For: byte (disambiguation)

A byte (pronounced "bite", IPAEng: baɪt) is the basic unit of measurement of information storage in computer science. In many computer architectures it is a unit of memory addressing, most often consisting of eight bits. A byte is one of the basic integral data types in some programming languages, especially system programming languages.

A byte is an ordered collection of bits, with each bit denoting a single binary value of 1 or 0. The size of a byte can vary and is generally determined by the underlying computer operating system or hardware, although the 8-bit byte is the standard in modern systems. Historically, byte size was determined by the number of bits required to represent a single character from a Western character set. Its size was generally determined by the number of possible characters in the supported character set and was chosen to be a divisor of the computer's word size. Historically bytes have ranged from five to twelve bits.

The popularity of IBM's System/360 architecture starting in the 1960s and the explosion of microcomputers based on 8-bit microprocessors in the 1980s has made eight bits by far the most common size for a byte. The term octet is widely used as a more precise synonym where ambiguity is undesirable (for example, in protocol definitions).

There has been considerable confusion about the meanings of metric -- or SI prefixes -- used with the word "byte", especially concerning prefixes such as kilo- (k or K) and mega- (M) as shown in the chart Prefixes for bit and byte. Since computer memory comes in a Power of two rather than 10, a large portion of the software and computer industry use binary estimates of the SI-prefixed quantities, while producers of computer storage devices prefer the SI values. This is why a computer hard drive advertised with a "100 GB" decimal storage capacity actually contains no more than 93 GB of 8-bit (power of 2) addressable storage. Because of the confusion, a contract specifying a quantity of bytes must define what the prefixes mean in terms of the contract (i.e., the alternative binary equivalents or the actual decimal values, or a binary estimate based on the actual values).

To make the meaning of the table absolutely clear: A kibibyte (KiB) is made up of 1,024 bytes. A mebibyte (MiB) is made up of 1,024 × 1,024 i.e. 1,048,576 bytes. The figures in the column using 1,024 raised to powers of 1, 2, 3, 4 and so on are in units of bytes.

Meanings

The word "byte" has two closely related meanings:

1. A contiguous sequence of a fixed number of bits (binary digits). The use of a byte to mean 8 bits has become nearly ubiquitous.
2. 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. The UNIVAC 1100/2200 series computers (now Unisys) addressed in both 6-bit (Fieldata) and 9-bit (ASCII) modes within its 36-bit word.