In a digital data bit string, the MSB is a bit of the highest digit, and the LSB is a bit of the lowest digit.
Digital data is binary, and like ordinary numerical notation, the left end is the highest digit, while the right end is the lowest digit. For example, 99 in the decimal system is expressed as (MSB)01100011(LSB) in the binary system. In this case, the MSB is 0 and the LSB is 1.
Considering inter-block interfaces, with a parallel interface, all the bits on the transmitting side and the receiving side correspond, so there is no problem in particular. However, with a serial interface, all the bits are transmitted and received through a single data line, so that the bit order definitions in the protocols of the transmitting side and the receiving side must match. In the case of the above data (number), the data is transmitted and received in the order of "01100011" in the case of MSB first, and in the order of "11000110" in the case of LSB first. As long as the receiving side matches the transfer mode (MSB first or LSB first) with the transmitting side, parallel data can be reproduced through serial-parallel conversion of the receive data.
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A bit is the smallest unit in a binary system, and a binary number is a set of digits represented by one of two states, 1 or 0. Therefore, in the binary system, the handling of binary numbers is described as "digital," which is derived from the word "digit."
However, since the binary notation system usually involves large numbers of digits, the hexadecimal notation system, which treats 4 bits as 1 unit, is frequently employed. When hexadecimal notation is used in programs, each number is usually prefixed with "0x" and suffixed with "H" (C language and assembler definition). Octal notation, which treats 3 bits as a unit, was often used in the past.
The "byte," a unit consisting of 8 bits, is also used frequently in digital notation. The "word," is another notation convention that consists of 16 bits or more. The number of bits making up a word is not universal and depends on the system.
However, in the case of memory, the capacity depends on the data bit structure, and the address count is called words, the data bit structure is described as × n bits, using a notation such as 2 Mwords × 8 bits (= 16 Mb). In SRAM, in the case of this structure, there are 21 address lines (A0 to A20), and 8 data lines (D0 to D7).