Nibble

A nibble uses four digits in binary positional notation (base 2) to encode one of sixteen ($2^4$) possible values, representing the same amount of information stored by two digits in the quaternary numeral system (base 4) or just a single hexadecimal ("hex") digit (base 16). That functional equivalence allows nibbles to typically be represented by a single hex digit, and likewise, byte values by a pair of hex digits.

Four-bit computers use nibble-sized data for storage and operations, in the form of the word unit. Such computers were used in early microprocessors, pocket calculators and pocket computers, and continue to be used in some microcontrollers. In this context, 4-bit groups were sometimes also called characters or digits rather than nibbles.

History

The term nibble originates from its representing half a byte, with byte a homophone of the English word bite. David B. Benson may have coined its use to describe four-bit values as a joke while working as a professor at Washington State University in 1958.

In 1977, an early use of the spelling nybble for the term was recorded within the consumer-banking technology group at Citibank. It created a pre-ISO 8583 standard for transactional messages between ATMs and Citibank's data centers that used the basic data unit nabble.

In the early 1980s, the alternative spelling nybble reflected the spelling of byte, as noted in editorials of Kilobaud Microcomputing and Byte.

Historically, nybble was used in several cases for data units longer than four bits. On the Apple II, much of the disk drive control and group coded recording was implemented in software, and writing data to a disk was done by converting 256-byte pages into sets of 5-bit (later, 6-bit) nibbles whereas loading disk data required the reverse. Moreover, 1982 documentation for the Integrated Woz Machine refers consistently to an "8 bit nibble". The term byte once had the same ambiguity and meant a set of bits but not necessarily 8, hence the distinction of bytes and octets or of nibbles and quartets (or quadbits). Today, the terms byte and nibble almost always refer to 8-bit and 4-bit collections, respectively, and are very rarely used to express any other sizes.