its variant Windows-1252 (often mislabeled as ISO-8859-1 even by Microsoft software) and original 7-bit ASCII remain the most common character encodings in use today.

[edit] Unicode fix

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 (and UTF-7) 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.

[edit] Trivia

• The blend word ASCIIbetical has evolved to describe the collation of data in ASCII-code order rather than "standard" alphabetical order.^[5]
• The abbreviation ASCIIZ or ASCIZ refers to a null-terminated ASCII string (also known as a C string).
• Asteroid 3568 ASCII is named after the character encoding.

[edit] See also

• ANSI
• ASCII art
• ASCII games
• Text file
• Bob Bemer
• EBCDIC
• Unicode
• Latin Unicode
• ASCII ribbon
• Binary

[edit] ASCII extensions

(where all ASCII printable characters are identical to ASCII)

• Extended ASCII
• UTF-8
• ISO 8859
• ISCII
• VISCII
• Windows code pages
• Mac Roman

[edit] ASCII variants

(where some ASCII printable characters have been replaced)

• ISO 646
• ATASCII - Atari Standard Code for Information Interchange
• PETSCII - PET Standard Code of Information Interchange, also known as CBM ASCII
• ZX Spectrum character set
• YUSCII - Yugoslav Standard Code for Information Interchange
• Galaksija - character set of Galaksija kit computer

[edit] Further reading

• Tom Jennings. World Power Systems:Texts:Annotated history of character codes. Retrieved on November 6, 2006.

[edit] References

[edit] External links

• The ASCII subset of Unicode
• A history of ASCII, its roots and predecessors by Tom Jennings, October 29, 2004 (accessed December 17, 2005)
• A pronunciation guide for ASCII characters
• ASCII Chart, how to send documents "in ASCII", etc
• Online Encoder/Decoder for ASCII, HEX, Binary, Base64, etc with MD2, MD4, MD5, SHA1+2, CRC, and other hashing algorithms
• Interactive AJAX-style ASCII (and Unicode) Decoder Table
• Printable ASCII Table
• Another Printable ASCII Table
• ASCII to 8-bit binary converter and source code
• ASCII to ASCII CODE converter and source code
• Convert PDF, PostScript, WMF and EMF to ASCII
• ANSI INCITS 4-1986 (R2002) Information Systems - Coded Character Sets - 7-Bit American National Standard Code for Information Interchange (7-Bit ASCII)
• ASCII-Table and ASCII-Charconverter