Binary coded decimal is a binary representation of decimal numbers where each nibble (4 bits) in a word represents a separate decimal digit.
For example, in BCD, the value 128 would be represented in a 16-bit value as:
Binary: 0 0 0 0 0 0 0 1 0 0 1 0 1 0 0 0 Decimal: 0 1 2 8
(Note that, in traditional binary integer representation, the same binary number represents the decimal value 296).
In signed BCD values, the highest-order nibble is usually used to hold the sign (0 for positive, non-0 for negative).