Аски

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Сваки од ASCII знакова приказаних овде једноставним битмапираним фонтом имају своје кодне вредности, од 89 до 101. Између великих и малих слова налазе се шест знакова интерпункције.

ASCII (American Standard Code for Information Interchange, (енгл. Амерички Стандардни Код за Размену Информација), изговара се "аски", је сет карактера и кодна страница базирана на римском алфабету какав га користи енглески језик и још неки западноевропски језици. Најчешће се користи у рачунарству као и у опреми за комуникацију за представљање текста и за контролне уређаје који раде са текстом.

[уреди] Преглед

Слично другим кодовима за репрезентацију писаних знакова, ASCII специфира кореспонденцију између дигиталних битова и симбола/глифова писаног језика, и омогућује дигиталним уређајима да комуницирају међусобно и да обрађују, чувају и комуницирају информацијама које садрже знакове. ASCII знаковни код, или његова компатибилно проширење (види ниже), се користи готово код свих уобичајених рачунара, нарочито персоналним рачунарима и радним станицама. Препоручени MIME назив за ово кодирање је "US-ASCII".

ASCII је, стриктно прописано, седмобитни код, што значи да користи седам бинарних цифара (распон од 0 до 127 у декадном систему) за представљање знаковних информација. У време када је ASCII био уведен, многи рачунари су користили осмобитне групе (бајтове, или нешто прецизније, октете) као најмању јединицу за представљање информације; осми бит је био обично коришћен као бит парности за проверу грешака у преносу информација или друге функције специфичне за неки уређај. Машине које нису користиле парност обично су постављале осми бит на нулу, мада је било и неких система које су постављале осми бит на јединицу.

АSCII не прописује ни на који начин информације о концептуалној структури или изгледу парчета текста. То захтева друге стандарде као што су маркуп језици. Концептуална структура може се додати користећи XML а изглед коришћењем HTML-а, за релативно једноставне ствари, SGML-ом за нешто компликонваије или PostScript-ом, Display PostScript-ом, или TeX-ом за напредне контроле изгледа фонта и поравњања.

ASCII је први пут објавила као стандард 1963. године Америчка асоцијација за стандарде (енгл. American Standards Association, ASA), која се касније преименовала у ANSI. Постоје разне ASCII варијанте, али тренутно најраспрострањенија је ANSI X3.4-1986, такође стандардизована као ECMA-6, ISO/IEC 646:1991 International Reference Version, ITU-T Recommendation T.50 (09/92), и RFC 20. Уграђена је у њену највероватније најбољу замену, Unicode, као првих 128 знакова. Неки рачунарски стручњаци сматрају ASCII најуспешнијим софтверским стандардом икада пропагираном.

Историјски гледано, ASCII се развио из телеграфских кодова и његова прва комерцијална употреба је била као седмобитни телепринтерски код који је производио Bell data services. The Bell System had been planning to use a 6-bit code derived from Fieldata that added punctuation and lower-case letter to the earlier 5-bit Baudot teleprinter code but was persuaded to instead join the ASA subcommittee that was developing ASCII. Baudot helped in the automation of sending and receiving of telegraphic messages, and took many features from Morse code; it was however, a constant length code unlike Morse code. Compared to earlier telegraph codes, the proposed Bell code and ASCII were both reordered for more convenient sorting (ie, alphabetization) of lists, and added features for devices other than teleprinters. Some ASCII features, including the 'ESCape sequence', were due to Bob Bemer.

[уреди] ASCII контролни карактери

Прва тридесетдва кода у табели ASCII-кодова (од 0 до 31, децимално) су резервисана за такозване контролне карактере; ови кодови практично не садрже никакву информацију о карактеру, него управљају излазним уређајима (нпр. штампачима, скенерима). Тако на пример код који има вредност 10 представља команду "line feed", која налаже штампачу да помери папир "на горе" за један ред, или пак код са вредношћу 27 који симулира притисак на тастер "escape", лоциран у горњем левом углу тастатуре.

Код 127 (свих 7 битова укључено) је још један специјални карактер познат као "delete" или "rubout". Иако је његова функција is similar to that of other control characters, this pattern was used so that it could be used to "erase" a section of paper tape, a popular storage medium until the 80's, by punching all possible holes at a particular character position.

Many of the ASCII control codes are to mark data packets, or to control a data transmission protocol (i.e., ENQuiry (effectively, "any stations out there?"), ACKnowledge, Negative AcKnowledge, Start Of Header, Start Of Text, End Of Text, etc). ESCape and SUBstitute permit a communications protocol to, for instance, mark binary data so that if it contains codes with the same pattern as a protocol character, the code will be processed as data.

The separator characters (Record Separator, etc.) were intended for use with magnetic tape systems.

XON and XOFF are common interpretations of two of the Device Control characters and are generally used to throttle data flow to a slow device, such as a printer, from a fast device, such as a computer so data does not overrun and be lost.

