Byte: Encyclopedia - Byte

Byte

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.

Byte - Meanings

The word "byte" has several meanings, all closely related:

1. A contiguous sequence of a fixed number of bits. On modern computers, an eight-bit byte or octet is by far the most common. This was not always the case. Certain older models have used six-, seven-, or nine-bit bytes - for instance on the 36-bit architecture of the PDP-10. Another example of a non eight-bit sequence is the 12-bit slab of the NCR-315. An eight-bit byte can hold 256 possible values (2⁸ = 256) – enough to store an unsigned integer ranging from 0 to 255, a signed integer from -128 to 127, or a character of a seven-bit (such as ASCII) or eight-bit character encoding.
2. A contiguous sequence of bits that comprises a sub-field of a longer sequence known as a word. On some computers it is possible to address bytes of arbitrary length. This usage is reflected, for example, in LDB and DPB assembly instructions for field extraction on a PDP-10, which survive as bytewise operations in Common Lisp; and in the six-bit bytes of the IBM 1401.
3. 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.

Byte - History

The term byte was coined by Werner Buchholz in 1956 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; 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.

Byte - 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. This 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.

Byte - Abbreviation

Byte can be abbreviated to B (E.g. MB means megabyte)

Bit can be abbreviated to b (E.g. Mb means megabit)

French-speaking countries sometimes use "o" for "octet". This is unacceptable in SI because of the risk of confusion with the zero.

Byte - Names for larger units

Note: the names "kilobyte", "megabyte", etc. may be used to mean either the SI or binary multipliers. For further discussion, see Binary prefix.

Category: Units of information

Adapted from the Wikipedia article "Byte", under the G.N U Free Docmentation License. Please also see http://en.wikipedia.org/wiki