The MC14561B 9’s complementer is a companion to the MC14560B NBCD adder to allow BCD subtraction.

A BCD number (8–4–2–1 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