UTF-1

UTF-1 is one way of transforming ISO 10646/Unicode into a stream of bytes. Its design does not provide self-synchronization, which makes searching for substrings and error recovery difficult. It reuses the ASCII printing characters for multi-byte encodings, making it unsuited for some uses (for instance Unix filenames cannot contain the byte value used for forward slash decode due to its use of division and multiplication by a number which is not a power of 2. Due to these issues, it did not gain acceptance and was quickly replaced by UTF-8.

Design

UTF-1 is a multi-byte encoding like UTF-8; a single Unicode code point can be encoded in one, two, three, or five bytes. The ASCII range is encoded as one byte (all code points from U+0000 to U+009F are).

UTF-1 does not use the C0 and C1 control codes or the space character in multi-byte encodings, the bytes 0 - 0x20 or 0x7F - 0x9F always stand for the corresponding code point. This design with 66 protected characters tried to be ISO 2022 compatible.

UTF-1 uses "modulo 190" arithmetic (256 − 66 = 190). For comparison, UTF-8 protects all 128 ASCII characters and needs one bit for this, and a second bit to make it self-synchronizing, resulting in "modulo 64" arithmetic (8 − 2 = 6; 2⁶ = 64). BOCU-1 protects only the minimal set required for MIME-compatibility (0x00, 0x07–0x0F, 0x1A–0x1B, and 0x20), resulting in "modulo 243" arithmetic (256 − 13 = 243).

code point	UTF-8	UTF-1
U+007F	7F	7F
U+0080	C2 80	80
U+009F	C2 9F	9F
U+00A0	C2 A0	A0 A0
U+00BF	C2 BF	A0 BF
U+00C0	C3 80	A0 C0
U+00FF	C3 BF	A0 FF
U+0100	C4 80	A1 21
U+015D	C5 9D	A1 7E
U+015E	C5 9E	A1 A0
U+01BD	C6 BD	A1 FF
U+01BE	C6 BE	A2 21
U+07FF	DF BF	AA 72
U+0800	E0 A0 80	AA 73
U+0FFF	E0 BF BF	B5 48
U+1000	E1 80 80	B5 49
U+4015	E4 80 95	F5 FF
U+4016	E4 80 96	F6 21 21
U+D7FF	ED 9F BF	F7 2F C3
U+E000	EE 80 80	F7 3A 79
U+F8FF	EF A3 BF	F7 5C 3C
U+FDD0	EF B7 90	F7 62 BA
U+FDEF	EF B7 AF	F7 62 D9
U+FEFF	EF BB BF	F7 64 4C
U+FFFD	EF BF BD	F7 65 AD
U+FFFE	EF BF BE	F7 65 AE
U+FFFF	EF BF BF	F7 65 AF
U+10000	F0 90 80 80	F7 65 B0
U+38E2D	F0 B8 B8 AD	FB FF FF
U+38E2E	F0 B8 B8 AE	FC 21 21 21 21
U+FFFFF	F3 BF BF BF	FC 21 37 B2 7A
U+100000	F4 80 80 80	FC 21 37 B2 7B
U+10FFFF	F4 8F BF BF	FC 21 39 6E 6C
U+7FFFFFFF	FD BF BF BF BF BF	FD BD 2B B9 40

Although modern Unicode ends at U+10FFFF, both UTF-1 and UTF-8 were designed to encode the complete 31 bits of the original Universal Character Set (UCS-4), and the last entry in this table shows this original final code point.

See also

• Comparison of Unicode encodings
• Universal Character Set

References

• ISO/IEC JTC 1/SC2/WG2 (1993-01-21). "ISO IR 178: UCS Transformation Format One (UTF-1)" (PDF) (PDF, 256 KB) (1 ed.). Registration number 178. Archived from the original (PDF) on 2015-03-18. {{cite web}}: Unknown parameter |dead-url= ignored (|url-status= suggested) (help)CS1 maint: numeric names: authors list (link)
• Czyborra, Roman (1998-11-30). "Unicode Transformation Formats: UTF-8 & Co". Archived from the original on 2016-06-07. Retrieved 2016-06-07. {{cite web}}: Unknown parameter |dead-url= ignored (|url-status= suggested) (help)