Misplaced Pages

This is an old revision of this page, as edited by 213.253.39.186 (talk) at 08:08, 6 March 2002 (defined 'first', mentioned network byte order). The present address (URL) is a permanent link to this revision, which may differ significantly from the current revision.

Endianness refers to a computer architecture's ordering of parts of a unit in memory.

When some computers store the 32-bit integer value 0xDEADBEEF in memory, 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 PowerPC.

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 MOS Technologies 6502 and Intel x86.

Some architectures can be configured either way; these include ARM and MIPS.

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, old Cray supercomputers stored the integer BE EF DE AD. See also NUXI problem.

The Internet Protocol defines a standard "big-endian" network byte order, where binary values are encoded into packets, and sent out over the network, most significant byte first.

Serial devices also have 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.

The terms "big-endian" and "little-endian" are derived from terms used in the voyage to Lilliput and Blefuscu from Gulliver's Travels by "Dr. Lemuel Gulliver" (pen name of Jonathan Swift).

Additional Resources

• Endian FAQ (http://www.rdrop.com/~cary/html/endian_faq.html)