UTF, Unicode, ASCII, rune – character set and format

The Plan 9 character set and representation are based on the Unicode Standard and on the ISO multibyte UTF-8 encoding (Universal Character Set Transformation Format, 8 bits wide). The Unicode Standard represents its characters in 16 bits; UTF-8 represents such values in an 8-bit byte stream. Throughout this manual, UTF-8 is shortened to UTF.
In Plan 9, a rune is a 16-bit quantity representing a Unicode character. Internally, programs may store characters as runes. However, any external manifestation of textual information, in files or at the interface between programs, uses a machine-independent, byte-stream encoding called UTF.
UTF is designed so the 7-bit ASCII set (values hexadecimal 00 to 7F), appear only as themselves in the encoding. Runes with values above 7F appear as sequences of two or more bytes with values only from 80 to FF.
The UTF encoding of the Unicode Standard is backward compatible with ASCII: programs presented only with ASCII work on Plan 9 even if not written to deal with UTF, as do programs that deal with uninterpreted byte streams. However, programs that perform semantic processing on ASCII graphic characters must convert from UTF to runes in order to work properly with non-ASCII input. See rune(3).
Letting numbers be binary, a rune x is converted to a multibyte UTF sequence as follows:
01. x in [00000000.0bbbbbbb] → 0bbbbbbb
10. x in [00000bbb.bbbbbbbb] → 110bbbbb, 10bbbbbb
11. x in [bbbbbbbb.bbbbbbbb] → 1110bbbb, 10bbbbbb, 10bbbbbb
Conversion 01 provides a one-byte sequence that spans the ASCII character set in a compatible way. Conversions 10 and 11 represent higher-valued characters as sequences of two or three bytes with the high bit set. Plan 9 does not support the 4, 5, and 6 byte sequences proposed by X-Open. When there are multiple ways to encode a value, for example rune 0, the shortest encoding is used.
In the inverse mapping, any sequence except those described above is incorrect and is converted to rune hexadecimal 0080.

ascii(1), tcs(1), rune(3), The Unicode Standard.