Combined I/O interface circuit.
Address bits A2-A31, that is 30 bits are
used to select the overall interface.
Address bits A0-A1, that is 2 bits select
one of the three registers: DATAIN,
DATAOUT, and the status register.
Status register contains the flags SIN
and SOUT in bits 0 and 1.
Data lines PA0-PA7 connect the input
device to the DATAIN register.
Data lines PB0-PB7 connect the
DATAOUT register to the output device.
This separate input and output data lines
for connection to an I/O device.