A summary of the 8255A's addressing is:
                 __   __   __  
       A1   A0   RD   WR   CS       Transfer Description
       
       0    0    0    1    0      Port A to data bus
       0    1    0    1    0      Port B to data bus
       1    0    0    1    0      Port C to data bus  
       0    0    1    0    0      Data bus to port A
       0    1    1    0    0      Data bus to port B
       1    0    1    0    0      Data bus to port C
       1    1    1    0    0      Data bus to control register if D7=1;
                                  if D7=0 input from the data bus is
                                  treated as a Set/Reset instruction.
       x    x    x    x    1      D7-D0 go to high impedance state
       1    1    0    1    0      illegal combination
       x    x    1    1    0      D7-D0 go to high impedance state