I've been trying to convert a full adder to a full subtractor using an inverter. I tried using not gates at the B input ( in A B Cin ) and also using it in the outputs but the final answers are not matching.

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    \$\begingroup\$ I suggest that you write out the truth tables for the full adder and full subtractor, compare the minterms that produce a `1' output, and look for input variables that are complemented between the two sets. \$\endgroup\$ Feb 15, 2019 at 12:40

1 Answer 1


Inverting all bits gives you the ones complement and I assume you want to work in twos-complement format.

To make a number negative in twos-complement format you can do a ones-complement (invert all bits) followed by addition of 1.

Thus you can convert an adder to a subtractor using the two's complement but that requires another adder :-).

However there is a trick you can use sometimes: if your adder has an unused carry-in port you can use that to perform the extra "add-one".


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