|
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 or tetrade. In a networking or telecommunication context, the nibble is often called a semi-octet, quadbit, or quartet. A nibble has 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. 4-bit computer architectures use groups of four bits as their fundamental unit. Such architectures were used in early microprocessors and pocket calculators and continue to be used in some microcontrollers. ==History== The term 'nibble' originates from its representing 'half a byte', with 'byte' a homophone of the English word 'bite'.〔 In 2014, David B. Benson, a professor emeritus at 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 binary-coded decimal (BCD) decimal digit around 1958, when talking to a programmer of Los Alamos Scientific Laboratory.〔(Comment by David B. Benson on Wikipedia discussion page Talk:Nibble ), July 27, 2014.〕 The alternative spelling 'nybble' reflects the spelling of 'byte', as noted in editorials of ''Kilobaud'' and ''Byte'' in the early 1980s. 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' or 'nibblet' is used to refer to a 2-bit collection or half a nibble but rarely so. 抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』 ■ウィキペディアで「nibble」の詳細全文を読む スポンサード リンク
|