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ASCII ( ), abbreviated from American Standard Code for Information Interchange, is a (the prefers the name US-ASCII). ASCII codes represent text in computers, , and other devices that use text. Most modern character-encoding schemes are based on ASCII, though they support many additional characters. ASCII was the most common character encoding on the World Wide Web until December 2007, when it was surpassed by , which includes ASCII as a subset.

ASCII developed from . Its first commercial use was as a seven- code promoted by Bell data services. Work on the ASCII standard began on October 6, 1960, with the first meeting of the 's (ASA) X3.2 subcommittee. The first edition of the standard was published during 1963, underwent a major revision during 1967, and experienced its most recent update during 1986. 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.

Originally based on the , ASCII encodes 128 specified into seven-bit binary integers as shown by the ASCII chart on the right. The characters encoded are numbers 0 to 9, lowercase letters a to z, uppercase letters A to Z, basic , that originated with , and a . For example, lowercase would become 1101010 and 106. ASCII includes definitions for 128 characters: 33 are non-printing (many now obsolete) ξ1 that affect how text and space are processedInternational Organization for Standardization (December 1, 1975). " The set of control characters for ISO 646". Internet Assigned Numbers Authority Registry. Alternate U.S. version: [2]. Accessed 2008-04-14. and 95 printable characters, including the (which is considered an invisible graphic "RFC 20: ASCII format for Network Interchange", ANSI X3.4-1968, October 16, 1969. ξ2 ).


History
The American Standard Code for Information Interchange (ASCII) was developed under the auspices of a committee of the American Standards Association (ASA), 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 and ultimately the .


Bit width
The X3.2 subcommittee designed ASCII based on the earlier encoding systems. Like other , ASCII specifies a correspondence between digital bit patterns and symbols (i.e. and ). This allows devices to communicate with each other and to process, store, and communicate character-oriented information such as written language. Before ASCII was developed, the encodings in use included 26 characters, 10 , and from 11 to 25 special graphic symbols. To include all these, and control characters compatible with the (CCITT) (ITA2) standard, , and early , more than 64 codes were required for ASCII.

The committee debated the possibility of a function (like in ), which would allow more than 64 codes to be represented by a . In a shifted code, some character codes determine choices between options for the following character codes. It allows compact encoding, but is less reliable for as 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.

The committee considered an eight-bit code, since eight bits () would allow two four-bit patterns to efficiently encode two digits with . However, it 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, it also allowed for a for if desired. machines (with octets as the native data type) that did not use parity checking typically set the eighth bit to 0. ξ3


Organization
The code itself was patterned so that most control codes were together, and all graphic codes were together, for ease of identification. The first two columns (32 positions) were reserved for control characters. The had to come before graphics to make easier, so it became position 20; for the same reason, many special signs commonly used as separators were placed before digits. The committee decided it was important to support , and chose to pattern ASCII so it could be reduced easily to a usable 64-character set of graphic codes, as was done in the code. letters were therefore not interleaved with uppercase. To keep options available for lowercase letters and other graphics, the special and numeric codes were arranged before the letters, and the letter A was placed in position 41 to match the draft of the corresponding British standard. The digits 0–9 were arranged so they correspond to values in binary prefixed with 011, making conversion with straightforward.

