Nibble

A character table ordered by nibbles.

In computing, a nibble (often, nybble) is a four-bit aggregation,^[1] or half an octet. As a nibble contains 4 bits, there are sixteen (2⁴) possible values, so a nibble corresponds to a single hexadecimal digit (thus, it is often referred to as a "hex digit" or "hexit").

A full byte (octet) is represented by two hexadecimal digits; therefore, it is common to display a byte of information as two nibbles. The nibble is often called a "semioctet" or a "quartet" in a networking or telecommunication context. Sometimes the set of all 256 byte values is represented as a table 16×16, which gives easily readable hexadecimal codes for each value.

The term "nibble" originates from the fact that the term "byte" is a homophone of the English word "bite". A nibble is a small bite, which in this context is construed as "half a bite". The alternative spelling "nybble" parallels the spelling of "byte", as noted in editorials in Kilobaud and Byte in the early eighties.^{[citation needed]}

The nibble is used to describe the amount of memory used to store a digit of a number stored in packed decimal format within an IBM mainframe. This technique is used to make computations faster and make debugging easier. An 8-bit byte is split in half and each nibble is used to store one digit. The last nibble of the variable is reserved for the sign. Thus a variable which can store up to nine digits would be "packed" into 5 bytes. Ease of debugging resulted from the numbers being readable in a hex dump where two hex numbers are used to represent the value of a byte, as 16×16 = 2⁸.

Historically, there have been cases where the term "nybble" was used for a set of bits fewer than 8, but not necessarily 4. In the Apple II microcomputer line, much of the disk drive control was implemented in software. Writing data to a disk was done by converting 256-byte pages into sets of 5-bit or, later, 6-bit nibbles; loading data from the disk required the reverse. Note that the term byte also had this ambiguity; at one time, byte meant a set of bits but not necessarily 8. Today, the terms "byte" and "nibble" generally refer to 8- and 4-bit collections, respectively, and are not often used for other sizes. The term "semi-nibble" is used to refer to a 2-bit collection, or half a nibble.

The sixteen nibbles and their equivalents in other numeral systems:

Every nibble matches a logic gate (file) (zoom in)

See also

• Binary numeral system
• Computer word

References

1. ^ Hall, D. V. (1980). Microprocessors and Digital Systems. McGraw-Hill. ISBN 0-07-025571-7. 

External links

• Computer Math Forum
• Apple Assembly Line, May 1981, Volume 1, Number 8