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. 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.
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. 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.
Currently, the dominant UCS encoding is UTF-8, which is a variable-width encoding designed for backward compatibility with ASCII, and for avoiding the complications of endianness and byte-order marks in UTF-16 and UTF-32. More than half of all Web pages are encoded in UTF-8. The Internet Engineering Task Force (IETF) requires all Internet protocols to identify the encoding used for character data, and the supported character encodings must include UTF-8. The Internet Mail Consortium (IMC) recommends that all e-mail programs be able to display and create mail using UTF-8. It is also increasingly being used as the default character encoding in operating systems, programming languages, APIs, and software applications.
See also Comparison of Unicode encodings.
Read more about this topic: Universal Character Set
Famous quotes containing the words forms, universal, character and/or set:
“And what avails it that science has come to treat space and time as simply forms of thought, and the material world as hypothetical, and withal our pretension of property and even of self-hood are fading with the rest, if, at last, even our thoughts are not finalities, but the incessant flowing and ascension reach these also, and each thought which yesterday was a finality, to-day is yielding to a larger generalization?”
—Ralph Waldo Emerson (1803–1882)
“Necessity does everything well. In our condition of universal dependence, it seems heroic to let the petitioner be the judge of his necessity, and to give all that is asked, though at great inconvenience.”
—Ralph Waldo Emerson (1803–1882)
“For character too is a process and an unfolding ... among our valued friends is there not someone or other who is a little too self confident and disdainful; whose distinguished mind is a little spotted with commonness; who is a little pinched here and protruberent there with native prejudices; or whose better energies are liable to lapse down the wrong channel under the influence of transient solicitations?”
—George Eliot [Mary Ann (or Marian)
“My curse on plays
That have to be set up in fifty ways,
On the day’s war with every knave and dolt,
Theater business, management of men.”
—William Butler Yeats (1865–1939)