Two$$s Complement Subtractors

Figure $3$ shows the circuit for a two$$s complement subtractor. Replacing the NOT gates in the

subtractor with an XOR, as shown in Figure $4,$ results in a circuit that can add $($C$0=0)$ or

subtract $($C$0=1).$ These assume that a two$$s complement number system is used to represent

signed numbers.

Figure $3$ Two$$s Complement Subtractor

Figure $4$ Two$$s Complement Adder$/$Subtractor

A B S $=$ A $+$ B Cout $($C$4)$ OVR?

$01010001$

$01110001$

$01111111$

$01101101$

$10100011$

$10011110$

$10101110$

$110111002$

FA$3$ FA$2$ FA$1$ FA$0$

A