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Choosing Between Addition and Subtraction [TOC]
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In the next step, the circuit will be expanded to allow choosing between
addition and subtraction.

First, consider the following circuit:

---- IMAGE (width=600) -----------
session02/s02_15.png
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With the `Op` input, you can select whether `A` is inverted. It can be easily
shown with truth tables that the following holds: In the case of $Op = 0$, $S =
A$, and in the case of $Op = 1$, $S = \lnot A$.

This circuit uses a 4-bit wide XOR gate to invert the bit pattern:

---- IMAGE (width=600) -----------
session02/s02_16.png
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Use this method of selective inversion of a bit pattern to choose between addition and subtraction.

---- IMAGE (width=600) -----------
session02/s02_17.png
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