Thebyte is aunit of digital information that most commonly consists of eightbits. Historically, the byte was the number of bits used to encode a singlecharacter of text in a computer[1][2] and for this reason it is the smallestaddressable unit ofmemory in manycomputer architectures. To disambiguate arbitrarily sized bytes from the common8-bit definition,network protocol documents such as theInternet Protocol (RFC791) refer to an 8-bit byte as anoctet.[3] Those bits in an octet are usually counted with numbering from 0 to 7 or 7 to 0 depending on thebit endianness.
The size of the byte has historically beenhardware-dependent and no definitive standards existed that mandated the size. Sizes from 1 to 48 bits have been used.[4][5][6][7] Thesix-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 hadmemory 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 assyllables[a] orslab, before the termbyte became common.
The modernde facto standard of eight bits, as documented in ISO/IEC 2382-1:1993, is a convenientpower of two permitting thebinary-encoded values 0 through 255 for one byte, as 2 to the power of 8 is 256.[8] The international standardIEC 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 termbyte was coined byWerner Buchholz in June 1956,[4][13][14][b] during the early design phase for theIBM 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 ofbite to avoid accidental mutation tobit.[1][13][19][c]
Another origin ofbyte for bit groups smaller than a computer's word size, and in particular groups offour bits, is on record by Louis G. Dooley, who claimed he coined the term while working withJules Schwartz and Dick Beeler on an air defense system calledSAGE atMIT Lincoln Laboratory in 1956 or 1957, which was jointly developed byRand, MIT, and IBM.[20][21] Later on, Schwartz's languageJOVIAL actually used the term, but the author recalled vaguely that it was derived fromAN/FSQ-31.[22][21]
Early computers used a variety of four-bitbinary-coded decimal (BCD) representations and thesix-bit codes for printable graphic patterns common in theU.S. Army (FIELDATA) andNavy. These representations included alphanumeric characters and special graphical symbols. These sets were expanded in 1963 to seven bits of coding, called theAmerican Standard Code for Information Interchange (ASCII) as theFederal 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 ofcontrol 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 ofSystem/360 the eight-bitExtended Binary Coded Decimal Interchange Code (EBCDIC), an expansion of theirsix-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 introduceddigital telephony on long-distancetrunk lines. These used the eight-bitμ-law encoding. This large investment promised to reduce transmission costs for eight-bit data.
In Volume 1 ofThe Art of Computer Programming (first published in 1968),Donald Knuth usesbyte in his hypotheticalMIX computer to denote a unit which "contains anunspecified 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 wordbyte 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 ofeight-bitmicroprocessors in the 1970s popularized this storage size. Microprocessors such as theIntel 8080, the direct predecessor of the8086, could also perform a small number of operations on thefour-bit pairs in a byte, such as the decimal-add-adjust (DAA) instruction. A four-bit quantity is often called anibble, alsonybble, which is conveniently represented by a singlehexadecimal digit.
The termoctet unambiguously specifies a size of eight bits.[18][12] It is used extensively inprotocol definitions.
Historically, the termoctad oroctade 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 ofPhilips mainframe computers.
In theInternational System of Quantities (ISQ), B is also the symbol of thebel, a unit of logarithmic power ratio named afterAlexander 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, thedecibel (dB), forsignal strength andsound 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 foroctet is defined as the symbol for octet in IEC 80000-13 and is commonly used in languages such asFrench[27] andRomanian, and is also combined with metric prefixes for multiples, for example ko and Mo.
More than one system exists to defineunit multiples based on the byte. Some systems are based onpowers of 10, following theInternational System of Units (SI), which defines for example the prefixkilo as 1000 (103); other systems are based onpowers of two. Nomenclature for these systems has led to confusion. Systems based on powers of 10 use standardSI prefixes (kilo,mega,giga, ...) and their corresponding symbols (k, M, G, ...). Systems based on powers of 2, however, might use binary prefixes (kibi,mebi,gibi, ...) and their corresponding symbols (Ki, Mi, Gi, ...) or they might use the prefixes K, M, and G, creating ambiguity when the prefixes M or G are used.
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.
