位序 / 比特序（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.

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 the numbering scheme is called "LSB 0".^[1] This bit numbering method has the advantage that for any unsigned integral data type the value of the number can be calculated by usingexponentiation with the bit number and a base of 2.

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 the numbering scheme is called "MSB 0".

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-RISCuse "MSB 0".^[2]

The recommended style for Request for Comments documents is "MSB 0" bit numbering.^[3]

Bit numbering is usually transparent to the software, but some programming languages like Ada allow specifying the appropriate bit order for data type representation.^[4]