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The byte is a unit of digital information that most commonly consists of eight bits. Historically, the byte was the number of bits used to encode a single character of text in a computer<ref name=“Buchholz_1962” /><ref name=“Bemer_1959” /> and for this reason it is the smallest addressable unit of memory in many computer architectures. To disambiguate arbitrarily sized bytes from the common 8-bit definition, network protocol documents such as The Internet Protocol (
) refer to an 8-bit byte as an octet.<ref>
</ref> Those bits in an octet are usually counted with numbering from 0 to 7 or 7 to 0 depending on the bit endianness. The first bit is number 0, making the eighth bit number 7.
The size of the byte has historically been hardware-dependent and no definitive standards existed that mandated the size. Sizes from 1 to 48 bits have been used.<ref name=“Buchholz_1956_1” /><ref name=“CDC_1965_3600” /><ref name=“Rao_1989” /><ref name=“Tafel_1971” /> The six-bit character code was an often-used implementation in early encoding systems, and computers using six-bit and nine-bit bytes were common in the 1960s. These systems often had memory words of 12, 18, 24, 30, 36, 48, or 60 bits, corresponding to 2, 3, 4, 5, 6, 8, or 10 six-bit bytes. In this era, bit groupings in the instruction stream were often referred to as syllables
or slab, before the term byte became common.
The modern de facto standard of eight bits, as documented in ISO/IEC 2382-1:1993, is a convenient power of two permitting the binary-encoded values 0 through 255 for one byte—2 to the power of 8 is 256.<ref name=“ISO_IEC_2382-1_1993” /> The international standard IEC 80000-13 codified this common meaning. Many types of applications use information representable in eight or fewer bits and processor designers commonly optimize for this usage. The popularity of major commercial computing architectures has aided in the ubiquitous acceptance of the 8-bit byte.<ref name=“CHM_1964” /> Modern architectures typically use 32- or 64-bit words, built of four or eight bytes, respectively.
The unit symbol for the byte was designated as the upper-case letter B by the International Electrotechnical Commission (IEC) and Institute of Electrical and Electronics Engineers (IEEE).<ref name=“MIXF” /> Internationally, the unit octet, symbol o, explicitly defines a sequence of eight bits, eliminating the potential ambiguity of the term “byte”.<ref name=“TCPIP” /><ref name=“ISO_2382-4” />
Etymology and history
The term byte was coined by Werner Buchholz in June 1956,<ref name=“Buchholz_1956_1” /><ref name=“Buchholz_1977” /><ref name=“Timeline_1956” />
Many sources erroneously indicate a birthday of the term byte in July 1956, but Werner Buchholz claimed that the term would have been coined in June 1956. In fact, the earliest document supporting this dates from 1956-06-11. Buchholz stated that the transition to 8-bit bytes was conceived in August 1956, but the earliest document found using this notion dates from September 1956.}} during the early design phase for the IBM Stretch<ref name=“Buchholz_1956_2” /><ref name=“Buchholz_1956_3” /><ref name=“Buchholz_1962” /><ref name=“Buchholz_1977” /><ref name=“Timeline_1956” /><ref name=“ESR” /><ref name=“Bemer_2000” /> computer, which had addressing to the bit and variable field length (VFL) instructions with a byte size encoded in the instruction.<ref name=“Buchholz_1977” /> It is a deliberate respelling of bite to avoid accidental mutation to bit.<ref name=“Buchholz_1962” /><ref name=“Buchholz_1977” /><ref name=“Blaauw_1959” />
Another origin of byte for bit groups smaller than a computer's word size, and in particular groups of four bits, is on record by Louis G. Dooley, who claimed he coined the term while working with Jules Schwartz and Dick Beeler on an air defense system called SAGE at MIT Lincoln Laboratory in 1956 or 1957, which was jointly developed by Rand, MIT, and IBM.<ref name=“Dooley_1995_Byte” /><ref name=“Ram_Byte” /> Later on, Schwartz's language JOVIAL actually used the term, but the author recalled vaguely that it was derived from AN/FSQ-31.<ref name=“Schwartz_Brooks_ACM” /><ref name=“Ram_Byte” />
Early computers used a variety of four-bit binary-coded decimal (BCD) representations and the six-bit codes for printable graphic patterns common in the U.S. Army (FIELDATA) and Navy. These representations included alphanumeric characters and special graphical symbols. These sets were expanded in 1963 to seven bits of coding, called the American Standard Code for Information Interchange (ASCII) as the Federal Information Processing Standard, which replaced the incompatible teleprinter codes in use by different branches of the U.S. government and universities during the 1960s. ASCII included the distinction of upper- and lowercase alphabets and a set of control characters to facilitate the transmission of written language as well as printing device functions, such as page advance and line feed, and the physical or logical control of data flow over the transmission media.<ref name=“Bemer_2000” /> During the early 1960s, while also active in ASCII standardization, IBM simultaneously introduced in its product line of System/360 the eight-bit Extended Binary Coded Decimal Interchange Code (EBCDIC), an expansion of their six-bit binary-coded decimal (BCDIC) representations
used in earlier card punches.<ref name=“ibm.com” /> The prominence of the System/360 led to the ubiquitous adoption of the eight-bit storage size,<ref name=“Bemer_2000” /><ref name=“Buchholz_1956_3” /><ref name=“Buchholz_1977” /> while in detail the EBCDIC and ASCII encoding schemes are different.
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Bits: Binary Digit = Bit, 1024-bit, 512-bit, 256-bit, 128-bit, 72-bit, 64-bit, 32-bit, 24-bit, 16-bit, 8-bit, 4-bit; Units of information: Metric bit units (kilobit, megabit, gigabit, terabit, petabit, exabit, zettabit, yottabit); IEC bit units (kibibit, mebibit, gibibit, tebibit, pebibit, exbibit, zebibit, yobibit). Byte, Word (computer architecture). (navbar_bits - see also navbar_bytes)
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Bytes: Byte = 8-bits (1 Character), 1024-byte, 512-byte, 256-byte, 128-byte, 64-byte, 32-byte, 16-byte, 8-byte, 4-byte; Units of information: Metric byte units (kilobyte - KB, megabyte - MB, gigabyte - GB, terabyte - TB, petabyte - PB, exabyte, zettabyte - ZB, yottabyte); IEC byte units (kibibyte, mebibyte, gibibyte, tebibyte, pebibyte, exbibyte, zebibyte, yobibyte). Bits, Word (computer architecture). (navbar_bytes - see also navbar_bytes)