Port 063  -  8255A-5 Mode Control Register

        Normaly set to 99h to cause port B to be an output port
        (a read obtains the last value send) and ports A and C
        to be input ports.

                1xxx xxxx  1=Active
                x11x xxxx  Port A mode
                           00=mode 0 (PC usage)
                           01=mode 1
                           10=mode 2
                xxx1 xxxx  Port A 1=input, 0=output
                xxxx 1xxx  Port C (bits 7-4) 1=input, 0=output
                xxxx x1xx  Port B mode
                           0=mode 0 (PC usage)
                           1=mode 1
                xxxx xx1x  Port B 1=input, 2=output
                xxxx xxx1  Port C (bits 3-0) 1=input, 0=output