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Incomputing, anibble,[1] or spellednybble to matchbyte, is aunit of information that is an aggregation of four-bits; half of a byte/octet.[1][2][3] The unit is alternatively callednyble,nybl,half-byte[4] ortetrade.[5][6] Innetworking ortelecommunications, the unit is often called asemi-octet,[7]quadbit,[8] orquartet.[9][10]
As a nibble can represent sixteen (24) possible values, a nibble value is often shown as ahexadecimaldigit (hex digit).[11]. A byte is two nibbles, and therefore, a value can be shown as two hex digits.
Four-bit computers use nibble-sized data for storage and operations; as theword unit. Such computers were used in earlymicroprocessors,pocket calculators andpocket computers. They continue to be used in somemicrocontrollers. In this context, 4-bit groups were sometimes also calledcharacters[12] rather than nibbles.[1]
The termnibble originates from its representing "half a byte", withbyte ahomophone of theEnglish wordbite.[4] In 2014, David B. Benson, a professor emeritus atWashington State University, remembered that he playfully used (and may have possibly coined) the termnibble as "half a byte" and unit of storage required to hold abinary-coded decimal (BCD) digit around 1958, when talking to a programmer fromLos Alamos Scientific Laboratory. The alternative spellingnybble reflects the spelling ofbyte, as noted in editorials ofKilobaud andByte in the early 1980s. Another early recorded use of the termnybble was in 1977 within the consumer-banking technology group at Citibank. It created a pre-ISO 8583 standard for transactional messages betweencash machines and Citibank'sdata centers that used the basic data unit 'nabble'.
Nibble is used to describe the amount of memory used to store a digit of a number stored inpacked decimal format (BCD) within an IBM mainframe. This technique is used to make computations faster and debugging easier. An 8-bit byte is split in half and each nibble is used to store one decimal digit. The last (rightmost) 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 ahex dump where twohex numbers are used to represent the value of a byte, as16×16 = 28. For example, a five-byte BCD value of31 41 59 26 5C represents a decimal value of +314159265.
Historically, there are cases wherenybble was used for a group of bits greater than 4. On theApple II, much of the disk drive control andgroup-coded recording was implemented in software. Writing data to a disk was done by converting 256-byte pages into sets of5-bit (later,6-bit) nibbles and loading disk data required the reverse.[13][14][15] Moreover, 1982 documentation for theIntegrated Woz Machine refers consistently to an "8 bit nibble".[16] The termbyte once had the same ambiguity and meant a set of bits but not necessarily 8, hence the distinction ofbytes andoctets or ofnibbles andquartets (orquadbits). Today, the termsbyte andnibble almost always refer to 8-bit and 4-bit collections respectively and are very rarely used to express any other sizes.
A nibble-sized value can be represented in different numeric bases:
| Binary | Decimal | Hexadecimal |
|---|---|---|
| 0000 | 0 | 0 |
| 0001 | 1 | 1 |
| 0010 | 2 | 2 |
| 0011 | 3 | 3 |
| 0100 | 4 | 4 |
| 0101 | 5 | 5 |
| 0110 | 6 | 6 |
| 0111 | 7 | 7 |
| 1000 | 8 | 8 |
| 1001 | 9 | 9 |
| 1010 | 10 | A |
| 1011 | 11 | B |
| 1100 | 12 | C |
| 1101 | 13 | D |
| 1110 | 14 | E |
| 1111 | 15 | F |
The low and high nibbles of a byte are its two halves that are the less and the more significant bits within the byte, respectively. In a graphical representation of bits within a byte, the leftmost bit could represent the most significant bit (MSB), corresponding to ordinary decimal notation in which the digit at the left of a number is the most significant. In such an illustration, the four bits on the left end of the byte form the high nibble, and the remaining four bits form the low nibble.[17] For example,
the high nibble is01102 (616), and the low nibble is00012 (116). The total value ishigh-nibble × 1610 + low-nibble (6 × 16 + 1 = 9710).
[...] The characteristic eightbit field is sometimes referred to as abyte, a four bit field can be referred to as a nibble. [...]
Each of these letters corresponds to one of the integers from zero to fifteen, therefore requiring 4 bits (one "tetrade") in binary representation.
A data symbol represents one quartet (4 bits) of binary data.
Each hex digit (0–f) represents exactly 4 bits.
[...]Bit – The smallest unit of information which can be represented. (A bit may be in one of two states I 0 or 1). [...]Byte – A group of 8 contiguous bits occupying a single memory location. [...]Character – A group of 4 contiguous bits of data. [...](NB. ThisIntel 4004 manual uses the termcharacter referring to4-bit rather than 8-bitdata entities. Intel switched to use the more common termnibble for 4-bit entities in their documentation for the succeeding processor4040 in 1974 already.)