Definition of prefixes using powers of 10—in which 1kilobyte (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 prefixesronna- for 10009 andquetta- for 100010 were adopted by theInternational Bureau of Weights and Measures (BIPM) in 2022.[30][31]
Prior art, the IBM System 360 and the related tape systems set the byte at 8 bits.[citation needed] Early 5.25" disks used decimal[dubious –discuss] even though they used 128 byte and 256 byte sectors.[citation needed] Hard disks used mostly 256 byte and then 512 byte before 4096 byte blocks became standard.[citation needed] RAM was always sold in powers of 2.[citation needed]
A system of units based onpowers of 2 in which 1 kibibyte (KiB) is equal to 1,024 (i.e., 210) bytes is defined by international standard IEC 80000-13 and is supported by national and international standards bodies (BIPM,IEC,NIST). The IEC standard defines eight such multiples, up to 1 yobibyte (YiB), equal to 10248 bytes. The natural binary counterparts toronna- andquetta- were given in a consultation paper of the International Committee for Weights and Measures' Consultative Committee for Units (CCU) asrobi- (Ri, 10249) andquebi- (Qi, 102410), but have not yet been adopted by the IEC or ISO.[37]
An alternative system of nomenclature for the same units (referred to here as thecustomary convention), in which 1kilobyte (KB) is equal to 1,024 bytes,[38][39][40] 1megabyte (MB) is equal to 10242 bytes and 1gigabyte (GB) is equal to 10243 bytes is mentioned by a 1990sJEDEC standard. Only the first three multiples (up to GB) are mentioned by the JEDEC standard, which makes no mention of TB and larger. While confusing and incorrect,[41] the customary convention is used by theMicrosoft Windows operating system[42][better source needed] andrandom-access memory capacity, such as main memory andCPU cache size, and in marketing and billing by telecommunication companies, such asVodafone,[43]AT&T,[44]Orange[45] andTelstra.[46]
Forstorage capacity, the customary 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.[34]
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 includedouble word,half word,long word,quad word,slab,superword andsyllable. There are also informal terms. e.g.,half byte andnybble for 4 bits,octal K for 10008.
Percentage difference between decimal and binary interpretations of the unit prefixes grows with increasing storage size
Contemporary[e] computer memory has abinary architecture making a definition of memory units based on powers of 2 most practical. The use of the metric prefixkilo for binary multiples arose as a convenience, because1024 is approximately1000.[27] This definition was popular in early decades ofpersonal computing, with products like theTandon 51⁄4-inchDD floppy format (holding368640 bytes) being advertised as "360 KB", following the1024-byte convention. It was not universal, however. TheShugart SA-400 51⁄4-inchfloppy disk held 109,375 bytes unformatted,[47] and was advertised as "110 Kbyte", using the 1000 convention.[48] Likewise, the 8-inchDEC RX01 floppy (1975) held256256 bytes formatted, and was advertised as "256k".[49] Some devices were advertised using amixture of the two definitions: most notably, floppy disks advertised as "1.44 MB" have an actual capacity of1440 KiB, the equivalent of 1.47 MB or 1.41 MiB.
In 1995, theInternational Union of Pure and Applied Chemistry's (IUPAC) Interdivisional Committee on Nomenclature and Symbols attempted to resolve this ambiguity by proposing a set ofbinary prefixes for the powers of 1024, including kibi (kilobinary), mebi (megabinary), and gibi (gigabinary).[50][51]
In December 1998, theIEC addressed such multiple usages and definitions by adopting the IUPAC's proposed prefixes (kibi, mebi, gibi, etc.) to unambiguously denote powers of 1024.[52] 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).[53]
The IEC adopted the IUPAC proposal and published the standard in January 1999.[54][55] The IEC prefixes are part of theInternational System of Quantities. The IEC further specified that the kilobyte should only be used to refer to1000 bytes.[56]
Lawsuits arising from alleged consumer confusion over the binary and decimal definitions of multiples of the byte have generally ended in favor of the manufacturers, with courts holding that the legal definition of gigabyte or GB is 1 GB =1000000000 (109) bytes (the decimal definition), rather than the binary definition (230, i.e.,1073741824). Specifically, the United States District Court for the Northern District of California held that "the U.S. Congress has deemed the decimal definition of gigabyte to be the 'preferred' one for the purposes of 'U.S. trade and commerce' [...] The California Legislature has likewise adopted the decimal system for all 'transactions in this state.'"[57]
Earlier lawsuits had ended in settlement with no court ruling on the question, such as a lawsuit against drive manufacturerWestern Digital.[58][59] Western Digital settled the challenge and added explicit disclaimers to products that the usable capacity may differ from the advertised capacity.[58] Seagate was sued on similar grounds and also settled.[58][60]
TheC andC++ programming languages definebyte as an "addressable unit of data storage large enough to hold any member of the basic character set of the execution environment" (clause 3.6 of the C standard). The C standard requires that the integral data typeunsigned char must hold at least 256 different values, and is represented by at least eight bits (clause 5.2.4.2.1). Various implementations of C and C++ reserve 8, 9, 16, 32, or 36 bits for the storage of a byte.[67][68][f] In addition, the C and C++ standards require that there be no gaps between two bytes. This means every bit in memory is part of a byte.[69]
Java's primitive data typebyte is defined as eight bits. It is a signed data type, holding values from −128 to 127.
