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In computing, a nibble (often nybble or even nyble to match the vowels of byte) is a four-bit aggregation, or half an octet. It is also known as half-byte, quadruple, or tetrade. In a networking or telecommunication context, the nibble is often called a semi-octet or a quartet. As a nibble contains 4 bits, there are sixteen (24) possible values. A nibble can be represented by a single hexadecimal digit and called a "hex digit".
A full byte (octet) is represented by two hexadecimal digits; therefore, it is common to display a byte of information as two nibbles. Sometimes the set of all 256 byte values is represented as a table 16×16, which gives easily readable hexadecimal codes for each value.
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In 2014, David B. Benson, a former professor of Washington State University, remembered that he playfully used (and may have possibly coined) the term nibble as "half a byte" and unit of storage required to hold a BCD decimal digit around 1958, when talking to a programmer of Los Alamos Scientific Laboratory.
Another early recorded use of the term 'nybble' was in 1977 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 centres that used the basic informational unit 'NABBLE'.
The 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 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. For example, a five-byte BCD value of
represents a decimal value of
Historically, there are cases where nybble was used for a group 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 (later, 6-bit) nibbles and loading disk data required the reverse. Note that the term byte once had this ambiguity and meant a set of bits but not necessarily 8, hence the distinction of bytes and octets. 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.
The term 'semi-nibble' is used to refer to a 2-bit collection or half a nibble but rarely so.
Table of nibbles
The sixteen nibbles and their equivalents in other numeral systems:
Low and high nibbles
The terms "low nibble" and "high nibble" are used to denote the nibbles containing, respectively, the less significant bits and the more significant bits within a byte. In graphical representations 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 illustrations the four bits on the left end of the byte form the high nibble, and the remaining four bits form the low nibble.
(Binary to Hexadecimal)
|0000 0100 0010 = 042|
|0010 1010 1001 = 2A9|
|0010 0000 1001 = 209|
|1110 0100 1001 = E49|
|0011 1001 0110 = 396|
|0001 0000 0001 = 101|
|0011 0101 0100 = 354|
|0001 0110 0100 = 164|
Extracting a nibble from a byte
In the C programming language:
#define HI_NIBBLE(b) (((b) >> 4) & 0x0F) #define LO_NIBBLE(b) ((b) & 0x0F)
b must be a variable or constant of an integral data type, and only the least-significant byte of
b is used.
In Common Lisp:
(defun hi-nibble (b) (ldb (byte 4 4) b)) (defun lo-nibble (b) (ldb (byte 4 0) b))
- Hall, D. V. (1980). Microprocessors and Digital Systems. McGraw-Hill. ISBN 0-07-025571-7.