I am trying to design a calculator circuit that adds 2 four-bit 2's-complement binary numbers as its input and outputs an 8-bit 2's-complement number when the control bits equal 01.
Here's what I have done so far using a software called Logisim. I am really stuck when I found out that I cannot put the other 4 output buttons (colored in blue) in my full adder. I am stumped on how to approach this problem now.
I have tested a few 2's-complement numbers and got the output I wanted and some 2's-complement numbers that did not go so well like (having 0100 as the first input and 0110 as the second input did not give me 1010 but gave me 1011.
I just need help, suggestions and guidance on how to move forward.