# binary coded decimal (BCD)

The BCD code, Binary Code Decimal (BCD), also known as 8-4-2-1 code, is used for binary coding of decimal numbers. During encoding, each decimal digit is dual-coded individually. In the BCD code, four bits are encoded. This corresponds to 16 different values to which letters or digits can be assigned. Bit groups of four bits are also called tetrad, nibble or half byte.

The BCD code assigns the digits 0 to 9 to the first ten values. No letters are assigned to the other 6 valences, as in the hexadecimal system, for example, where these have the letters A to F. In the place value system of the BCD code, the remaining six values are called pseudotetrades and are sometimes assigned carryovers or mathematical characters.

A single decimal number is assigned a four-digit dual number combination in the BCD code. In terms of significance, the dual number on the left has the highest significance and is the Most Significant Bit( MSB). The dual number on the right has the lowest significance. It represents the Least Significant Bit ( LSB).

If a multi-digit decimal number is represented in BCD code, then each individual digit is dual coded. Thus, the conversion of the digit 418 results in the dual number 0100 0001 1000. By shifting the pseudotetrade, the Aiken code and the Excess-3 code were derived from the BCD code, which can also be assigned to the BCD codes. These codes work with an offset compared to the BCD code, which can be used advantageously in some mathematical operations. Other codes derived from the BCD code are the BCDIC code, Binary Coded Decimal Interchange Code (BCDIC) and the EBCDIC code, Extended Binary Coded Decimal Interchange Code (EBCDIC).

The BCD system is still used in numeric display modules, also in the time signal or when sending short messages.