Calculator (add)

Cool! Maybe try multiplication as well, as you just need multiplexers and more adders :)

To substract, you only need to invert the negative number (so if you do A - B, you need to invert B) and set to one the Cin input of the first adder (the most in left in the picture of your adders). Then invert the output. To invert a signal, juste use a XOR gate in all input. The first input of the gate will be the number bit, the other is 1 if you will substract and 0 if you will add. Remember, if you will add, you have to set the Cin input (discussed above) to 0.

To show a negative number (if the MSB is set to 1), invert the number and add 1 (you need to use another sets of adders). Then turn on some leds on the screen to show a - .

First, read the negative number representation to understand why we need to add one.