Nibble

The term nibble originates from its representing half a byte, with byte a homophone of the English word bite.^[4]

David B. Benson, at the time a Washington State University professor, might have originated the term nibble in 1958 as a joke.^[13]

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 cash machines and Citibank's data centers that used the basic data unit nabble.^{[citation needed]}

In the early 1980s, the alternative spelling nybble reflected the spelling of byte, as noted in editorials of Kilobaud and Byte.^{[citation needed]}

Historically, nybble was used in many cases for a group of bits greater than 4. On the Apple II, much of the disk drive control and group-coded recording was implemented in software. Writing data to a disk was done by converting 256-byte pages into sets of 5-bit (later, 6-bit) nibbles and loading disk data required the reverse.^[14][15][16] Moreover, 1982 documentation for the Integrated Woz Machine refers consistently to an "8 bit nibble".^[17] 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.^{[citation needed]}

Nibble is used to describe the amount of memory used to store a digit of a number stored in packed 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 that 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. For example, a five-byte BCD value of 31 41 59 26 5C represents a decimal value of +314159265.^{[citation needed]}

Packed nibbles can also describe binary numbers. The low and high nibbles of a byte are its two halves, which are the least and the most significant bits within the byte, respectively.^[18] For example in binary,

ninety-seven = 97₁₀ = (0110 0001)₂

the high nibble is 0110₂ (6₁₆), and the low nibble is 0001₂. The total value is high-nibble × 16₁₀ + low-nibble (6 × 16 + 1 = 97₁₀).

A nibble-sized value can be represented in different numeric bases:

• Binary number – Number expressed in the base-2 numeral system
• Syllable – Platform-specific data size used for some historical digital hardware
• Word – Base memory unit handled by a computer