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UTF-9 and UTF-18

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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.[1]

However, unlike some of the "specifications" given in other April 1 RFCs, UTF-9 and UTF-18 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[edit]

Similarly to UTF-8, which uses a variable-width encoding with 8-bit code units, UTF-9 uses a system of putting the octets of a Unicode code point in the low 8 bits of each output 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 octet.

This table shows the UTF-9 encoding scheme (the x characters are replaced by the bits of the code point):

of nonets
Bits for
code point
code point
code point
Nonet 1 Nonet 2 Nonet 3
1 8 U+0000 U+00FF 0xxxxxxxx
2 16 U+0100 U+FFFF 1xxxxxxxx 0xxxxxxxx
3 21 U+10000 U+10FFFF 1000xxxxx 1xxxxxxxx 0xxxxxxxx

The details of the encoding scheme used in UTF-9 differ from UTF-8 in a non-ideal way, as UTF-9 is not self-synchronizing: the end of a longer sequence can be confused with a shorter sequence. For instance, U+0041 is represented in octal as 101 and U+E0041 as 416 400 101. This stems from the lack of distinction between the beginning of a sequence and the subsequent continuation nonets, as both simply have their most significant bit set, and the lack of distinction between a one-nonet sequence and the last nonet of a multi-nonet sequence. In contrast, in UTF-8, the three different kinds of octets are trivially distinguishable from each other, making the scheme self-synchronizing. Searching within a UTF-9 encoded string or splitting one requires special care, as it is always necessary to search backwards to find the beginning of the current sequence.

UTF-18 is a fixed length encoding using an 18 bit integer per code point. This allows the representation of four planes, which are mapped to the four 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 allow the use of surrogates for these code points, arguing that surrogates, as used in UTF-16, are complicated. It is unlikely that planes 3–13 will be assigned by Unicode in the foreseeable future. Thus, UTF-18, like UCS-4, guarantees a fixed width for all code points currently in use (although not for all glyphs).

See also[edit]

  • April Fools' Day RFC
  • Comparison of Unicode encodings
  • UCS-2
  • UCS-4
  • UTF-8
  • UTF-16


  1. "Mark Crispin's Web Page". Retrieved 2006-09-17. Points out April Fool's Day for two of his RFCs.

External links[edit]

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