There are two types of BCD (binary coded decimal) number

[i] unpacked BCD

ii] packed BCD

i) Unpacked BCD: The lower four bits of the number represent the BCD number, the rest of the bits are zero.

For example

Unpacked BCD for 9 is 0000 1001.

[ii] In packed BCD a single byte has two BCD numbers in it, one in the lower 4 bits and one in the upper 4 bits. For example. 0101 1001 is packed BCD for 59H.