4-bit adder/subtractor that is controlled by the carry/borrow bit.
The first nybble bits are on the right and the second nybble bits are on the left.
The overflow/underflow bit is connected to an OR gate buffer.
0 - addition operation, the oscilloscope will read 0 unless there is an overflow
1 - subtraction operation, the oscilloscope will read 1 unless there is an underflow
