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Nibble

In informatica il nibble (chiamato alternativamente anche nyble o nybble per assonanza con il termine byte) è un'unità di misura usata per indicare un gruppo di 4 bit. Il termine è entrato in uso per indicare la metà di un byte a causa dell'omofonia di quest'ultimo con la parola inglese bite, che significa "morso": in inglese, infatti, nibble significa "morsettino".

Utilizzo

Rappresenta comunemente la metà di un byte, ed è per questo che viene indicato anche come "mezzo byte" (half-byte in inglese); vi è corrispondenza biunivoca, a livello di significato numerico, tra un nibble ed una cifra esadecimale, potendo esprimere con 4 bit i valori compresi tra [0000, 1111] cioè [0, 15] ossia [0, F] in esadecimale. Con 2 nibble si possono rappresentare i 256 (16²) possibili valori di un byte.

Storia

Il termine nibble comincia ad essere utilizzato negli anni '50 del XX secolo: sembra che il primo ad utilizzarlo sia stato il professor David B. Benson nel 1958 presso il Los Alamos Scientific Laboratory.[1]

Il nibble era molto più usato in passato, quando erano molto diffuse le CPU a 4 bit. Con un nibble era possibile rappresentare una cifra della codifica BCD o una cifra in codifica "packed decimal" tipica dei mainframe IBM.

Alcune architetture ormai completamente scomparse utilizzavano addirittura dei nibble da 3 bit.

Le unità floppy Disk II degli Apple II lavoravano inizialmente con nibble di 5 o 6 bit mentre nella documentazione di quei computer era spesso indicato il nibble come gruppo di 8 bit. Per evitare fraintendimenti sul numero di bit espressi da un nibble vengono perciò usati oggi altri termini più espliciti come semi-ottetto, quadbit o quartetto.[1]

Nell'assembly dei microcontrollori PICmicro è ancora utilizzata un'istruzione denominata SWAPF che scambia tra di loro i 2 nibble di un registro.[2] Inoltre è presente un flag che nelle operazioni aritmetiche agisce sul riporto del digit (Digit Carry).

Tabella di nibble

I 16 nibble e i loro equivalenti in altri sistemi numerici:

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

Note

  1. ^ a b Nate Williams, What Is a Nibble in Computing, and What Does It Equal?, su history-computer.com. URL consultato l'08/09/2023.
  2. ^ "MPASM/MPLINK PICmicro MCU Quick Chart (PDF), Microchip.

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