The BCD8421 code is so called because each of the four bits is given a ‘weighting’ according to its column value in the binary system. The least significant bit (lsb) has the weight or value 1, the next bit, going left, the value 2. The next bit has the value 4, and the most significant bit (msb) the value 8, as shown in Table 1.6.1.
So the 8421BCD code for the decimal number 610 is 01108421. Check this from Table 1.6.1.
For numbers greater than 9 the system is extended by using a second block of 4 bits to represent tens and a third block to represent hundreds etc.
2410 in 8 bit binary would be 00011000 but in BCD8421 is 0010 0100.
99210 in 16 bit binary would be 00000011111000002 but in BCD8421 is 1001 1001 0010.
Therefore BCD acts as a half way stage between binary and true decimal representation, often preparing the result of a pure binary calculation for display on a decimal numerical display. Although BCD can be used in calculation, the values are not the same as pure binary and must be treated differently if correct results are to be obtained. The facility to make calculations in BCD is included in some microprocessors.
One of the main drawbacks of BCD is that, because sixteen values are available from four bits, but only ten are used, there are several redundant values whichever BCD system is used. This is wasteful in terms of circuitry, as the fourth bit (the 8s column) is under used.