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UTF-16 and UCS-2 produce a sequence of 16-bit code units. Since most communication and storage protocols are defined for bytes, and each unit thus takes two 8-bit bytes, the order of the bytes may depend on the endianness (byte order) of the computer architecture.
In UTF-16, a BOM (U+FEFF) may be placed as the first bytes of a file or character stream to indicate the endianness (byte order) of all the 16-bit code units of the file or stream. If an attempt is made to read this stream with the wrong endianness, the bytes will be swapped, thus delivering the character U+FFFE , which is defined by Unicode as ...
UTF-16 and UTF-32 do not have endianness defined, so a byte order must be selected when receiving them over a byte-oriented network or reading them from a byte-oriented storage. This may be achieved by using a byte-order mark at the start of the text or assuming big-endian (RFC 2781).
The sequence also has no meaning in any arrangement of UTF-32 encoding, so, in summary, it serves as a fairly reliable indication that the text stream is encoded as UTF-16 in big-endian byte order. Conversely, if the first two bytes are 0xFF, 0xFE, then the text stream may be assumed to be encoded as UTF-16LE because, read as a 16-bit little ...
Simple character encoding schemes include UTF-8, UTF-16BE, UTF-32BE, UTF-16LE, and UTF-32LE; compound character encoding schemes, such as UTF-16, UTF-32 and ISO/IEC 2022, switch between several simple schemes by using a byte order mark or escape sequences; compressing schemes try to minimize the number of bytes used per code unit (such as SCSU ...
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:
For UTF-8, the BOM is optional, while it is a must for the UTF-16 and the UTF-32 encodings. (Note: UTF-16 and UTF-32 without the BOM are formally known under different names, they are different encodings, and thus needs some form of encoding declaration – see UTF-16BE, UTF-16LE, UTF-32LE and UTF-32BE.) The use of the BOM character (U+FEFF ...
UTF-16 uniquely encodes all Unicode characters in the Basic Multilingual Plane (BMP) using 16 bits but the remaining Unicode (e.g. emojis) is encoded with a 32-bit (four byte) code – while the rest of the industry (Unix-like systems and the web), and now Microsoft chose UTF-8 (which uses one byte for the 7-bit ASCII character set, two or ...