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The Universal Coded Character Set (UCS, Unicode) is a standard set of characters defined by the international standard ISO/IEC 10646, Information technology — Universal Coded Character Set (UCS) (plus amendments to that standard), which is the basis of many character encodings, improving as characters from previously unrepresented writing systems are added.
UTF-16 arose from an earlier obsolete fixed-width 16-bit encoding now known as UCS-2 (for 2-byte Universal Character Set), [2] [3] once it became clear that more than 2 16 (65,536) code points were needed, [4] including most emoji and important CJK characters such as for personal and place names.
Microsoft was one of the first companies to implement Unicode in their products. Windows NT was the first operating system that used "wide characters" in system calls.Using the (now obsolete) UCS-2 encoding scheme at first, it was upgraded to the variable-width encoding UTF-16 starting with Windows 2000, allowing a representation of additional planes with surrogate pairs.
The numbers in the names of the encodings indicate the number of bits per code unit (for UTF encodings) or the number of bytes per code unit (for UCS encodings and UTF-1). UTF-8 and UTF-16 are the most commonly used encodings. UCS-2 is an obsolete subset of UTF-16; UCS-4 and UTF-32 are functionally equivalent. UTF encodings include:
The Unicode Consortium and the ISO/IEC JTC 1/SC 2/WG 2 jointly collaborate on the list of the characters in the Universal Coded Character Set.The Universal Coded Character Set, most commonly called the Universal Character Set (abbr. UCS, official designation: ISO/IEC 10646), is an international standard to map characters, discrete symbols used in natural language, mathematics, music, and other ...
[citation needed] UTF-8 is a sparse encoding: a large fraction of possible byte combinations do not result in valid UTF-8 text. Binary data and text in any other encoding are likely to contain byte sequences that are invalid as UTF-8, so existence of such invalid sequences indicates the file is not UTF-8, while lack of invalid sequences is a ...
To encode characters outside of the BMP (unreachable in plain UCS-2), such as Emoji, UTF-16 uses surrogate pairs, which when decoded with UCS-2 would appear as two valid but unmapped code points. A single SMS GSM message using this encoding can have at most 70 characters (140 octets).
As a further example, prior to the publication of XML 1.0 Second Edition on October 6, 2000, XML 1.0 was based on an older version of ISO 10646 and prohibited using characters above U+FFFD, except in character data, thus making a reference like 𐀀 (U+10000) illegal. In XML 1.1 and newer editions of XML 1.0, such a reference is allowed ...