BYTE

For the computer industry magazine, see Byte (magazine).

Quantities of bytes
Popular use and (SI standard meaning)			Binary prefix standards from IEC 60027-2
Name	Symbol	Quantity	Name	Symbol	Quantity
kilobyte	kB	2¹⁰ (10³)	kibibyte	KiB	2¹⁰
megabyte	MB	2²⁰ (10⁶)	mebibyte	MiB	2²⁰
gigabyte	GB	2³⁰ (10⁹)	gibibyte	GiB	2³⁰
terabyte	TB	2⁴⁰ (10¹²)	tebibyte	TiB	2⁴⁰
petabyte	PB	2⁵⁰ (10¹⁵)	pebibyte	PiB	2⁵⁰
exabyte	EB	2⁶⁰ (10¹⁸)	exbibyte	EiB	2⁶⁰
zettabyte	ZB	2⁷⁰ (10²¹)	zebibyte	ZiB	2⁷⁰
yottabyte	YB	2⁸⁰ (10²⁴)	yobibyte	YiB	2⁸⁰

A byte is commonly used as a unit of storage measurement in computers, regardless of the type of data being stored. It is also one of the basic integral data types in many programming languages.

Contents

1 Meanings
2 History
3 Alternate words
4 Abbreviation/Symbol
5 Names for larger units

Meanings

The word "byte" has numerous closely related meanings:

A contiguous sequence of a fixed number of bits (binary digits). In recent years, the use of a byte to mean 8 bits is nearly ubiquitous.
A contiguous sequence of bits within a binary computer, that comprises the smallest addressable sub-field of the computer's natural word-size. That is, the smallest unit of binary data on which meaningful computation, or natural data boundaries, could be applied. For example, the CDC 6000 series scientific mainframes divided their 60-bit floating-point words into 10 six-bit bytes. These bytes conveniently held Hollerith data from punched cards, typically the upper-case alphabet and decimal digits. CDC also often referred to 12-bit quantities as bytes, each holding two 6-bit display code characters, due to the 12-bit I/O architecture of the machine. The PDP-10 used assembly instructions LDB and DPB to extract bytes—these operations survive today in Common Lisp. Bytes of six, seven, or nine bits were used on some computers, for example within the 36-bit word of the PDP-10.
A contiguous sequence of binary bits in a serial data stream, such as in modem or satellite communications, or from a disk-drive head, which is the smallest meaningful unit of data. These bytes might include start bits, stop bits, or parity bits, and thus could vary from 7 to 12 bits to contain a single 7-bit ASCII code.
A datatype or synonym for a datatype in certain programming languages. C, for example, defines byte as a storage unit capable of at least being large enough to hold any character of the execution environment (clause 3.5 of the C standard). Since the C char integral data type can hold at least 8 bits (clause 5.2.4.2.1), a byte in C is at least capable of holding 256 different values (signed or unsigned char doesn't matter). Java's primitive byte data type is always defined as consisting of 8 bits and being a signed data type, holding values from -128 to 127.

The term "byte" came from "bite," as in the smallest amount of data a computer could "bite" at once. The spelling change not only reduced the chance of a "bite" being mistaken for a "bit," but also was consistent with the penchant of early computer scientists to make up words and change spellings.

Early microprocessors, such as Intel's 8008 (the direct predecessor of the 8080, and then the Pentium) could perform a small number of operations on four bits, such as the DAA (decimal adjust) instruction, and the "half carry" flag, that were used to implement decimal arithmetic routines. These four-bit quantities were called "nibbles," in homage to the then-common 8-bit "bytes."

History

The term byte was coined by Werner Buchholz in 1957 during the early design phase for the IBM Stretch computer. Originally it was defined in instructions by a 4-bit byte-size field, allowing from one to sixteen bits (the production design reduced this to a 3-bit byte-size field, allowing from one to eight bits in a byte); typical I/O equipment of the period used six-bit units. A fixed eight-bit byte size was later adopted and promulgated as a standard by the System/360. The word was coined by mutating the word bite so it would not be accidentally misspelled as bit. However back in the sixties the luminaries at IBM Education Department in the UK were teaching that a bit was a Binary digIT and a byte was a BinarY TuplE (from n-tuple, i.e. [quin]tuple, [sex]tuple, [sep]tuple, [oc]tuple ...). A byte was also often referred to as "an 8-bit byte" reinforcing the notion that it was a tuple of n bits, and that other sizes were possible.

Alternate words

The eight-bit byte is often called an octet in formal contexts such as industry standards, as well as in networking and telecommunication, in order to avoid any confusion about the number of bits involved. However, 8-bit bytes are now firmly embedded in such common standards as Ethernet and HTML. Octet is also the word used for the eight-bit quantity in many non-English languages, where the pun on bite does not translate.

Half of an eight-bit byte (four bits) is sometimes called a nibble (sometimes spelled nybble) or a hex digit. The nibble is often called a semioctet in a networking or telecommunication context and also by some standards organizations.

Other sources have also said that the word, Byte comes from the following: BinarY TablE

Abbreviation/Symbol

IEEE 1541 and [Metric-Interchange-Format] specify "B" as the symbol for byte (e.g. MB means megabyte), whilst IEC 60027 seems silent on the subject. Furthermore, B means bel (see decibel), another (logarithmic) unit used in the same field.

IEEE 1541 specifies "b" as the symbol for bit; however the IEC 60027 and Metric-Interchange-Format specify "bit" (e.g. Mbit for megabit) for the symbol, achieving maximum disambiguation from byte.

French-speaking countries sometimes use an uppercase "o" for "octet". This is not allowed in SI because of the risk of confusion with the zero and the convention that capitals are reserved for unit names derived from proper names, e.g., A=ampere, J=joule; s=second, m=metre.

Therefore, lowercase 'o' is good, and already in use with multiples ko, Mo Octet (computing).

Names for larger units

The prefixes used for byte measurements are usually written the same as the SI prefixes used for other measurements, but they often have somewhat different values, powers of 1024 rather than 1000, as illustrated in the table below. See binary prefix for further discussion of this difference.

Prefix Name SI Meaning Binary meaning Size difference
m milli 10^-3 = 0.001 n/a n/a
k kilo 10³ = 1000 2¹⁰ = 1024 2.40%
M mega 10⁶ = 1000² 2²⁰ = 1024² 4.86%
G giga 10⁹ = 1000³ 2³⁰ = 1024³ 7.37%
T tera 10¹² = 1000⁴ 2⁴⁰ = 1024⁴ 9.95%
P peta 10¹⁵ = 1000⁵ 2⁵⁰ = 1024⁵ 12.59%

Prefix	Name	SI Meaning	Binary meaning	Size difference
m	milli	10^-3 = 0.001	n/a	n/a
k	kilo	10³ = 1000	2¹⁰ = 1024	2.40%
M	mega	10⁶ = 1000²	2²⁰ = 1024²	4.86%
G	giga	10⁹ = 1000³	2³⁰ = 1024³	7.37%
T	tera	10¹² = 1000⁴	2⁴⁰ = 1024⁴	9.95%
P	peta	10¹⁵ = 1000⁵	2⁵⁰ = 1024⁵	12.59%

Other allowed physical SI prefixes (e.g. those that shrink) make no sense for theoretical units like the byte, as the "milli" example above shows.

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