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[1][2] 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 (RFC 791) refer to an 8-bit byte as an octet.[3] Those bits in an octet are usually counted with numbering from 0 to 7 or 7 to 0 depending on the bit endianness.

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.[4][5][6][7] 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, and persisted, in legacy systems, into the twenty-first century. In this era, bit groupings in the instruction stream were often referred to as syllables[a] 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, as 2 to the power of 8 is 256.[8] 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.[9] 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).[10] Internationally, the unit octet explicitly defines a sequence of eight bits, eliminating the potential ambiguity of the term "byte".[11][12] The symbol for octet, 'o', also conveniently eliminates the ambiguity in the symbol 'B' between byte and bel.

Etymology and history

The term byte was coined by Werner Buchholz in June 1956,[4][13][14][b] during the early design phase for the IBM Stretch[15][16][1][13][14][17][18] computer, which had addressing to the bit and variable field length (VFL) instructions with a byte size encoded in the instruction.[13] It is a deliberate respelling of bite to avoid accidental mutation to bit.[1][13][19][c]

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.[20][21] Later on, Schwartz's language JOVIAL actually used the term, but the author recalled vaguely that it was derived from AN/FSQ-31.[22][21]

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.[18] 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[d] used in earlier card punches.[23] The prominence of the System/360 led to the ubiquitous adoption of the eight-bit storage size,[18][16][13] while in detail the EBCDIC and ASCII encoding schemes are different.

In the early 1960s, AT&T introduced digital telephony on long-distance trunk lines. These used the eight-bit μ-law encoding. This large investment promised to reduce transmission costs for eight-bit data.

In Volume 1 of The Art of Computer Programming (first published in 1968), Donald Knuth uses byte in his hypothetical MIX computer to denote a unit which "contains an unspecified amount of information ... capable of holding at least 64 distinct values ... at most 100 distinct values. On a binary computer a byte must therefore be composed of six bits".[24] He notes that "Since 1975 or so, the word byte has come to mean a sequence of precisely eight binary digits...When we speak of bytes in connection with MIX we shall confine ourselves to the former sense of the word, harking back to the days when bytes were not yet standardized."[24]

The development of eight-bit microprocessors in the 1970s popularized this storage size. Microprocessors such as the Intel 8080, the direct predecessor of the 8086, could also perform a small number of operations on the four-bit pairs in a byte, such as the decimal-add-adjust (DAA) instruction. A four-bit quantity is often called a nibble, also nybble, which is conveniently represented by a single hexadecimal digit.

The term octet unambiguously specifies a size of eight bits.[18][12] It is used extensively in protocol definitions.

Historically, the term octad or octade was used to denote eight bits as well at least in Western Europe;[25][26] however, this usage is no longer common. The exact origin of the term is unclear, but it can be found in British, Dutch, and German sources of the 1960s and 1970s, and throughout the documentation of Philips mainframe computers.

Unit symbol

The unit symbol for the byte is specified in IEC 80000-13, IEEE 1541 and the Metric Interchange Format[10] as the upper-case character B.

In the International System of Quantities (ISQ), B is also the symbol of the bel, a unit of logarithmic power ratio named after Alexander Graham Bell, creating a conflict with the IEC specification. However, little danger of confusion exists, because the bel is a rarely used unit. It is used primarily in its decadic fraction, the decibel (dB), for signal strength and sound pressure level measurements, while a unit for one-tenth of a byte, the decibyte, and other fractions, are only used in derived units, such as transmission rates.

The lowercase letter o for octet is defined as the symbol for octet in IEC 80000-13 and is commonly used in languages such as French[27], and is also combined with metric prefixes for multiples, for example ko and Mo.

Multiple-byte units

Multiple-byte units
Decimal Binary
Value SI Value IEC JEDEC
1000 103 kBkilobyte 1024 210 KiBkibibyte KBkilobyte
10002 106 MBmegabyte 10242 220 MiBmebibyte MBmegabyte
10003 109 GBgigabyte 10243 230 GiBgibibyte GBgigabyte
10004 1012 TBterabyte 10244 240 TiBtebibyte TBterabyte
10005 1015 PBpetabyte 10245 250 PiBpebibyte —
10006 1018 EBexabyte 10246 260 EiBexbibyte —
10007 1021 ZBzettabyte 10247 270 ZiBzebibyte —
10008 1024 YByottabyte 10248 280 YiByobibyte —
10009 1027 RBronnabyte 10249 290 RiBrobibyte —
100010 1030 QBquettabyte 102410 2100 QiBquebibyte —
Orders of magnitude of data

Unit multiples of the byte are defined in a metric system based on the powers of 10, following the International System of Units (SI), which defines, for example, the prefix kilo as 1000 (103), as well as a binary system based on powers of two. Historically, the binary system used the identical prefixes of the metric system, but quantified differently. The nomenclature of the latter system has led to confusion. Systems based on powers of 10 use standard SI prefixes (kilo, mega, giga, ...) and their corresponding symbols (k, M, G, ...). The modern binary system uses prefixes kibi, mebi, gibi, etc., and their corresponding symbols (Ki, Mi, Gi, ...).[e] Historical usage for the binary system still uses the prefixes K, M, and G.

While the difference between the decimal and binary interpretations is relatively small for the kilobyte (about 2% smaller than the kibibyte), the systems deviate increasingly as units grow larger (the relative deviation grows by 2.4% for each three orders of magnitude). For example, a power-of-10-based terabyte is about 9% smaller than power-of-2-based tebibyte.

