I need some really basic help here. Can I use a 4bit adder chip as a subtracter by using the 2's complement for the number to be subtracted?

# Electronic – Subtraction using adder circuit

digital-logic

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## Best Answer

In 2's complement, negation can be achieved by inverting a number and adding one (ie -A = ~A + 1). To subtract a number

`B`

from`A`

, invert`B`

, add 1 to it, then proceed to add that sum to`A`

.In order to transform a normal adder IC into a subtractor, you need to invert the second operand (

`B`

) and add 1 (by setting`Cin`

= 1 ). An Adder subtractor can be achieved by using the following circuitry.Note that when the control signal

`SUB`

is low,Therefore, the computed sum will be

`A + B + SUB = A + B`

.But if

`SUB = 1`

Meaning the computed sum will now be

`A + ~B + SUB = A + ~B + 1 = A - B`

, hence achieving subtraction.