6551 Interface
                             ==============
           The 6551 ACIA is used in CUBE and Archimedes systems

+--------+----------------------------+-----------------------------+
|        |      Write                 |         Read                |
+========+============================+=============================+
| base+0 |  Transmit Data Register    |    Receive Data Register    |
+--------+----------------------------+-----------------------------+
| base+1 |        Reset ACIA          |     Status                  |
|        |                            | b7: Interupt Request        |
|        |                            | b6: DSR level               |
|        |                            | b5: DCD level               |
|        |                            | b4: Transmit Reg Empty      |
|        |                            | b3: Receive Register Full   |
|        |                            | b2: Receive data overrun    |
|        |                            | b1: Framing Error           |
|        |                            | b0: Receive Parity error    |
+--------+----------------------------+-----------------------------+
| base+2 |                    Command Register                      |
|        | b7-6: 00=odd parity, 01=even parity,                     |
|        |       10=mark parity, 11=space parity                    |
|        | b5:   0=parity disabled, 1=parity enabled                |
|        | b4:   0=no echo, 1=echo                                  |
|        | b3-2: Transmit Control                                   |
|        | b1:   TxIRQ enable/disable                               |
|        | b0:   TxIRQ disable/enable                               |
+--------+----------------------------+-----------------------------+
| base+3 |                    Control Register                      |
|        | b7:   0=1 stop bit, 1= 2 stop bits                       |
|        | b6-5: 00=8bit, 01=7bit, 10=6bit, 11=5bit                 |
|        | b4:   0=external receive clock, 1=internal baud generator|
|        | b3-b0: baud rate:                                        |
|        |       0000 : 16x ext. clock       1000 : 1200            |
|        |       0001 : 50 baud              1001 : 1800            |
|        |       0010 : 75                   1010 : 2400            |
|        |       0011 : 110                  1011 : 3600            |
|        |       0100 : 134.5                1100 : 4800            |
|        |       0101 : 150                  1101 : 7200            |
|        |       0110 : 300                  1110 : 9600            |
|        |       0111 : 600                  1111 : 19,200          |
+--------+----------------------------+-----------------------------+



Base  Device
----------------------------------------------
FE10  CUBE Internal serial port