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?
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In 2's complement, negation can be achieved by inverting a number and adding one (ie -A = ~A + 1). To subtract a number
B, add 1 to it, then proceed to add that sum to
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
Therefore, the computed sum will be
A + B + SUB = A + B.
SUB = 1
Meaning the computed sum will now be
A + ~B + SUB = A + ~B + 1 = A - B, hence achieving subtraction.