Many of the non-alphanumeric characters were positioned to correspond to their shifted position on typewriters; an important subtlety is that these were based on mechanical typewriters, not electric typewriters. Mechanical typewriters followed the standard set by the Remington No. 2 (1878), the first typewriter with a shift key, and the shifted values of 23456789- were "#$%_&'() early typewriters omitted 0 and 1, using O (capital letter o) and l (lowercase letter L) instead, but 1! and 0) pairs became standard once 0 and 1 became common. Thus, in ASCII !"#$% were placed in second column, rows 1–5, corresponding to the digits 1–5 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. This was accommodated by removing _ (underscore) from 6 and shifting the remaining characters left, which corresponded to many European typewriters that placed the parentheses with 8 and 9. This discrepancy from typewriters led to , notably the , which used the left-shifted layout corresponding to ASCII, not to traditional mechanical typewriters. Electric typewriters, notably the more recently introduced (1961), used a somewhat different layout that has become standard on computersfollowing the (1981), especially (1984)and thus shift values for symbols on modern keyboards do not correspond as closely to the ASCII table as earlier keyboards did. The /? pair also dates to the No. 2, and the ,< .> pairs were used on some keyboards (others, including the No. 2, did not shift , (comma) or . (full stop) so they could be used in uppercase without unshifting). However, ASCII split the ;: pair (dating to No. 2), and rearranged mathematical symbols (varied conventions, commonly -* = ) to :* ; -=.

Some common characters were not included, notably ½¼¢, while ^`~ were included as diacritics for international use, and <> for mathematical use, together with the simple line characters \| (in addition to common /). The @ symbol was not used in continental Europe and the committee expected it would be replaced by an accented À in the French variation, so the @ was placed in position 40, right before the letter A.

The control codes felt essential for data transmission were the start of message (SOM), end of address (EOA), (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 between their bit patterns.


Publication
With the other special characters and control codes filled in, ASCII was published as ASA X3.4-1963, leaving 28 code positions without any assigned meaning, reserved for future standardization, and one unassigned control code. There was some debate at the time whether there should be more control characters rather than the lowercase alphabet. The indecision did not last long: during May 1963 the CCITT Working Party on the New Telegraph Alphabet proposed to assign lowercase characters to columns 6 and 7,Brief Report: Meeting of CCITT Working Party on the New Telegraph Alphabet, May 13–15, 1963. and TC 97 SC 2 voted during October to incorporate the change into its draft standard.Report of ISO/TC/97/SC 2 – Meeting of October 29–31, 1963. The X3.2.4 task group voted its approval for the change to ASCII at its May 1963 meeting.Report on Task Group X3.2.4, June 11, 1963, Pentagon Building, Washington, DC. Locating the lowercase letters in columns 6 and 7 caused the characters to differ in bit pattern from the upper case by a single bit, which simplified character matching and the construction of keyboards and printers.

The X3 committee made other changes, including other new characters (the and characters),Report of Meeting No. 8, Task Group X3.2.4, December 17 and 18, 1963 renaming some control characters (SOM became start of header (SOH)) and moving or removing others (RU was removed). 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 first two are occasionally retronamed ANSI X3.4-1967, and ANSI X3.4-1968).

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


Use
ASCII itself was first used commercially during 1963 as a seven-bit teleprinter code for 's TWX (TeletypeWriter eXchange) network. TWX originally used the earlier five-bit , which was also used by the competing teleprinter system. introduced features such as the . His British colleague 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".Bob Bemer (n.d.). Bemer meets Europe. Trailing-edge.com. Accessed 2008-04-14. Employed at at that time Because of his extensive work on ASCII, Bemer has been called "the father of ASCII."

On March 11, 1968, U.S. President mandated that all computers purchased by the United States federal government support ASCII, stating:Lyndon B. Johnson (March 11, 1968). Memorandum Approving the Adoption by the Federal Government of a Standard Code for Information Interchange. The American Presidency Project. Accessed 2008-04-14.

I have also approved recommendations of the 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 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.


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

ASCII was incorporated into the character set as the first 128 symbols, so the 7-bit ASCII characters have the same numeric codes in both sets. This allows to be with 7-bit ASCII, as a UTF-8 file containing only ASCII characters is identical to an ASCII file containing the same sequence of characters. Even more importantly, is ensured as software that recognizes only 7-bit ASCII characters as special and does not alter bytes with the highest bit set (as is often done to support 8-bit ASCII extensions such as ISO-8859-1) will preserve UTF-8 data unchanged.


