ASCII

ASCII (American Standard Code for Information Interchange), që zakonisht lexohet si 'aski', është bashkësi e karakterëve të bazuar në Alfabetin Romak apo alfabetin e gjuhës së sotme angleze apo atyre të Europës perendimore. Më së shumti përdoret për kompjuterë dhe për pajisje të tjera që e përdorin atë për të reprezentuar tekstin apo për ato pajisje për konrollë që përdorin tekstin.

 Hyrje 
Si çdo kodim tjetër kompjuterik që përdoret për paraqitjen e karakterëve, ASCII krijon një lidhshmëri në mes të mostrave të bit-ave dhe simboleve të gjuhës së shkruar. Në këtë mënyrë i lejojnë pajisjeve digjitale të komunikojnë njëra me tjetrën dhe të procesojnë, ruajnë dhe të bartin informata që kanë të bëjnë me karaktera. Enkodimi ASCII apo edhe ASCII e zgjeruar, përdoret pothuajse nga të gjithë kompjuterët, e posaçërisht nga kompjuterat personal. Emri MIME i preferuar për këtë enkodim është "US-ASCII".

ASCII është kod 7-bitësh, që do të thotë se përdoren 7 bite (në një fushë prej 0 deri në 127 në sistemin decimal) për të paraqitur karakterin. Në kohën kur ASCII u paraqit, shumë kompjuterë kishin të bënin me bajte apo grupe 8-bitëshe (oktete) si njësi më të vogla të informatës. Biti i tetë zakonisht është përdorur si bit për paritet për të bërë kontrollimin e gabimeve gjatë komunikimit apo funksioneve të ndryshme të pajisjeve. Te makinat të cilat nuk e kanë përdorur bitin e tetë për kontrollim të paritetit, zakonisht e kanë kthyer atë bit në zero, edhe pse disa sisteme sikurse PRIME, e kanë kthyer në një.

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, or TeX for advanced layout and font control.

ASCII was first published as a standard in 1963 by the American Standards Association (ASA), which later became ANSI. 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 where they belonged. There are many variations of ASCII, but its present, most widely-used form is ANSI X3.4-1986, also standardized as ECMA-6, ISO/IEC 646:1991 International Reference Version, ITU-T Recommendation T.50 (09/92), and RFC 20.  It is embedded in its probable replacement, Unicode, as the 'lowest' 128 characters. ASCII is considered by some the most successful software standard ever promulgated.

Historically, ASCII developed from telegraphic codes and its first commercial use was as a 7-bit teleprinter code promoted by 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.

 ASCII control characters 
The first thirty-two codes (numbers 0–31 decimal) in ASCII are reserved for control characters: codes that were not originally intended to carry 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 found on the top left of common keyboards.

Code 127 (all seven bits on) is another special character known as "delete" or "rubout". Though its function 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 80s, 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 (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 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.

BinaryDecimalHexAbbreviationPrintableRepresentationKeyboardAccessName/Meaning0000 0000000NUL␀^@Null character0000 0001101SOH␁^AStart of Header0000 0010202STX␂^BStart of Text0000 0011303ETX␃^CEnd of Text0000 0100404EOT␄^DEnd-of-transmission character0000 0101505ENQ␅^EEnquiry0000 0110606ACK␆^FACK (Acknowledgement)0000 0111707BEL␇^GBell0000 1000808BS␈^HBackspace0000 1001909HT␉^IHorizontal Tab0000 1010100ALF␊^JLine feed0000 1011110BVT␋^KVertical Tab0000 1100120CFF␌^LForm feed0000 1101130DCR␍^MCarriage return0000 1110140ESO␎^NShift Out0000 1111150FSI␏^OShift In0001 00001610DLE␐^PData Link Escape0001 00011711DC1␑^QDevice Control 1 — oft. XON 0001 00101812DC2␒^RDevice Control 20001 00111913DC3␓^SDevice Control 3 — oft. XOFF0001 01002014DC4␔^TDevice Control 40001 01012115NAK␕^UNegative Acknowledgement0001 01102216SYN␖^VSynchronous Idle0001 01112317ETB␗^WEnd of Trans. Block0001 10002418CAN␘^XCancel0001 10012519EM␙^YEnd of Medium0001 1010261ASUB␚^ZSubstitute0001 1011271BESC␛^[ or ESCEscape0001 1100281CFS␜^\File Separator0001 1101291DGS␝^]Group Separator0001 1110301ERS␞^^Record Separator0001 1111311FUS␟^_Unit Separator0111 11111277FDEL␡DEL or BackspaceDelete

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".

 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).

BinaryDecimalHexGraphic0010 00003220(blank) (␠)0010 00013321!0010 00103422"0010 00113523#0010 01003624$0010 01013725%0010 01103826&0010 01113927'0010 10004028(0010 10014129)0010 1010422A*0010 1011432B+0010 1100442C,0010 1101452D-0010 1110462E.0010 1111472F/0011 0000483000011 0001493110011 0010503220011 0011513330011 0100523440011 0101533550011 0110543660011 0111553770011 1000563880011 1001573990011 1010583A:0011 1011593B;0011 1100603C<0011 1101613D=0011 1110623E>0011 1111633F? BinaryDecimalHexGraphic0100 00006440@0100 00016541A0100 00106642B0100 00116743C0100 01006844D0100 01016945E0100 01107046F0100 01117147G0100 10007248H0100 10017349I0100 1010744AJ0100 1011754BK0100 1100764CL0100 1101774DM0100 1110784EN0100 1111794FO0101 00008050P0101 00018151Q0101 00108252R0101 00118353S0101 01008454T0101 01018555U0101 01108656V0101 01118757W0101 10008858X0101 10018959Y0101 1010905AZ0101 1011915B[0101 1100925C\0101 1101935D]0101 1110945E^0101 1111955F_ BinaryDecimalHexGraphic0110 00009660`0110 00019761a0110 00109862b0110 00119963c0110 010010064d0110 010110165e0110 011010266f0110 011110367g0110 100010468h0110 100110569i0110 10101066Aj0110 10111076Bk0110 11001086Cl0110 11011096Dm0110 11101106En0110 11111116Fo0111 000011270p0111 000111371q0111 001011472r0111 001111573s0111 010011674t0111 010111775u0111 011011876v0111 011111977w0111 100012078x0111 100112179y0111 10101227Az0111 10111237B{0111 11001247C|0111 11011257D}0111 11101267E~

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.

 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.

 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.

 Shih edhe 
 Binary and text files
 EBCDIC
 Extended ASCII
 ISCII
 ISO 646
 ISO 8859
 Tabela-ASCII
 Textfiles.com
 Unicode
 UTF-8
 VISCII

Computer (family)-specific ASCII variants:
 ATASCII
 PETSCII
 ZX Spectrum character set

ASCII in space:
 Asteroid 3568 ASCII

 External links 
 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)
 Annotated History of Character Codes by Tom Jennings, World Power Systems
 Learning by Simulations Interactive simulation of ASCII encoding

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

Kategoria:Shkencë kompjuterike
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