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In computing, endianness is the ordering used to represent some kind of data as a sequence of smaller units. more...

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Typical cases are the order in which integer values are stored as bytes in computer memory (relative to a given memory addressing scheme) and the transmission order over a network or other medium. When specifically talking about bytes, endianness is also referred to simply as byte order.

Endianness as a general concept

Generally speaking, endianness is a particular facet of a representation format. As such, it applies to the representation(s) of integers used by computer processors, to encoding schemes such as UTF-16 and UTF-32 or to the conceptual encoding implied by some low-level algorithms (see for instance MD5 and SHA hash functions). Likewise it applies to network transmissions, where it is established by the employed protocol.

A note on some non-idiomatic usages: some authors extend the usage of the word "endianness", and of related terms, to entities such as street addresses, date formats and others. It should be noticed however that such usages —basically reducing endianness to a mere synonym of ordering of the parts— are non-standard usage (e.g., ISO 8601:2004 talks about "descending order year-month-day", not about "big-endian format"), do not have widespread usage, and are generally (other than for date formats) employed in a metaphorical sense.

Endianness and hardware

Most computer processors simply store integers as sequences of bytes, so that, conceptually, the encoded value can be obtained by simple concatenation. For an n-byte integer value this allows n! (n factorial) possible representations (one for each byte permutation). The two most common of them are

increasing numeric significance with increasing memory addresses, known as little-endian, and; its opposite, called big-endian.;

Intel x86 processors (and their clones) use the little-endian format (for this reason also called Intel format).

Motorola processors have generally used big-endian. PowerPC (which includes Apple's Macintosh line prior to the Intel switch) and System/370 also adopt big-endian. SPARC historically used big-endian, though version 9 is bi-endian (see below).

Bi-endian hardware

Some architectures —including ARM, PowerPC (but not the PPC970/G5), DEC Alpha, SPARC V9, MIPS, PA-RISC and IA64— feature switchable endianness. That can improve performance or simplify the logic of networking devices and software. The word bi-endian, said of hardware, denotes the capability to compute or pass data in either of two different endian formats (usually big-endian and little-endian). Note that in a bi-endian processor, instruction accesses may still be limited to a single endianness even though data accesses are fully bi-endian.

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