Nibble

In computer architecture, this is known as a nibble (sometimes semi-octet, nibble or half-byte b>) to a set of four binary digits (bits) or half an octet.

Its interest is due to the fact that each number in hexadecimal (0, 1, 2,..., 9, A, B, C, D, E, F) can be represented with a quartet, since 2 raised to the 4 is 16 (2⁴=16). Also the nibble is the basis of the BCD coding system.

Architectures that use four bits as their fundamental unit were used for the creation of the first microprocessors and pocket calculators.

Example

The following shows the correspondence between the 16 hexadecimal figures and their corresponding binary representations in the form of a nibble (_hex = hexadecimal, _oct = octal, _dec = decimal):

According to the previous correspondence, it is possible to encode decimal or hexadecimal numbers in BCD as shown in the following examples:

• 0110 1101= 6D (decimal = 109);

• 0001 0001 0010 = 112 (decimal = 274);

• 0101 1001 0001 0000 0111 = 5 9107 (decimal = 364807);

A full byte is represented by two hexadecimal digits, therefore it is common to visualize a byte of information as two nibbles. The nibble is often called a half-octet or nibble in a networking or telecommunications context. In English there is a food pun on nibble (meaning nibble), compared to bite/byte and bit (small piece).

The nibble is used to describe the amount of memory used to store a digit of a number stored in BCD on an IBM mainframe. This technique is used to reduce space requirements, making computation faster and debugging easier. An 8-bit byte is divided into halves and each nibble is used to store one digit. The last nibble of the variable is reserved for the sign. So a variable that can store more than nine digits would be "packaged" in 5 bytes. Easy to debug were numbers that are readable in a hex dump, where two hexadecimal numbers are used to represent the value of a byte, since 16×16 = 2⁸ = 256.

Historically, there have been cases where the term "nybble" has been used for a set of bits less than 8, but not necessarily 4. In the Apple II line, many of the disk control drivers were implemented in software. Writing data to disk was done by converting 256 byte pages into 5 bit arrays, or later into 6 bit nibbles . Data loaded from disk needed the opposite. Note that the term byte also has this ambiguity, in that byte means a set of bits but not necessarily 8.

Today, the terms "byte" and "nibble" generally refer to collections of 8 and 4 bits respectively and are not often used for other sizes. The nibble was also used when the first microprocessors appeared in the early 1970s, since these devices worked with microinstructions which were made up of groups of 4 bits. However, when the commercialization of microprocessors arrived, they could already work with groups of 8 bits and that is how the popularity of the byte began in the field of digital systems. and of computing. In some languages, a nibble is called a tetrade —from the Greek tetra ("four"). This usage reflects the number of bits—four—in a half byte (considering 1 byte = 8 bits).