UTF-1

UTF-1 is a way of transforming ISO 10646/Unicode into a stream of bytes. Due to the design it is not possible to resynchronise if decoding starts in the middle of a character (this makes truncation hard, among other things) and simple byte-oriented search routines cannot be reliably used with it. UTF-1 is also fairly slow due to its use of division by a number which is not a power of 2. Due to these issues, UTF-1 never gained wide acceptance and has been almost totally 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 octets. While the ASCII range is encoded as one octet, as in UTF-8, the ASCII octets 0x21 - 0x7E (decimal 33 - 126) are also used in UTF-1 multi-byte encodings; therefore UTF-1 is unsuited for many Internet protocols, including MIME.

UTF-1 does not use the C0 and C1 control codes in other encodings – any 0x00–0x20 or 0x7F–0x9F octet stands for the corresponding code points in ISO-8859-1 (U+0000–0020 and U+007F–009F, respectively). This design with 66 protected octets tried to be ISO 2022 compatible.

The UTF-1 encoding scheme uses "modulo 190" arithmetic (256 − 66 = 190); it was designed to encode the complete 31 bits of the original Universal Character Set (UCS-4). For comparison, UTF-8 protects all 128 ASCII octets, and needs two bits in trail bytes of multi-byte encodings for this purpose, 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).

codepoint	UTF-16BE	UTF-16LE	UTF-8	UTF-1
U+007F	007F	7F00	7F	7F
U+0080	0080	8000	C280	80
U+009F	009F	9F00	C29F	9F
U+00A0	00A0	A000	C2A0	A0A0
U+00BF	00BF	BF00	C2BF	A0BF
U+00C0	00C0	C000	C380	A0C0
U+00FF	00FF	FF00	C3BF	A0FF
U+0100	0100	0001	C480	A121
U+015D	015D	5D01	C59D	A17E
U+015E	015E	5E01	C59E	A1A0
U+01BD	01BD	BD01	C6BD	A1FF
U+01BE	01BE	BE01	C6BE	A221
U+07FF	07FF	FF07	DFBF	AA72
U+0800	0800	0008	E0A080	AA73
U+0FFF	0FFF	FF0F	E0BFBF	B548
U+1000	1000	0010	E18080	B549
U+4015	4015	1540	E48095	F5FF
U+4016	4016	1640	E48096	F62121
U+D7FF	D7FF	FFD7	ED9FBF	F72FC3
U+E000	E000	00E0	EE8080	F73A79
U+F8FF	F8FF	FFF8	EFA3BF	F75C3C
U+FDD0	FDD0	D0FD	EFB790	F762BA
U+FDEF	FDEF	EFFD	EFB7AF	F762D9
U+FEFF	FEFF	FFFE	EFBBBF	F7644C
U+FFFD	FFFD	FDFF	EFBFBD	F765AD
U+FFFE	FFFE	FEFF	EFBFBE	F765AE
U+FFFF	FFFF	FFFF	EFBFBF	F765AF
U+10000	D800DC00	00D800DC	F0908080	F765B0
U+38E2D	D8A3DE2D	A3D82DDE	F0B8B8AD	FBFFFF
U+38E2E	D8A3DE2E	A3D82EDE	F0B8B8AE	FC21212121
U+FFFFF	DBBFDFFF	BFDBFFDF	F3BFBFBF	FC2137B27A
U+100000	DBC0DC00	C0DB00DC	F4808080	FC2137B27B
U+10FFFF	DBFFDFFF	FFDBFFDF	F48FBFBF	FC21396E6C

See also

• Comparison of Unicode encodings
• Universal Character Set

References

• ISO IR 178 (PDF, 256 KB, the retired UTF-1 specification)