ASCII control characters
ASCII reserves the first 32 codes (numbers 0–31 decimal) for : codes originally intended not to represent printable information, but rather to control devices (such as ) that make use of ASCII, or to provide 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". RFC 2822 refers to control characters that do not include carriage return, line feed or as non-whitespace control characters.RFC 2822 (April 2001). "NO-WS-CTL". 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 , address page and document layout and formatting.

The original ASCII standard used only short descriptive phrases for each control character. The ambiguity this caused was sometimes intentional, for example where a character would be used slightly differently on a terminal link than on a , and sometimes accidental, for example with the meaning of "delete".

Probably the most influential single device on the interpretation of these characters was the ASR, which was a printing terminal with an available reader/punch option. Paper tape was a very popular medium for long-term program storage until the 1980s, less costly and in some ways less fragile than magnetic tape. In particular, the Teletype Model 33 machine assignments for codes 17 (Control-Q, DC1, also known as XON), 19 (Control-S, DC3, also known as XOFF), and 127 () became de facto standards. The Model 33 was also notable for taking the description of Control-G (BEL, meaning audibly alert the operator) literally as the unit contained an actual bell which it rang when it received a BEL character. Because the keytop for the O key also showed a left-arrow symbol (from ASCII-1963, which had this character instead of ), 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 became neglected.

When a Teletype 33 ASR equipped with the automatic paper tape reader received a Control-S (XOFF, an abbreviation for transmit off), it caused the tape reader to stop; receiving Control-Q (XON, "transmit on") caused the tape reader to resume. This technique became adopted by several early computer operating systems as a "handshaking" signal warning a sender to stop transmission because of impending overflow; it 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. The 33 ASR also could be configured to employ Control-R (DC2) and Control-T (DC4) to start and stop the tape punch; on some units equipped with this function, the corresponding control character lettering on the keycap above the letter was TAPE and TAPE respectively.

Code 127 is officially named "delete" but the Teletype label was "rubout". Since the original standard did not give 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 useful specifically 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.

Some software assigned special meanings to ASCII characters sent to the software from the terminal. Operating systems from , for example, interpreted DEL as an input character as meaning "remove previously-typed input character", and this interpretation also became common in Unix systems. Most other systems used BS for that meaning and used DEL to mean "remove the character at the cursor". That latter interpretation is the most common now.

Many more of the control codes have been given meanings quite different from their original ones. The "escape" character (ESC, code 27), for example, was intended originally to allow sending other control characters as literals instead of invoking their meaning. This is the same meaning of "escape" encountered in URL encodings, strings, and other systems where certain characters have a reserved meaning. Over time this meaning has been co-opted and has eventually been changed. In modern use, an ESC sent to the terminal usually indicates the start of a command sequence, usually in the form of a so-called "" (or, more properly, a "") beginning with ESC followed by a "[" (left-bracket) character. An ESC sent from the terminal is most often used as an character used to terminate an operation, as in the and . In (GUI) and systems, ESC generally causes an application to abort its current operation or to (terminate) altogether.

The inherent ambiguity of many control characters, combined with their historical usage, created problems when transferring "plain text" files between systems. The best example of this is the problem on various . Teletype machines required that a line of text be terminated with both "Carriage Return" (which moves the printhead to the beginning of the line) and "Line Feed" (which advances the paper one line without moving the printhead). The name "Carriage Return" comes from the fact that on a manual the carriage holding the paper moved while the position where the typebars struck the ribbon remained stationary. The entire carriage had to be pushed (returned) to the right in order to position the left margin of the paper for the next line.

operating systems (, , , , , etc.) used both characters to mark the end of a line so that the console device (originally ) would work. By the time so-called "glass TTYs" (later called CRTs or terminals) came along, the convention was so well established that backward compatibility necessitated continuing the convention. When cloned to create he followed established convention. Until the introduction of in 1981, had no hand in this because their 1970s operating systems used EBCDIC instead of ASCII and they were oriented toward punch-card input and line printer output on which the concept of carriage return was meaningless. IBM's (also marketed as by Microsoft) inherited the convention by virtue of being a clone of , and inherited it from MS-DOS.

