Bit numbering

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.

Least significant bit

The binary representation of decimal 149, with the LSB highlighted. The MSB in an 8-bit binary number represents a value of 128 decimal. The LSB represents a value of 1.

In computing, the least significant bit (LSB) 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 low-order bit or right-most bit, due to the convention in positional notation of writing less significant digits further to the right. It is analogous to the least significant digit of a decimal integer, which is the digit in the ones (right-most) position.

It is common to assign each bit a position number, ranging from zero to N-1, where N is the number of bits in the binary representation used. Normally, this is simply the exponent for the corresponding bit weight in base-2 (such as in 231..20). Although a few CPU manufacturers assign bit numbers the opposite way (which is not the same as different endianness), the term least significant bit itself remains unambiguous as an alias for the unit bit.

By extension, the least significant bits (plural) are the bits of the number closest to, and including, the LSB.

The least significant bits have the useful property of changing rapidly if the number changes even slightly. For example, if 1 (binary 00000001) is added to 3 (binary 00000011), the result will be 4 (binary 00000100) and three of the least significant bits will change (011 to 100). By contrast, the three most significant bits (MSBs) stay unchanged (000 to 000).

Least significant bits are frequently employed in pseudorandom number generators, steganographic tools, hash functions and checksums.

Least significant bit in digital steganography

In digital steganography, sensitive messages may be concealed by manipulating and storing information in the least significant bits of an image or a sound file. In the context of an image, if a user were to manipulate the last two bits of a color in a pixel, the value of the color would change at most +/- 3 value places, which is likely to be indistinguishable by the human eye. The user may later recover this information by extracting the least significant bits of the manipulated pixels to recover the original message.

This allows for the storage or transfer of digital information to be kept concealed.

Least significant byte

LSB can also stand for least significant byte. The meaning is parallel to the above: it is the byte (or octet) in that position of a multi-byte number which has the least potential value. If the abbreviation's meaning least significant byte isn't obvious from context, it should be stated explicitly to avoid confusion with least significant bit.

To avoid this ambiguity, the less abbreviated terms "lsbit" or "lsbyte" may be used.

Most significant bit

In computing, the most significant bit (MSB, also called the high-order bit) is the bit position in a binary number having the greatest value. The MSB is sometimes referred to as the high-order bit or left-most bit due to the convention in positional notation of writing more significant digits further to the left.

The MSB can also correspond to the sign bit of a signed binary number in one's or two's complement notation, "1" meaning negative and "0" meaning positive.

It is common to assign each bit a position number ranging from zero to N-1 where N is the number of bits in the binary representation used. Normally, this is simply the exponent for the corresponding bit weight in base-2 (such as in 231..20). Although a few CPU manufacturers assign bit numbers the opposite way (which is not the same as different endianness), the MSB unambiguously remains the most significant bit. This may be one of the reasons why the term MSB is often used instead of a bit number, although the primary reason is probably that different number representations use different numbers of bits.

By extension, the most significant bits (plural) are the bits closest to, and including, the MSB.

The unsigned binary representation of decimal 149, with the MSB highlighted. The MSB in an 8-bit binary number represents a value of 128 decimal. The LSB represents a value of 1.

Most significant byte

MSB can also stand for "most significant byte". The meaning is parallel to the above: it is the byte (or octet) in that position of a multi-byte number which has the greatest potential value.

To avoid this ambiguity, the less abbreviated terms "MSbit" or "MSbyte" are often used.

Unsigned integer example

This table illustrates an example of decimal value of 149 and the location of LSB. In this particular example, the position of unit value (decimal 1 or 0) is located in bit position 0 ( n=0). MSB stands for Most Significant Bit, while LSB stands for Least Significant Bit.

Binary (Decimal: 149)	1	0	0	1	0	1	0	1
Bit weight for given bit position n ( 2n )	27	26	25	24	23	22	21	20
Bit position label	MSB	___	___	___	___	___	___	LSB

Position of LSB is independent of how the bit position is transmitted (Some system transmit MSB first, others transmit LSB first), which is a question more of a topic of Endianness.

Most Significant Bit First vs Least Significant Bit First

The expressions Most Significant Bit First and Least Significant Bit First are indications on the ordering of the sequence of the bits in the bytes sent over a wire in a transmission protocol or in a stream (e.g. an audio stream).

Most Significant Bit First means that the most significant bit will arrive first: hence e.g. the hexadecimal number 0x12, 00010010 in binary representation, will arrive as the sequence 0 0 0 1 0 0 1 0 .

Least Significant Bit First means that the least significant bit will arrive first: hence e.g. the same hexadecimal number 0x12, again 00010010 in binary representation, will arrive as the (reversed) sequence 0 1 0 0 1 0 0 0.

LSB 0 bit numbering

LSB 0: A container for 8-bit binary number with the highlighted least significant bit assigned the bit number 0

When the bit numbering starts at zero for the least significant bit (LSB) the numbering scheme is called "LSB 0". This bit numbering method has the advantage that for any unsigned number the value of the number can be calculated by using exponentiation with the bit number and a base of 2. The value of an unsigned binary integer is therefore

$\sum _{i=0}^{N-1}b_{i}\cdot 2^{i}$

where b_i denotes the value of the bit with number i, and N denotes the number of bits in total.

MSB 0 bit numbering

MSB 0: A container for 8-bit binary number with the highlighted most significant bit assigned the bit number 0

Similarly, when the bit numbering starts at zero for the most significant bit (MSB) the numbering scheme is called "MSB 0".

The value of an unsigned binary integer is therefore

$\sum _{i=0}^{N-1}b_{i}\cdot 2^{N-1-i}$

Other

ALGOL 68's elem operator is effectively "MSB 1 bit numbering" as the bits are numbered from left to right, with the first bit (bits elem 1) being the "most significant bit", and the expression (bits elem bits width) giving the "least significant bit". Similarly, when bits are coerced (typecast) to an array of Boolean ([ ]bool bits), the first element of this array (bits[lwb bits]) is again the "most significant bit".

For MSB 1 numbering, the value of an unsigned binary integer is

$\sum _{i=1}^{N}b_{i}\cdot 2^{N-i}$

PL/I numbers BIT strings starting with 1 for the leftmost bit.

The Fortran BTEST function uses LSB 0 numbering.

Usage

Little-endian 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 (RfC) documents is "MSB 0" bit numbering.

Bit numbering is usually transparent to the software, but some programming languages like Ada and hardware description languages like VHDL and verilog allow specifying the appropriate bit order for data type representation.

See also

• ARINC 429
• Binary numeral system
• Signed number representations
• Two's complement
• Endianness
• Binary logarithm
• Unit in the last place (ULP)
• Find first set
• MAC address: Bit-reversed notation

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")