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Template:Table Unicode In computing, UTF-16 is a variable-length (16 or 32 bits) character encoding. It is able to represent the complete Unicode basic multilingual plane with exactly two bytes and every other plane with exactly four bytes. UCS-2 is a fixed-length (16 bits) subset of UTF-16, able to represent the basic multilingual plane only.

"UTF-16 is officially defined in Annex Q of ISO/IEC 10646-1. It is also described in The Unicode Standard version 3.0 and higher, as well as in the IETF's RFC 2781." UTF-16 represents a character that has been assigned within the lower 65536 code points of Unicode or ISO/IEC 10646 as a single code value equivalent to the character's code point: 0 for 0, hexadecimal FFFD for FFFD, for example.

Method for code points in Plane 1, Plane 2

UTF-16 represents a character above hexadecimal FFFF using a pair of 16-bit words, known as a surrogate pair, using "flag" code values from the range D800–DFFF. For example, the character at code point hexadecimal 10000 becomes the code value sequence D800 DC00, and the character at hexadecimal 10FFFD, the upper limit of Unicode, becomes the code value sequence DBFF DFFD. Unicode and ISO/IEC 10646 do not assign characters to any of the code points in the D800–DFFF range, so an individual code value from a surrogate pair does not ever represent a character.

Big-endian/Little-endian

These code values are then serialized as 16-bit words, one word per code value. Because the endianness of these words varies according to the computer architecture, UTF-16 specifies three encoding schemes: UTF-16, UTF-16LE, and UTF-16BE.

The UTF-16 encoding scheme mandates that the byte order must be declared by prepending a Byte Order Mark before the first serialized character. This BOM is the encoded version of the Zero-Width No-Break Space character, Unicode number FEFF in hex, manifesting as the byte sequence FE FF for big-endian, or FF FE for little-endian. A BOM at the beginning of UTF-16 encoded data is considered to be a signature separate from the text itself; it is for the benefit of the decoder.

The UTF-16LE and UTF-16BE encoding schemes are identical to the UTF-16 encoding scheme, but rather than using a BOM, the byte order is implicit in the name of the encoding (LE for little-endian, BE for big-endian). A BOM at the beginning of UTF-16LE or UTF-16BE encoded data is not considered to be a BOM; it is part of the text itself.

The IANA has approved UTF-16, UTF-16BE, and UTF-16LE for use on the Internet, by those exact names (case insensitively). The aliases UTF_16 or UTF16 may be meaningful in some programming languages or software applications, but they are not standard names in Internet protocols.

Major operating system usage

UTF-16 is the native internal representation of text in the Microsoft Windows NT/Windows 2000/Windows XP/Windows CE, Qualcomm BREW, and Symbian operating systems; the Java and .NET bytecode environments; Mac OS X's Cocoa and Core Foundation frameworks; and the Qt cross-platform graphical widget toolkit.

UCS-2

UCS-2 is an obsolete specification that can only encode the BMP, and consequently has been superseded by UTF-16. In some materials, the terms UCS-2 and UTF-16 are erroneously conflated and used interchangeably; UTF-16 is often mislabeled UCS-2.

NT systems prior to Windows 2000 only support UCS-2.

Examples

* Appropriate font and software are required to see the correct glyphs.

Example UTF-16 Encoding Procedure

The character at code point U+64321 (hexadecimal) is to be encoded in UTF-16. Since it is above U+FFFF, it must be encoded with a surrogate pair, as follows:

v  = 0x64321
v′ = v - 0x10000
   = 0x54321
   = 0101 0100 0011 0010 0001
vh = 0101010000 // higher 10 bits of v′
vl = 1100100001 // lower  10 bits of v′
w1 = 0xD800 // the resulting 1st word is initialized with the lower bracket
w2 = 0xDC00 // the resulting 2nd word is initialized with the higher bracket
w1 = w1 | vh
   = 1101 1000 0000 0000 | 01 0101 0000
   = 1101 1001 0101 0000
   = 0xD950
w2 = w2 | vl
   = 1101 1100 0000 0000 | 11 0010 0001
   = 1101 1111 0010 0001
   = 0xDF21

The correct UTF-16 encoding for this character is thus the following word sequence:

0xD950 0xDF21

Since the character is above U+FFFF, the character cannot be encoded in UCS-2.

See also

• Comparison of Unicode encodings
• UTF-8

External links

• Unicode Technical Note #12: UTF-16 for Processing
• Unicode FAQ: What is the difference between UCS-2 and UTF-16?
• Ten Minute Tutor guide to UTF-16
• Unicode Character Name Index

• Unicode
• Character sets