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| Architectures that follow this rule are called ''little-endian'' and include the ] and ]. | Architectures that follow this rule are called ''little-endian'' and include the ] and ]. | ||
| Some architectures can be configured either way; these include ], ] and ]. | Some architectures can be configured either way; these include ], ] and ]. The word '''bytesexual''', said of hardware, denotes willingness to compute or pass data in either big-endian or little-endian format (depending, presumably, on a mode bit somewhere). | ||
| Still other (generally older) architectures, called ''middle-endian'', may have a more complicated ordering such that the bytes within a 16-bit unit are ordered differently from the 16-bit units within a 32-bit word. | Still other (generally older) architectures, called ''middle-endian'', may have a more complicated ordering such that the bytes within a 16-bit unit are ordered differently from the 16-bit units within a 32-bit word. | ||
Revision as of 13:51, 16 December 2003
When an integer or any other data is represented with multiple bytes, the actual ordering of those bytes in memory, or the sequence in which they are transmitted over some medium, is subject to convention. This is similar to the situation in written languages, where some are written left-to-right, while others are written right-to-left. The convention is called endianness, describing the method either big-endian or little-endian. Endianness is also referred to as byte sex.
Endianness in computers
When some computers store a 32-bit integer value in memory, for example 0xDEADBEEF, they store it as bytes in the following order: DE AD BE EF, that is, most significant byte first (that is to say, most significant byte is stored at the lowest byte address in store within this word).
Architectures that follow this rule are called big-endian and include Motorola 68000 and SPARC.
Other computers store 0xDEADBEEF as EF BE AD DE, that is, least significant byte first. Architectures that follow this rule are called little-endian and include the MOS Technologies 650x and Intel x86.
Some architectures can be configured either way; these include ARM, PowerPC and MIPS. The word bytesexual, said of hardware, denotes willingness to compute or pass data in either big-endian or little-endian format (depending, presumably, on a mode bit somewhere).
Still other (generally older) architectures, called middle-endian, may have a more complicated ordering such that the bytes within a 16-bit unit are ordered differently from the 16-bit units within a 32-bit word.
For instance, BE EF DE AD.
Endianness in communications
The Internet Protocol defines a standard "big-endian" network byte order, where binary values are in general encoded into packets, and sent out over the network, most significant byte first. This occurs regardless of the native endianness of the host CPU.
Serial devices also have bit-endianness: the bits in a byte can be sent little-endian (least significant bit first) or big-endian (most significant bit first). This decision is made in the very bottom of the data link layer of the OSI model.
Discussion
Big-endian numbers are easier to read when debugging a program but less intuitive (because the high byte is at the smaller address); similarly little-endian numbers are more intuitive but harder to debug. The choice of big-endian vs. little-endian for a CPU design has begun a lot of flame wars. Emphasizing the futility of this argument, the very terms big-endian and little-endian were taken from the Big-Endians and Little-Endians of Jonathan Swift's Gulliver's Travels, two peoples in conflict over which end to crack an egg in the voyage to Lilliput and Blefuscu. See the Endian FAQ, including the significant essay "On holy wars and a plea for peace", Danny Cohen 1980.
- Processor families that use big-endian storage: SPARC, Motorola 68000, IBM 370
- Processor families that use little-endian format: x86, VAX
- Processor families that use either (determined by software): MIPS, DEC Alpha, PowerPC
- The PDP family of processors, which were word- rather than byte-addressable, used the unusual pattern of B-A-D-C (that is, byte-swap within words).
NUXI problem
The NUXI problem is the problem of transferring data between computers with differing byte order. For example, the string "UNIX" might look like "NUXI" on a machine with a different "byte sex". The problem is caused by the difference in endianness, for instance. e.g. when transferring data from a little-endian to a big-endian, or vice-versa.
This section was originally based on material from FOLDOC, used with permission.