What is "endianness"

n. (context computing English) The property of a computer of being either big-endian or little-endian.

Endianness is the order of the bytes comprising a digital word in computer memory. It also describes the order of byte transmission over a digital link. Words may be represented in big-endian or little-endian format. When storing a word in big-endian format the most significant byte, which is the byte containing the most significant bit, is stored first and the following bytes are stored in decreasing significance order with the least significant byte, which is the byte containing the least significant bit, thus being stored at last place. Little-endian format reverses the order of the sequence and stores the least significant byte at the first location with the most significant byte being stored last. The order of bits within a byte can also have endianness (as discussed later); however, a byte is typically handled as a numerical value or character symbol and so bit sequence order is obviated.

Both big and little endianness forms are widely used in digital electronics. The choice of endianness for a new design is often arbitrary, but later technology revisions and updates perpetuate the existing endianness and many other design attributes to maintain backward compatibility. As examples, the IBM z/Architecture mainframes use big-endian while the Intel x86 processors use little-endian. The designers chose endianness in the 1960s and 1970s respectively.

Big-endian is the most common format in data networking; fields in the protocols of the Internet protocol suite, such as IPv4, IPv6, TCP, and UDP, are transmitted in big-endian order. For this reason, big-endian byte order is also referred to as network byte order. Little-endian storage is popular for microprocessors, in part due to significant influence on microprocessor designs by Intel Corporation. Mixed forms also exist, for instance the ordering of bytes in a 16-bit word may differ from the ordering of 16-bit words within a 32-bit word. Such cases are sometimes referred to as mixed-endian or middle-endian. There are also some bi-endian processors that operate in either little-endian or big-endian mode.