Universal Character Set

The Universal Character Set (UCS), defined by the ISO/IEC 10646 International Standard, is a standard set of characters upon which many character encodings are based. The UCS contains nearly a hundred thousand abstract characters, each identified by an unambiguous name and an integer number called its code point.

Characters (letters, numbers, symbols, ideograms, logograms, etc.) from the many languages, scripts, and traditions of the world are represented in the UCS with unique code points. The inclusiveness of the UCS is continually improving as characters from previously unrepresented writing systems are added.

Since 1991, the Unicode Consortium has worked with ISO to develop The Unicode Standard ("Unicode") and ISO/IEC 10646 in tandem. The repertoire, character names, and code points of Version 2.0 of Unicode exactly match those of ISO/IEC 10646-1:1993 with its first seven published amendments. After the publication of Unicode 3.0 in February 2000, corresponding new and updated characters entered the UCS via ISO/IEC 10646-1:2000.

The UCS has over 1.1 million code points available for use, but only the first 65,536 (the Basic Multilingual Plane, or BMP) had entered into common use before 2000. This situation began changing when the People's Republic of China (PRC) ruled in 2000 that all computer systems sold in its jurisdiction would have to support GB18030. This required computer systems intended for sale in the PRC to move beyond the BMP.

The system deliberately leaves many code points not assigned to characters, even in the BMP. It does this to allow for future expansion or to minimize conflicts with other encoding forms.

Encoding forms of the Universal Character Set

ISO 10646 defines several character encoding forms for the Universal Character Set. The simplest, UCS-2, uses a single code value (defined as one or more numbers representing a code point) between 0 and 65,535 for each character, and allows exactly two bytes (one 16-bit word) to represent that value. UCS-2 thereby permits a binary representation of every code point in the BMP, as long as the code point represents a character. UCS-2 cannot represent code points outside the BMP.

The first amendment to the original edition of the UCS defined UTF-16, an extension of UCS-2, to represent code points outside the BMP. A range of code points in the S (Special) Zone of the BMP remains unassigned to characters. UCS-2 disallows use of code values for these code points, but UTF-16 allows their use in pairs. Each pair consists of an "RC-element" (a two-octet sequence comprising the R-octet and the C-octet from the four octet sequence that corresponds to a cell in the coding space of a coded character set) from the high-half zone and an "RC-element" from the low-half zone. Unicode also adopted UTF-16, but in Unicode terminology, the high-half zone elements become "high surrogates" and the low-half zone elements become "low surrogates".

Another encoding, UCS-4, uses a single code value between 0 and (theoretically) hexadecimal 7FFFFFFF for each character (although the UCS stops at 10FFFF and ISO/IEC 10646 has stated that all future assignments of characters will also take place in that range). UCS-4 allows representation of each value as exactly four bytes (one 32-bit word). UCS-4 thereby permits a binary representation of every code point in the UCS, including those outside the BMP. As in UCS-2, every encoded character has a fixed length in bytes, which makes it simple to manipulate, but of course it requires twice as much storage as UCS-2.

Occasionally, articles about Unicode will mistakenly refer to UCS-2 as "UCS-16". UCS-16 does not exist; the authors who make this error usually intend to refer to UCS-2 or to UTF-16.

History of ISO 10646

The International Organization for Standardization (ISO) set out to compose the universal character set in 1989, and published the draft of ISO 10646 in 1990. Hugh McGregor Ross was one of its principal architects. That standard differed markedly from the current one. It defined 128 groups of 256 planes of 256 rows of 256 cells, for an apparent total of 2,147,483,648 characters, but actually the standard could code only 679,477,248 characters, as the policy forbade byte values of control characters (0x00 to 0x1F and 0x80 to 0x9F, in hexadecimal notation) anywhere. The Latin capital letter A, for example, had a location in group 0x20, plane 0x20, row 0x20, cell 0x41.

One could code the characters of this primordial ISO 10646 standard in one of three ways:

UCS-4, four bytes for every character, enabling the simple encoding of all characters;
UCS-2, two bytes for every character, enabling the encoding of the first plane, 0x20, the Basic Multilingual Plane, containing the first 36,864 codepoints, straightforwardly, and other planes and groups by switching to them with ISO 2022 escape sequences;
UTF-1, which encodes all the characters in sequences of bytes of varying length (1 to 5 bytes, each of which contain no control characters).

