A  B  C   !A   S1 S0
---------------------
0  0  0    1    0  0
0  0  1    1    0  1
0  1  0    1    0  1
0  1  1    1    1  0
1  0  0    0    0  1
1  0  1    0    1  0
1  1  0    0    1  0
1  1  1    0    1  1