Unfortunately, requiring two characters to mark the end of a line introduces unnecessary complexity and questions as to how to interpret each character when encountered alone. To simplify matters data streams, including files, on used line feed (LF) alone as a line terminator. and systems, and systems, adopted this convention from Multics. The original , , and , on the other hand, used carriage return (CR) alone as a line terminator; however, since Apple replaced these operating systems with the Unix-based operating system, they now use line feed (LF) as well.

Computers attached to the included machines running operating systems such as TOPS-10 and using CR-LF line endings, machines running operating systems such as Multics using LF line endings, and machines running operating systems such as that represented lines as a character count followed by the characters of the line and that used rather than ASCII. The protocol defined an ASCII "Network Virtual Terminal" (NVT), so that connections between hosts with different line-ending conventions and character sets could be supported by transmitting a standard text format over the network. Telnet used ASCII along with CR-LF line endings, and software using other conventions would translate between the local conventions and the NVT. The adopted the Telnet protocol, including use of the Network Virtual Terminal, for use when transmitting commands and transferring data in the default ASCII mode. This adds complexity to implementations of those protocols, and to other network protocols, such as those used for E-mail and the World Wide Web, on systems not using the NVT's CR-LF line-ending convention.

Older operating systems such as TOPS-10, 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. For this reason, EOF, or , was used colloquially and conventionally as a for Control-Z instead of SUBstitute. The end-of-text code (), also known as , was inappropriate for a variety of reasons, while using Z as the control code to end a file is analogous to it ending the alphabet and serves as a very convenient . A historically common and still prevalent convention uses the ETX code convention to interrupt and halt a program via an input data stream, usually from a keyboard.

In C library and conventions, the is used to terminate text ; such can be known in abbreviation as ASCIZ or ASCIIZ, where here Z stands for "zero".


ASCII control code chart
00
01
02
03
04

05

06
07
08
09
0A
0B
0C
0D
0E
0F
10
11 (oft. )
12Device Control 2
13Device Control 3 (oft. )
14Device Control 4
15
16
17
18
19
1A
1B
1C
1D
1E
1F
7F

Other representations might be used by specialist equipment, for example graphics or numbers.


ASCII printable characters
Codes 20 to 7E, known as the printable characters, represent letters, digits, , and a few miscellaneous symbols. There are 95 printable characters in total.

Code 20, the , denotes the space between words, as produced by the space bar of a keyboard. Since the space character is considered an invisible graphic (rather than a control character) it is listed in the table below instead of in the previous section.

Code 7F corresponds to the non-printable "delete" (DEL) control character and is therefore omitted from this chart; it is covered in the previous section's chart. Earlier versions of ASCII used the up arrow instead of the (5E) and the left arrow instead of the (5F). ASA X3.4-1963.


ASCII printable code chart
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Aliases
A June 1992 RFCRFC 1345 (June 1992). and the registry of character sets 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, ISO646-US, US-ASCII (preferred name),Internet Assigned Numbers Authority (May 14, 2007). " Character Sets". Accessed 2008-04-14. us, IBM367, cp367, and csASCII.

Of these, the IANA encourages use of the name "US-ASCII" for Internet uses of ASCII (even if it is a , but the US is needed because of abuse of the ASCII term). One often finds this in the optional "charset" parameter in the Content-Type header of some MIME messages, in the equivalent "meta" element of some documents, and in the encoding declaration part of the prologue of some documents.


Variants
As computer technology spread throughout the world, different and corporations developed many variations of ASCII to facilitate the expression of non-English languages that used Roman-based alphabets. One could class some of these variations as "", although some misuse that term to represent all variants, including those that do not preserve ASCII's character-map in the 7-bit range. Furthermore, the ASCII extensions have also been mislabelled as ASCII.

Many other countries developed variants of ASCII to include non-English letters (e.g. , , , ), currency symbols (e.g. , ), etc.

