View source for UTF-1

{{Short description|Obsolete multibyte encoding for Unicode}}
{{technical|date=September 2024}}
{{no footnotes|date=September 2024}}
{{Infobox character encoding
| name = UTF-1
| mime = ISO-10646-UTF-1
| alias = 
| image = 
| caption = 
| standard = 
| lang = International
| status = Obscure, of mainly historical interest.
| classification = [[Unicode Transformation Format]], [[extended ASCII]], [[variable-width encoding]]
| encodes = [[ISO/IEC 10646]] ([[Unicode]])
| extends = [[US-ASCII]]
| prev = 
| next = [[UTF-8]]
}}
'''UTF-1''' is an obsolete method of transforming [[ISO/IEC 10646]]/[[Unicode]] into a stream of [[byte]]s. Its design does not provide [[self-synchronizing code|self-synchronization]], which makes searching for [[Substring|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). UTF-1 is also slow to encode or 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 ==
Similar to [[UTF-8]], UTF-1 is a [[variable-width encoding]] that is backwards-compatible with [[ASCII]]. Every [[Unicode]] [[code point]] is represented by either a single byte, or a sequence of two, three, or ''five'' bytes. All ASCII code points are a single byte (the code points {{mono|U+0080}} through {{mono|U+009F}} are also single bytes).

UTF-1 does not use the [[C0 and C1 control codes]] or the space character in multi-byte encodings: a byte in the range 0–[[Hexadecimal|0x]]20 or 0x7F–0x9F always stands for the corresponding code point. This design with 66 ''protected'' characters tried to be [[ISO/IEC 2022]] compatible.

UTF-1 uses "[[modulo operation|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 ({{nowrap|1=8 − 2 = 6}}; {{nowrap|1=2<sup>6</sup> = 64}}). [[Binary Ordered Compression for Unicode|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).

{| <!-- legacy: --> border="1" cellspacing="3" cellpadding="3" <!-- modern: --> class="wikitable" style="font-family: monospace, monospace"
! 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
|-
| [[Byte order mark|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 ==
* {{cite web |title=The Unicode Standard: Appendix F FSS-UTF |version=Version 1.1 |institution=Unicode, Inc. |url=https://www.unicode.org/versions/Unicode1.1.0/appF.pdf |type=PDF, 768 KiB}}
* {{cite web |title=ISO IR 178: UCS Transformation Format One (UTF-1) |author=ISO/IEC JTC 1/SC2/WG2 |author-link=ISO/IEC JTC 1/SC2/WG2 |date=1993-01-21 |edition=1 |id=Registration number 178 |url=http://kikaku.itscj.ipsj.or.jp/ISO-IR/178.pdf |archive-url=https://web.archive.org/web/20150318032101/http://kikaku.itscj.ipsj.or.jp/ISO-IR/178.pdf |archive-date=2015-03-18 |type=PDF, 256 KiB}}
* {{cite web |author-first=Roman |author-last=Czyborra |title=Unicode Transformation Formats: UTF-8 & Co. |date=1998-11-30 |url=http://czyborra.com/utf/#UTF-1 |access-date=2016-06-07 |url-status=live |archive-url=https://web.archive.org/web/20160607111732/http://czyborra.com/utf/#UTF-1 |archive-date=2016-06-07}}
* {{Cite IETF |rfc=3629 |std=63 |title=UTF-8, a transformation format of ISO 10646 |first1=F. |last1=Yergeau |publisher=[[Internet Engineering Task Force|IETF]] |date=November 2003 |doi=10.17487/RFC3629}}

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