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 is making the correct meta-information transformations. For example, the character(s) representing end-of-line ("new line") in text data files/streams vary from operating system to operating system. When moving files from one system to another, these characters must be recognized as end-of-line markers and converted appropriately.

In the table above, the fifth column contains glyphs reserved for representing control codes in a data stream, ie, when they must be printed or displayed rather than (or in addition to) causing action; your browser, (i.e., your HTML user agent) may require the installation of additional fonts in order to display them.

The sixth column shows the key combinations traditionally used to input control characters from a keyboard. In this column, a caret ("^") represents the "Control"/"Ctrl" key, which must be held down while pressing the next key, e.g. "^Z" means to hold down Ctrl while pressing the Z key. This notation is also sometimes used by software as a printable representation of control characters that could not be processed.

In some systems on the Internet, there is a history of the DEL control code being converted to BS in transit to a remote server. If the code was received in a text editor that did not know what do with it, the result was often "^H" appearing where the user intended to delete previous characters. "^H" persists in messages today as a deliberate humorous device, e.g. "there's a sucker^H^H^H^H^H^H^H potential customer born every minute".

[уреди] ASCII printable characters

Code 32 is the "space" character, denoting the space between words, which is produced by the large space bar of a keyboard. Codes 33 to 126 are called the printable characters, which represent letters, digits, punctuation marks, and a few miscellaneous symbols.

Seven bit ASCII provided seven "national" characters and, if the combined hardware and software permit, can use overstrikes to simulate some additional international characters: a BackSpace can be followed with the grave accent (which the American and British standards, but only the American and British standards, also call "opening single quotation mark"), a tilde, or a breath mark (inverted vel).

Note how uppercase characters can be converted to lowercase by adding 32 to their ASCII value; in binary, this can be accomplished simply by setting the sixth-least significant bit to 1.

[уреди] Aliases for ASCII

RFC 1345, published in June 1992, and the IANA registry of character sets, ongoing, recognize the following case-insensitive aliases for ASCII as being 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 are widely used. They are often found 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.

[уреди] Variants of ASCII

As computer technology spread throughout the world, many variations of ASCII were developed by corporations and standards bodies in order to facilitate the expression of non-English languages that still used Roman-based alphabets. Some of these variations can be considered to be ASCII extensions, although the term is sometimes misapplied to cover all variants, including those that don't preserve ASCII's character map in the 7-bit range.

ISO 646 (1972) was the first attempt to remedy the English bias, although it created compatibility problems, since it was still a seven-bit character set. No additional codes were available, so some were re-assigned in language-specific variants. It was thus impossible to know what character was represented by a code without knowing what variant was in use, and text processing systems were generally able to 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 eight-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. These code pages were supported in hardware by IBM PC manufacturers, as well as in operating systems such as MS-DOS.

Eight-bit standards such as ISO/IEC 8859 were true extensions of ASCII, leaving the original character mapping intact and just adding additional values above the 7-bit range. This enabled a broader range of languages to be represented, but these standards were still plagued with incompatibilities and limitations. Still, ISO/IEC 8859-1 and original 7-bit ASCII are the most common character encodings in use today.

Unicode and ISO/IEC 10646: the Universal Character Set, have a much wider array of characters, and their various encoding forms are rapidly supplanting ISO/IEC 8859 and ASCII in many environments. While ASCII is defined in terms of 7-bit codes, Unicode and the UCS are defined in terms of relatively abstract "code points": non-negative integer numbers that can be mapped, using different encoding forms and schemes, to sequences of 1 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. ASCII can therefore be thought of as being a 7-bit encoding scheme for a very small subset of Unicode and 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 happens to be identical 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 are not entirely compatible without conversions.

The portmanteau word ASCIIbetical has evolved to describe the collation of data in ASCII code order rather than "standard" alphabetical order (which requires some tricky computation, and varies with language).

ASCII contains many characters which were not commonly used, or at least spoken of, outside of the computing context; the "popularization" of these characters required that names be agreed upon for them. See the pronunciation guide in the external links, below.

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

[уреди] Види још

• ASCII art
• Binary and text files
• Extended ASCII
• ISO 8859
• Unicode
• UTF-8
• EBCDIC
• PETSCII
• ATASCII
• ISCII
• VISCII
• Asteroid 3568 ASCII, named in honour of the ASCII system

[уреди] Спољашње везе

• Standard ECMA-6: 7-bit Coded Character Set 6th edition (December 1991)
• IANA Registry: C0 Set of ISO 646
• IANA Registry: ISO 646, USA Version X3.4 - 1968
• ASCII: American Standard Code for Information Infiltration by Tom Jennings
• Jargon File: ASCIIbetical
• Bob Bemer's home page some historical notes about ASCII from one of its designers
• A pronunciation guide for ASCII characters (some are more whimsical than others; see especially the end of the list)

ASCII is also a name of one of the oldest and most prestigious computer magazines published in Japan. See ASCII (magazine)