The code used for their systems is probably unique among post-1970 codes in being based on ASCII-1963, instead of the more common ASCII-1967, such as found on the computer. 8-bit computers and computers also used ASCII variants.


7-bit
From early in its development,"Specific Criteria," attachment to memo from R. W. Reach, "X3-2 Meeting – September 14 and 15," September 18, 1961 ASCII was intended to be just one of several national variants of an international character code standard, ultimately published as (1972), which would share most characters in common but assign other locally useful characters to several 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 during 1967R. Maréchal, ISO/TC 97 – Computers and Information Processing: Acceptance of Draft ISO Recommendation No. 1052, December 22, 1967 caused ASCII's choices for the national use characters to seem to be de facto standards for the world, causing 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 did not make any 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 they were rarely used, so it was often impossible to know what variant to work with and therefore which character a code represented, and in general, text-processing systems could 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 using their national variant of ISO/IEC 646, rather than ASCII, had to write, and thus read, something such as

^@

instead of

[[^@]]

were created to solve this problem for , although their late introduction and inconsistent implementation in compilers limited their use. Many programmers kept their computers on US-ASCII, so plain-text in Swedish, German etc. (for example, in e-mail or ) contained "{, }" and similar variants in the middle of words, something those programmers got used to. For example, a Swedish programmer mailing another programmer asking if they should go for lunch, could get "N{ jag har sm|rg}sar." as the answer, which should be "Nä jag har smörgåsar." meaning "No I've got sandwiches."


8-bit
Eventually, as 8-, and (and later ) computers began to replace and 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. In most cases these 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.

Most early home computer systems developed their own 8-bit character sets containing line-drawing and game glyphs, and often filled in some or all of the control characters from 0–31 with more graphics. computers used the "upper" 128 characters for the Greek alphabet. The IBM PC defined , which replaced the control-characters with graphic symbols such as , and mapped additional graphic characters to the upper 128 positions. Operating systems such as supported these code pages, and manufacturers of supported them in hardware. developed the (DEC-MCS) for use in the popular as one of the first extensions designed more for international languages than for block graphics. The Macintosh defined and Postscript also defined a set, both of these contained both international letters and typographic punctuation marks instead of graphics, more like modern character sets.

The standard (derived from the DEC-MCS) finally provided a standard that most systems copied (at least as accurately as they copied ASCII, but with many substitutions). A popular further extension designed by Microsoft, (often mislabeled as ISO-8859-1), added the typographic punctuation marks needed for traditional text printing. ISO-8859-1, Windows-1252, and the original 7-bit ASCII were the most common character encodings until 2008 when became more common.

ISO/IEC 4873 introduced 32 additional control codes defined in the 80–9F range, as part of extending the 7-bit ASCII encoding to become an 8-bit system. ξ4


Unicode
and the ISO/IEC 10646 (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 called code points) and encoding (to 8-, 16- or 32-bit binary formats, called , and ).

To allow 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 ASCII (when prefixed with 0 as the eighth bit) is valid UTF-8.


Order
ASCII-code order is also called ASCIIbetical order. of data is sometimes done in this order rather than "standard" alphabetical order (). The main deviations in ASCII order are:
  • All uppercase come before lowercase letters; for example, "Z" comes before "a"
  • Digits and many punctuation marks come before letters; for example, "4" precedes "one"
  • Numbers are sorted naïvely as strings; for example, "10" precedes "2"

An intermediate orderreadily implementedconverts uppercase letters to lowercase before comparing ASCII values. Naïve number sorting can be averted by all numbers (e.g. "02" will sort before "10" as expected), although this is an external fix and has nothing to do with the ordering itself.


See also


Notes

Further reading
  • from:


External links


References
    ^ (2017). 9780470032145, John Wiley and Sons. .
    ^ (1980). 9780201144604, Addison-Wesley.
    ^ (1995). 9780849371592, . .
    ^ (2006). 9780321480910, Addison-Wesley.

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