Bit numbering
Encyclopedia
In computing
, bit numbering (or sometimes bit endianness
) is the convention used to identify the bit
positions in a binary number
or a container for such a value. The bit number starts with zero and is incremented by one for each subsequent bit position.
When the bit numbering starts at zero for the least significant bit
the numbering scheme is called "LSB 0". This bit numbering method has the advantage that for any unsigned integral data type the value of the number can be calculated by using exponentiation with the bit number and a base
of 2.
Similarly, when the bit numbering starts at zero for the most significant bit
the numbering scheme is called "MSB 0".
CPUs usually employ "LSB 0" bit numbering, however both bit numbering conventions can be seen in big-endian
machines. Some architectures like SPARC
and Motorola 68000
use "LSB 0" bit numbering, while S/390, PowerPC and PA-RISC
use "MSB 0".
The recommended style for Request for Comments
documents is "MSB 0" bit numbering.
Bit numbering is usually transparent to the software, but some programming languages like Ada
allow specifying the appropriate bit order for data type representation.
Computing
Computing is usually defined as the activity of using and improving computer hardware and software. It is the computer-specific part of information technology...
, bit numbering (or sometimes bit endianness
Endianness
In computing, the term endian or endianness refers to the ordering of individually addressable sub-components within the representation of a larger data item as stored in external memory . Each sub-component in the representation has a unique degree of significance, like the place value of digits...
) is the convention used to identify the bit
Bit
A bit is the basic unit of information in computing and telecommunications; it is the amount of information stored by a digital device or other physical system that exists in one of two possible distinct states...
positions in a binary number
Binary numeral system
The binary numeral system, or base-2 number system, represents numeric values using two symbols, 0 and 1. More specifically, the usual base-2 system is a positional notation with a radix of 2...
or a container for such a value. The bit number starts with zero and is incremented by one for each subsequent bit position.
LSB 0 bit numbering
Least significant bit
In computing, the least significant bit is the bit position in a binary integer giving the units value, that is, determining whether the number is even or odd. The lsb is sometimes referred to as the right-most bit, due to the convention in positional notation of writing less significant digits...
the numbering scheme is called "LSB 0". This bit numbering method has the advantage that for any unsigned integral data type the value of the number can be calculated by using exponentiation with the bit number and a base
Radix
In mathematical numeral systems, the base or radix for the simplest case is the number of unique digits, including zero, that a positional numeral system uses to represent numbers. For example, for the decimal system the radix is ten, because it uses the ten digits from 0 through 9.In any numeral...
of 2.
MSB 0 bit numbering
Most significant bit
In computing, the most significant bit is the bit position in a binary number having the greatest value...
the numbering scheme is called "MSB 0".
Usage
Little-endianEndianness
In computing, the term endian or endianness refers to the ordering of individually addressable sub-components within the representation of a larger data item as stored in external memory . Each sub-component in the representation has a unique degree of significance, like the place value of digits...
CPUs usually employ "LSB 0" bit numbering, however both bit numbering conventions can be seen in big-endian
Endianness
In computing, the term endian or endianness refers to the ordering of individually addressable sub-components within the representation of a larger data item as stored in external memory . Each sub-component in the representation has a unique degree of significance, like the place value of digits...
machines. Some architectures like SPARC
SPARC
SPARC is a RISC instruction set architecture developed by Sun Microsystems and introduced in mid-1987....
and Motorola 68000
Motorola 68000
The Motorola 68000 is a 16/32-bit CISC microprocessor core designed and marketed by Freescale Semiconductor...
use "LSB 0" bit numbering, while S/390, PowerPC and PA-RISC
PA-RISC
PA-RISC is an instruction set architecture developed by Hewlett-Packard. As the name implies, it is a reduced instruction set computer architecture, where the PA stands for Precision Architecture...
use "MSB 0".
The recommended style for Request for Comments
Request for Comments
In computer network engineering, a Request for Comments is a memorandum published by the Internet Engineering Task Force describing methods, behaviors, research, or innovations applicable to the working of the Internet and Internet-connected systems.Through the Internet Society, engineers and...
documents is "MSB 0" bit numbering.
Bit numbering is usually transparent to the software, but some programming languages like Ada
Ada (programming language)
Ada is a structured, statically typed, imperative, wide-spectrum, and object-oriented high-level computer programming language, extended from Pascal and other languages...
allow specifying the appropriate bit order for data type representation.
See also
- EndiannessEndiannessIn computing, the term endian or endianness refers to the ordering of individually addressable sub-components within the representation of a larger data item as stored in external memory . Each sub-component in the representation has a unique degree of significance, like the place value of digits...
- IntegerInteger (computer science)In computer science, an integer is a datum of integral data type, a data type which represents some finite subset of the mathematical integers. Integral data types may be of different sizes and may or may not be allowed to contain negative values....
- Binary numeral systemBinary numeral systemThe binary numeral system, or base-2 number system, represents numeric values using two symbols, 0 and 1. More specifically, the usual base-2 system is a positional notation with a radix of 2...
- Most significant bitMost significant bitIn computing, the most significant bit is the bit position in a binary number having the greatest value...
- Least significant bitLeast significant bitIn computing, the least significant bit is the bit position in a binary integer giving the units value, that is, determining whether the number is even or odd. The lsb is sometimes referred to as the right-most bit, due to the convention in positional notation of writing less significant digits...
External links
- Bit Numbers
- Bit numbering for different CPUs:
- Motorola 68000 ("Bit manipulation" and "Reversed Bit-Numbering" sections)
- IBM Cell Broadband Processors ("Byte ordering and bit numbering")