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UTF-9 and UTF-18
UTF-9 and UTF-18 (9- and 18-bit Unicode Transformation Format, respectively) were two April Fools' Day RFC joke specifications for encoding Unicode on systems where the nonet (nine bit group) is a better fit for the native word size than the octet, such as the 36-bit PDP-10 and the UNIVAC 1100/2200 series. Both encodings were specified in RFC 4042, written by Mark Crispin (inventor of IMAP) and released on April 1, 2005. The encodings suffer from a number of flaws and it is confirmed by their author that they were intended as a joke.〔(【引用サイトリンク】title=Mark Crispin's Web Page ) Points out April Fool's Day for two of his RFCs.〕
However, unlike some of the "specifications" given in other April 1 RFCs, they are actually technically possible to implement, and have in fact been implemented in PDP-10 assembly language. They are however not endorsed by the Unicode Consortium.
==Technical details==
Like the 8-bit code commonly called variable-length quantity, UTF-9 uses a system of putting an octet in the low 8 bits of each nonet and using the high bit to indicate continuation. This means that ASCII and Latin 1 characters take one nonet each, the rest of the BMP characters take two nonets each and non-BMP code points take three. Code points that require multiple nonets are stored starting with the most significant non-zero nonet.
UTF-18 is a fixed length encoding using an 18 bit integer per code point. This allows representation of 4 planes, which are mapped to the 4 planes currently used by Unicode (planes 0–2 and 14). This means that the two private use planes (15 and 16) and the currently unused planes (3–13) are not supported. The UTF-18 specification does not say why they did not allow surrogates to be used for these code points, though when talking about UTF-16 earlier in the RFC, it says "This transformation format requires complex surrogates to represent code points outside the BMP". After complaining about their complexity, it would have looked a bit hypocritical to use surrogates in their new standard. It is unlikely that planes 3–13 will be assigned by Unicode any time in the foreseeable future. Thus, UTF-18, like UCS-2 and UCS-4, guarantees a fixed width for all code points (although not for all glyphs).
抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』
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