0 x 0 = 0
0 x 1 = 0
1 x 0 = 0
1 x 1 = 1, and no carry or borrow bits
 
0 - 0 = 0
0 - 1 = 1, and borrow 1 from the next more significant bit
1 - 0 = 1
1 - 1 = 0
 
0 + 0 = 0
0 + 1 = 1
1 + 0 = 1
1 + 1 = 0  and carry 1 to the next more significant bit