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ISO/IEC 646 is the name of a set of ISO standards, described as ''Information technology — ISO 7-bit coded character set for information interchange''. Since its first edition in 1972 it has specified a 7-bit character code from which several national standards are derived. ISO/IEC 646 was also ratified by ECMA as ECMA-6. Characters in the ISO/IEC 646 Basic Character Set are ''invariant characters''.〔(【引用サイトリンク】url=http://www.ncip.info/invariant-character-handling.html )〕 Since that portion of ISO/IEC 646, that is the ''invariant character set'' shared by all countries, specified only those letters used in the ISO basic Latin alphabet, countries using additional letters needed to create national variants of ISO 646 to be able to use their native scripts. Since universal acceptance of the 8-bit byte did not exist at that time, the national characters had to be made to fit within the constraints of 7 bits, meaning that some characters that appear in ASCII do not appear in other national variants of ISO 646. == History == ISO/IEC 646 and its predecessor ASCII (ANSI X3.4) largely endorsed existing practice regarding character encodings in the telecommunications industry. As ASCII did not provide a number of characters needed for languages other than English, a number of national variants were made that substituted some less-used characters with needed ones. Due to the incompatibility of the various national variants, an International Reference Version (IRV) of ISO/IEC 646 was introduced, in an attempt to at least restrict the replaced set to the same characters in all variants. The original version (ISO 646 IRV) differed from ASCII only in that in code point 0024, ASCII's dollar sign ($) was replaced by the international currency symbol (¤). The final 1991 version of the code ISO 646:1991 is also known as ITU T.50, International Reference Alphabet or IRA, formerly International Alphabet No. 5, IA5. This standard allows users to exercise the 12 variable characters(i.e., 2 alternative graphic characters and 10 national defined characters). Among these exercises, ISO 646:1991 IRV(International Reference Version) is explicitly defined and identical to ASCII. The ISO 8859 series of standards governing 8-bit character encodings supersede the ISO 646 international standard and its national variants, by providing 96 additional characters with the additional bit and thus avoiding any substitution of ASCII codes. The ISO 10646 standard, directly related to Unicode, supersedes all of the ISO 646 and ISO 8859 sets with one unified set of character encodings using a larger 21-bit value. A legacy of ISO/IEC 646 is visible on Windows, where in some fonts or locales, the backslash character used in filenames is rendered as ¥ or other characters. Despite the fact that a different code for ¥ was available even on the original IBM PC, so much text was created with the backslash code used for ¥ that even modern Windows fonts have found it necessary to render the code that way. Another legacy is the existence of trigraphs in the C programming language. 抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』 ■ウィキペディアで「ISO/IEC 646」の詳細全文を読む スポンサード リンク
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