The serial port is a buffered, 8-bit synchronous shift register. A control
bit (CRA6) selects input or output mode.  In the Amiga system one shift
register is used for the keyboard, and the other is unassigned.  Note that
the RS-232 compatible serial port is controlled by the  paula chip ; see
chapter 8 for details.

 input mode 
 output mode 
 bidirectional feature