.NET programming languages, such asC#, definebyte as an unsigned type, and thesbyte as a signed data type, holding values from 0 to 255, and−128 to 127, respectively.
In data transmission systems, the byte is used as a contiguous sequence of bits in a serial data stream, representing the smallest distinguished unit of data. Forasynchronous communication a full transmission unit usually additionally includes a start bit, 1 or 2 stop bits, and possibly aparity bit, and thus its size may vary from seven to twelve bits for five to eight bits of actual data.[70] Forsynchronous communication the error checking usually uses bytes at the end of aframe.
^The term syllable was used for bytes containing instructions or constituents of instructions, not for data bytes.
^Many sources erroneously indicate a birthday of the termbyte in July 1956, butWerner Buchholz claimed that the term would have been coined inJune 1956. In fact, theearliest document supporting this dates from 1956-06-11. Buchholz stated that the transition to 8-bit bytes was conceived inAugust 1956, but the earliest document found using this notion dates fromSeptember 1956.
^Some later machines, e.g.,Burroughs B1700,CDC 3600, DEC PDP-6,DEC PDP-10 had the ability to operate on arbitrary bytes no larger than the word size.
Terms used here to describe the structure imposed by the machine design, in addition tobit, are listed below. Byte denotes a group of bits used to encode a character, or the number of bits transmitted in parallel to and from input-output units. A term other thancharacter is used here because a given character may be represented in different applications by more than one code, and different codes may use different numbers of bits (i.e., different byte sizes). In input-output transmission the grouping of bits may be completely arbitrary and have no relation to actual characters. (The term is coined frombite, but respelled to avoid accidental mutation tobit.) Aword consists of the number of data bits transmitted in parallel from or to memory in one memory cycle.Word size is thus defined as a structural property of the memory. (The termcatena was coined for this purpose by the designers of theBullGAMMA 60 [fr] computer.) Block refers to the number of words transmitted to or from an input-output unit in response to a single input-output instruction. Block size is a structural property of an input-output unit; it may have been fixed by the design or left to be varied by the program.
[...] Most important, from the point of view of editing, will be the ability to handle any characters or digits, from 1 to 6 bits long. Figure 2 shows the Shift Matrix to be used to convert a 60-bitword, coming from Memory in parallel, intocharacters, or 'bytes' as we have called them, to be sent to theAdder serially. The 60 bits are dumped intomagnetic cores on six different levels. Thus, if a 1 comes out of position 9, it appears in all six cores underneath. Pulsing any diagonal line will send the six bits stored along that line to the Adder. The Adder may accept all or only some of the bits. Assume that it is desired to operate on 4 bitdecimal digits, starting at the right. The 0-diagonal is pulsed first, sending out the six bits 0 to 5, of which the Adder accepts only the first four (0-3). Bits 4 and 5 are ignored. Next, the 4 diagonal is pulsed. This sends out bits 4 to 9, of which the last two are again ignored, and so on. It is just as easy to use all six bits inalphanumeric work, or to handle bytes of only one bit for logical analysis, or to offset the bytes by any number of bits. All this can be done by pulling the appropriate shift diagonals. An analogous matrix arrangement is used to change from serial to parallel operation at the output of the adder. [...]
