Nibble

This article is about the information storage unit. For other uses, see Nibble (disambiguation).

A character table ordered by nibbles.

In computing, a nibble (often nybble or even nyble to simulate the spelling of byte) 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.^{[citation needed]} Sometimes the set of all 256 byte values is represented as a table 16×16, which gives easily readable hexadecimal codes for each value.

History

One early recorded use of the term "nybble" was in 1977 within the consumer-banking technology group at Citibank that created a pre-ISO 8583 standard for transactional messages, between cash machines and Citibank's data centers, in which a nybble was the basic informational unit.

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 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.

Table of nibbles

The sixteen nibbles and their equivalents in other numeral systems:

	0hex	=	0dec	=	0oct		0	0	0	0
	1hex	=	1dec	=	1oct		0	0	0	1
	2hex	=	2dec	=	2oct		0	0	1	0
	3hex	=	3dec	=	3oct		0	0	1	1
	4hex	=	4dec	=	4oct		0	1	0	0
	5hex	=	5dec	=	5oct		0	1	0	1
	6hex	=	6dec	=	6oct		0	1	1	0
	7hex	=	7dec	=	7oct		0	1	1	1
	8hex	=	8dec	=	10oct		1	0	0	0
	9hex	=	9dec	=	11oct		1	0	0	1
	Ahex	=	10dec	=	12oct		1	0	1	0
	Bhex	=	11dec	=	13oct		1	0	1	1
	Chex	=	12dec	=	14oct		1	1	0	0
	Dhex	=	13dec	=	15oct		1	1	0	1
	Ehex	=	14dec	=	16oct		1	1	1	0
	Fhex	=	15dec	=	17oct		1	1	1	1

Extracting a nibble from a byte

In C programming language:

#define HINIBBLE(b) (((b) >> 4) & 0x0F)
#define LONIBBLE(b) ((b) & 0x0F)

where b must be variable or constant of an integer data type. (Of course, if b is more than a byte wide, only one of the bytes will be considered).

For example, HINIBBLE(0xAB)==0xA and LONIBBLE(0xAB)==0xB.

In Common Lisp:

(defun hi-nibble (b)
  (ldb (byte 4 4) b))
(defun lo-nibble (b)
  (ldb (byte 4 0) b))

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