Units based on powers of 10 (SI Prefixes)

Definition of prefixes using powers of 10—in which 1 kilobyte (symbol kB) is defined to equal 1,000 bytes—is recommended by the International Electrotechnical Commission (IEC).[28] The IEC standard defines eight such multiples, up to 1 yottabyte (YB), equal to 10008 bytes.[29] The additional prefixes ronna- for 10009 and quetta- for 100010 were adopted by the International Bureau of Weights and Measures (BIPM) in 2022.[30][31]

This definition is most commonly used for data-rate units in computer networks, internal bus, hard drive and flash media transfer speeds, ISP contracts, and for the capacities of most storage media, particularly hard drives[32] and SSDs, flash-based storage,[33] CDs, DVDs[34] and Blu-rays. Operating systems that use this definition include Android (and derivatives), macOS,[35] iOS,[35], WatchOS, Ubuntu,[36] and Debian[37], ChromeOS, HarmonyOS and the Gnome desktop environment. It is also consistent with the other uses of the SI prefixes in computing, such as CPU clock speeds or measures of performance.

The IBM System 360 and the related disk and tape systems set the byte at 8 bits and documented capacities in decimal units.[38] The early 8-, 5+1⁄4- and 3+1⁄2-inch floppies gave capacities in multiples of 1024, using "KB" rather than the more accurate "KiB". The later, larger, 8-, 5+1⁄4- and 3+1⁄2-inch floppies gave capacities in a hybrid notation, i.e., multiples of 1024,000, using "KB" = 1024 B and "MB" = 1024,000 B. Early 5+1⁄4-inch disks used decimal[dubious – discuss] even though they used 128-byte and 256-byte sectors.[39] Hard disks used mostly 256-byte and then 512-byte before 4096-byte blocks became standard.[40]

Units based on powers of 2 (IEC Prefixes)

A system of units based on powers of 2 in which 1 kibibyte (KiB) is equal to 1,024 (i.e., 210) bytes was created by the IEC to solve the confusion over incorrect usage of SI prefixes[41]. It is defined by international standard IEC 80000-13, which is supported by national and international standards bodies (BIPM, IEC, NIST). The IEC standard defines ten such multiples, up to 1 quebibyte (QiB), equal to 102410 bytes.[42] These unit symbols are rarely used in practice.[43] Notable exceptions are KDE and applications based on the Qt toolkit but the KDE settings app allows switching to SI.[44]

A historic convention of nomenclature for the same units, in which 1 kilobyte (KB) is equal to 1,024 bytes,[45][46][47] 1 megabyte (MB) is equal to 10242 bytes and 1 gigabyte (GB) is equal to 10243 bytes is mentioned by a 1990s JEDEC standard which is used for RAM. Only the first three multiples (up to GB) are mentioned by the JEDEC standard, which makes no mention of TB and larger. These units can only be recognized through comparison using different software. Sometimes the capitalization of the K in KB can be an indicator. While confusing and incorrect,[48] this convention is used by the Microsoft Windows operating system[49] and random-access memory capacity, such as main memory and CPU cache size, and in marketing and billing by some telecommunication companies, such as Vodafone,[50] AT&T,[51] Orange[52] and Telstra.[53] For storage capacity, the historic convention was used by macOS and iOS through Mac OS X 10.5 Leopard and iOS 10, after which they switched to units based on powers of 10.[35]

Parochial units

Various computer vendors have coined terms for data of various sizes, sometimes with different sizes for the same term even within a single vendor. These terms include double word, half word, long word, quad word, slab, superword and syllable. There are also informal terms. e.g., half byte and nybble for 4 bits, octal K for 10008.

History of the conflicting definitions

Percentage difference between decimal and binary interpretations of the unit prefixes grows with increasing storage size

Contemporary[f] computer memory has a binary architecture making a definition of memory units based on powers of 2 most practical. The use of the metric prefix kilo for binary multiples arose as a convenience, because 1024 is approximately 1000.[27] This definition was popular in early decades of personal computing, with products like the Tandon 51⁄4-inch DD floppy format (holding 368640 bytes) being advertised as "360 KB", following the 1024-byte convention. It was not universal, however. The Shugart SA-400 51⁄4-inch floppy disk held 109,375 bytes unformatted,[55] and was advertised as "110 Kbyte", using the 1000 convention.[56] Likewise, the 8-inch DEC RX01 floppy (1975) held 256256 bytes formatted, and was advertised as "256k".[57] Some devices were advertised using a mixture of the two definitions: most notably, floppy disks advertised as "1.44 MB" have an actual capacity of 1440 KiB, the equivalent of 1.47 MB or 1.41 MiB.

In 1995, the International Union of Pure and Applied Chemistry's (IUPAC) Interdivisional Committee on Nomenclature and Symbols attempted to resolve this ambiguity by proposing a set of binary prefixes for the powers of 1024, including kibi (kilobinary), mebi (megabinary), and gibi (gigabinary).[58][59]

In December 1998, the IEC addressed such multiple usages and definitions by adopting the IUPAC's proposed prefixes (kibi, mebi, gibi, etc.) to unambiguously denote powers of 1024.[60] Thus one kibibyte (1 KiB) is 10241 bytes = 1024 bytes, one mebibyte (1 MiB) is 10242 bytes = 1048576 bytes, and so on.

In 1999, Donald Knuth suggested calling the kibibyte a "large kilobyte" (KKB).[61]