^Tafel, Hans Jörg (1971).Einführung in die digitale Datenverarbeitung [Introduction to digital information processing] (in German). Munich:Carl Hanser Verlag. p. 300.ISBN3-446-10569-7.Byte = zusammengehörige Folge von i.a. neun Bits; davon sind acht Datenbits, das neunte ein Prüfbit NB. Defines a byte as a group of typically 9 bits; 8 data bits plus 1 parity bit.
^ISO/IEC 2382-1: 1993, Information technology - Vocabulary - Part 1: Fundamental terms. 1993.
byte: A string that consists of a number of bits, treated as a unit, and usually representing a character or a part of a character. NOTES: 1 The number of bits in a byte is fixed for a given data processing system. 2 The number of bits in a byte is usually 8.
We received the following from W Buchholz, one of the individuals who was working on IBM's Project Stretch in the mid 1950s. His letter tells the story.
Not being a regular reader of your magazine, I heard about the question in the November 1976 issue regarding the origin of the term "byte" from a colleague who knew that I had perpetrated this piece of jargon[see page 77 of November 1976 BYTE, "Olde Englishe"]. I searched my files and could not locate a birth certificate. But I am sure that "byte" is coming of age in 1977 with its 21st birthday. Many have assumed that byte, meaning 8 bits, originated with the IBM System/360, which spread such bytes far and wide in the mid-1960s. The editor is correct in pointing out that the term goes back to the earlier Stretch computer (but incorrect in that Stretch was the first, not the last, of IBM's second-generation transistorized computers to be developed). The first reference found in the files was contained in an internal memo written in June 1956 during the early days of developingStretch. A byte was described as consisting of any number of parallel bits from one to six. Thus a byte was assumed to have a length appropriate for the occasion. Its first use was in the context of the input-output equipment of the 1950s, which handled six bits at a time. The possibility of going to 8-bit bytes was considered inAugust 1956 and incorporated in the design of Stretchshortly thereafter. The first published reference to the term occurred in 1959 in a paper 'Processing Data in Bits and Pieces' byG A Blaauw,F P Brooks Jr andW Buchholz in theIRE Transactions on Electronic Computers, June 1959, page 121. The notions of that paper were elaborated inChapter 4 ofPlanning a Computer System (Project Stretch), edited by W Buchholz, McGraw-Hill Book Company (1962). The rationale for coining the term was explained there on page 40 as follows: Bytedenotes a group of bits used to encode a character, or the number of bits transmitted in parallel to and from input-output units. A term other than characteris used here because a given character may be represented in different applications by more than one code, and different codes may use different numbers of bits (ie, different byte sizes). In input-output transmission the grouping of bits may be completely arbitrary and have no relation to actual characters. (The term is coined frombite,but respelled to avoid accidental mutation to bit.) System/360 took over many of the Stretch concepts, including the basic byte and word sizes, which are powers of 2. For economy, however, the byte size was fixed at the 8 bit maximum, and addressing at the bit level was replaced by byte addressing. Since then the term byte has generally meant 8 bits, and it has thus passed into the general vocabulary. Are there any other terms coined especially for the computer field which have found their way into general dictionaries of English language?
1956 Summer:Gerrit Blaauw,Fred Brooks,Werner Buchholz,John Cocke and Jim Pomerene join theStretch team. Lloyd Hunter providestransistor leadership. 1956 July [sic]: In a report Werner Buchholz lists the advantages of a 64-bit word length for Stretch. It also supportsNSA's requirement for 8-bit bytes. Werner's term "Byte" first popularized in this memo.
NB. This timeline erroneously specifies the birth date of the term "byte" asJuly 1956, while Buchholz actually used the term as early asJune 1956.
[...] 60 is a multiple of 1, 2, 3, 4, 5, and 6. Hence bytes of length from 1 to 6 bits can be packed efficiently into a 60-bitword without having to split a byte between one word and the next. If longer bytes were needed, 60 bits would, of course, no longer be ideal. With present applications, 1, 4, and 6 bits are the really important cases. With 64-bit words, it would often be necessary to make some compromises, such as leaving 4 bits unused in a word when dealing with 6-bit bytes at the input and output. However, the LINK Computer can be equipped to edit out these gaps and to permit handling of bytes which are split between words. [...]
[...] The maximum input-output byte size for serial operation will now be 8 bits, not counting any error detection and correction bits. Thus, the Exchange will operate on an 8-bit byte basis, and any input-output units with less than 8 bits per byte will leave the remaining bits blank. The resultant gaps can be edited out later by programming [...]