In 1990, therefore, two initiatives for a universal character set existed: Unicode, with 16 bits for every character (65,536 possible characters), and ISO 10646. The software companies refused to accept the complexity and size requirement of the ISO standard and were able to convince a number of ISO National Bodies to vote against it. The ISO standardisers realised they could not continue to support the standard in its current state and negotiated the unification of their standard with Unicode. Two changes took place: the lifting of the limitation upon characters (prohibition of control character values), thus permitting characters like 0x0000101F; and the synchronisation of the repertoire of the Basic Multilingual Plane with that of Unicode.

Meanwhile, in the passage of time, the situation changed in the Unicode standard itself: 65,536 characters came to appear insufficient, and the standard from version 2.0 and onwards supports encoding of 1,112,064 characters by means of the UTF-16 surrogate mechanism. For that reason, ISO 10646 was limited to contain as many characters as could be encoded by UTF-16 and no more, that is, a little over a million characters instead of over 2,000 million. The UCS-4 encoding of ISO 10646 was incorporated into the Unicode standard with the limitation to the UTF-16 range and under the name UTF-32. As for UTF-1, no-one used it, because of its bad design (no way of distinguishing between single bytes, lead bytes and trail bytes, a problem similar to that of the Shift-JIS encoding of Japanese) and its poor performance (many division operations). Rob Pike and Ken Thompson, the designers of the Plan 9 operating system, devised a new, fast and well-designed mixed width encoding, which came to be called UTF-8.

Differences between ISO 10646 and Unicode

According to Alain LaBonté, the head of the Canadian delegation to ISO/IEC JTC1/SC22/WG20, the internationalization (i18n) working group in 2000:

ISO 10646 and Unicode have an identical repertoire and numbers — the same characters with the same numbers exist on both standards. The difference between them is that Unicode adds rules and specifications that are outside the scope of ISO 10646. ISO 10646 is a simple character map, an extension of previous standards like ISO 8859. In contrast, Unicode adds rules for collation, normalization of forms, and the bidirectional algorithm for scripts like Hebrew and Arabic. For interoperability between platforms, especially if bidirectional scripts are used, it is not enough to support ISO 10646; Unicode must be implemented.

To support these rules and algorithms, Unicode adds many properties to each character in the set such as properties determining a character’s default bidirectional class and properties to determine how the character combines with other characters. If the character represents a numeric value such as the European number ‘8’, or the vulgar fraction ‘¼’, that numeric value is also added as a property of the character. Unicode intends these properties to support interoperable text handling with a mixture of languages.

Some applications support ISO 10646 characters but do not fully support Unicode. One such application, Linux xterm, can properly display all ISO 10646 characters that have a one-to-one character-to-glyph mapping and a single directionality. It can handle some combining marks by simple overstriking methods, but cannot display Hebrew (bidirectional), Devanagari (one character to many glyphs) or Arabic (both features). Most GUI applications use standard OS text drawing routines which handle such scripts, although the applications themselves still do not always handle them correctly. For instance, selecting text in certain scripts in Mozilla Firefox causes the text to jump around.

Citing the Universal Character Set

ISO 10646, a general, informal citation for the ISO/IEC 10646 family of standards, is acceptable in most prose. And even though it is a separate standard, the term Unicode is used just as often, informally, when discussing the UCS. However, any normative references to the UCS as a publication should cite a particular part and version, using the form ISO/IEC 10646-{part}:{year}; for example: ISO/IEC 10646-1:1993.

Correlation to Unicode

ISO/IEC 10646-1:1993 ≈ Unicode 1.1
ISO/IEC 10646-1:2000 ≈ Unicode 3.0
ISO/IEC 10646-2:2001 ≈ Unicode 3.2
ISO/IEC 10646:2003 ≈ Unicode 4.0
ISO/IEC 10646:2003 plus Amendment 1 ≈ Unicode 4.1
ISO/IEC 10646:2003 plus Amendment 1, Amendment 2, and part of Amendment 3 ≈ Unicode 5.0
ISO/IEC 10646:2003 plus Amendment 1 to 4 ≈ Unicode 5.1

See §C.1 of The Unicode Standard and http://www.unicode.org/versions/Unicode5.1.0/ for more detail.