The MC14561B 9's complementer is a companion to the MC14560B NBCD adder to allow BCD subtraction. A BCD number (8421 code) is applied to the inputs (A1 = 20, A2 = 21, A3 = 22, A4 = 23). If the complement control (Comp) is low, the BCD number appears at the outputs unmodified. The complement disable (Comp) allows the complement control to be gated, or an inverted control signal to be used. If the complement input is high and the disable input low, the 9's complement of the number is displayed at the outputs. The zero control (Z), when high, forces the outputs low regardless of the state of the other inputs. When the MC14561B is used to perform BCD subtraction in conjunction with the MC14560B NBCD adder, the complement control becomes an add/subtract control.