I came to work forIBM, and saw all the confusion caused by the 64-character limitation. Especially when we started to think about word processing, which would require both upper and lower case. Add 26 lower case letters to 47 existing, and one got 73 -- 9 more than 6 bits could represent. I even made a proposal (in view ofSTRETCH, the very first computer I know of with an 8-bit byte) that would extend the number ofpunch card character codes to 256[1]. Some folks took it seriously. I thought of it as a spoof. So some folks started thinking about 7-bit characters, but this was ridiculous. With IBM's STRETCH computer as background, handling 64-character words divisible into groups of 8 (I designed the character set for it, under the guidance of Dr.Werner Buchholz, the man who DID coin the term "byte" for an 8-bit grouping). [2] It seemed reasonable to make a universal 8-bit character set, handling up to 256. In those days my mantra was "powers of 2 are magic". And so the group I headed developed and justified such a proposal [3]. That was a little too much progress when presented to the standards group that was to formalize ASCII, so they stopped short for the moment with a 7-bit set, or else an 8-bit set with the upper half left for future work. TheIBM 360 used 8-bit characters, although not ASCII directly. Thus Buchholz's "byte" caught on everywhere. I myself did not like the name for many reasons. The design had 8 bits moving around in parallel. But then came a new IBM part, with 9 bits for self-checking, both inside the CPU and in thetape drives. I exposed this 9-bit byte to the press in 1973. But long before that, when I headed software operations forCie. Bull in France in 1965-66, I insisted that 'byte' be deprecated in favor of "octet". You can notice that my preference then is now the preferred term. It is justified by new communications methods that can carry 16, 32, 64, and even 128 bits in parallel. But some foolish people now refer to a "16-bit byte" because of this parallel transfer, which is visible in theUNICODE set. I'm not sure, but maybe this should be called a "hextet". But you will notice that I am still correct. Powers of 2 are still magic!
The word byte was coined around 1956 to 1957 atMIT Lincoln Laboratories within a project calledSAGE (the North American Air Defense System), which was jointly developed byRand, Lincoln Labs, andIBM. In that era, computer memory structure was already defined in terms ofword size. A word consisted of x number ofbits; a bit represented a binary notational position in a word. Operations typically operated on all the bits in the full word. We coined the word byte to refer to a logical set of bits less than a full word size. At that time, it was not defined specifically as x bits but typically referred to as a set of4 bits, as that was the size of most of our coded data items. Shortly afterward, I went on to other responsibilities that removed me from SAGE. After having spent many years in Asia, I returned to the U.S. and was bemused to find out that the word byte was being used in the new microcomputer technology to refer to the basic addressable memory unit.
A question-and-answer session at anACM conference on the history of programming languages included this exchange:
[John Goodenough: You mentioned that the term "byte" is used inJOVIAL. Where did the term come from? ] [Jules Schwartz (inventor of JOVIAL): As I recall, theAN/FSQ-31, a totally different computer than the709, was byte oriented. I don't recall for sure, but I'm reasonably certain the description of that computer included the word "byte," and we used it. ] [Fred Brooks: May I speak to that?Werner Buchholz coined the word as part of the definition ofSTRETCH, and the AN/FSQ-31 picked it up from STRETCH, but Werner is very definitely the author of that word. ] [ Schwartz: That's right. Thank you. ]
^abKnuth, Donald (1997) [1968].The Art of Computer Programming: Volume 1: Fundamental Algorithms (3rd ed.). Boston: Addison-Wesley. p. 125.ISBN9780201896831.
^Brown, Richard J. C. (27 April 2022). "Reply to "Facing a shortage of the Latin letters for the prospective new SI symbols: alternative proposal for the new SI prefixes"".Accreditation and Quality Assurance.27 (3):143–144.doi:10.1007/s00769-022-01499-7.S2CID248397680.
^Amendment 2 to IEC International Standard IEC 60027-2: Letter symbols to be used in electrical technology – Part 2: Telecommunications and electronics.
^Barrow, Bruce (January 1997)."A Lesson in Megabytes"(PDF).IEEE. p. 5. Archived fromthe original(PDF) on 17 December 2005. Retrieved14 December 2024.
