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| A '''nibble''' (or |
A '''nibble''' (or more commonly, '''nybble''') is the ] term for a four-] aggregation<ref>{{cite book | ||
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Revision as of 16:26, 26 September 2007
For other uses, see Nibble (disambiguation).A nibble (or more commonly, nybble) is the computing term for a four-bit aggregation, or half an octet (an octet being an 8-bit byte). As a nibble contains 4 bits, there are sixteen (2) 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 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.
The term "nibble" originates from the fact that the term "byte" is a pun on the English word "bite". A nibble is a small bite, which in this context is "humorously" construed as "half a bite". The alternative spelling "nybble" parallels the spelling of "byte", as noted in an editorial Kilobaud or Byte magazines in the early eighties.
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 reduce space requirements, 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 = 2.
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.
In some languages, a nibble is called a tetrade — from the Greek tetra ("four"). This usage reflects the number of bits — four — in half a byte (considering 1 byte = 8 bits).
Trivia
- A related one-liner joke is: Computer programmers don't byte, they nibble a bit
See also
References
- Hall, D. V. (1980). Microprocessors and Digital Systems. McGraw-Hill. ISBN 0-07-025571-7.
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