Octet (computing)
An octet ( octet English ) is in the computer science and digital technology, the term for an orderly compilation ( a tuple ) of 8 bits.
History and Background
Since there is always one octet of 8 bits, it may therefore constitute 28 = 256 different states. The term is often used interchangeably with byte. In standards for data transmission ( IETF, ITU -T) is the term commonly used.
Not to be mistaken for the octet with a sign at the so-called multi -byte characters may well consist of multiple octets.
The foundation for the current standard requires that a byte consists of eight bits, and that the smallest unit of memory addressing is such a byte has been determined by the Company with the IBM S/360-Architektur April 7, 1964.
As a way of measuring the amount of data used by the derived unit of bit byte, with the proviso that here one byte as 8 bits is defined. Especially in standards or in your communications ( often in the stop bits, etc. to come) but is often used the more accurate term octet.
In computers often individual octets can be addressed in the main memory, but also the addressing of individual bits, half octet (nibbles ), double octets (16 bits) and the larger groups is possible. Today, 32 - and 64- bit machines available can the 32 or 64-bit ( four or eight octets ) addressed jointly.
An octet may represent 28 = 256 different values , which can be interpreted in different ways. One can, for example, is an unsigned integer value in the range from 0 to 255, a signed integer value in the range from -127 to 127 or -128 to 127 (depending on the encoding of negative numbers ) or a character or more store characters. All these data types are only different interpretations of the same bit pattern. For example corresponds to an octet with integer value 65 in the ASCII coding the character " A". In addition to the common ASCII exist even proprietary character encodings such as EBCDIC, which is used on IBM mainframes.
Often the values of octets in hexadecimal notation are given, for example, the decimal value " 65 " as " 4116 ", " 0x41 ", " $ 41 " or " 41h " to the last three spellings indicate that it is a hexadecimal value. Sometimes the value of an octet is specified as a binary number, then is usually left the bit with the highest value (most significant bit, MSB) and the right with the lowest (least significant bit, LSB). The count bit positions usually starts with 0, ie, for example, " the third bit " is " Bit 2 " and stands at the third position from the right.
Example for the notation of the value 65 (binary 01000001 ), bit positions and calculate the decimal:
Value of 128 64 32 16 8 4 2 1 Bit number 7 6 5 4 3 2 1 0 0 1 0 0 0 0 0 1 An octet may be 4 bits each, divided into two half-bytes, known as nibbles. If we write it in hexadecimal notation, this corresponds to a nibble of each hexadecimal digit.
According to the international standard IEC 60027-2, Chapter 3.8.2, a byte is an octet of bits, ie, contains exactly 8 bits.
Alternative use
In some cases, a tuple of 3 bits will be referred to as an octet, which is related to the fact that with 3 bits, eight different values , so an octal number representable. This designation was used for example for the three access permission bits (read, write, execute ) in Unix-like file systems (see chmod ). In recent descriptions of the term chmod but not used to avoid confusion with an 8 -bit octets.