6551 ACIA
                               =========
     The 6551 ACIA is used in CUBE, Archimedes, BenEater systems

+--------+-----------------------------+-----------------------------+
|        |            Write            |            Read             |
+========+=============================+=============================+
| base+0 |   Transmit Data Register    |    Receive Data Register    |
+--------+-----------------------------+-----------------------------+
| base+1 |        Reset ACIA           |     Status                  |
|        |                             | b7: 1=Interupt Request      |
|        |                             | b6: 1=DSR Off, 1=DSR On     |
|        |                             | b5: 1=DCD Off, 1=DCD On     |
|        |                             | b4: 1=Transmit Reg Empty    |
|        |                             | b3: 1=Receive Register Full |
|        |                             | b2: 1=Receive data overrun  |
|        |                             | b1: 1=Framing Error         |
|        |                             | b0: 1=Receive Parity error  |
|        |                             | Note: Hardware error causes |
|        |                             |       b4=1 always, CTS=Off  |
|        |                             |       corrupts TxData       |
+--------+-----------------------------+-----------------------------+
| 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: 00: TxIRQ=Off, RTS=Off            |
|        |                         01: TxIRQ=On,  RTS=On             |
|        |                         10: TxIRQ=Off, RTS=On             |
|        |                         11: TxIRQ=Off, RTS=On, Tx=BREAK   |
|        | b1:   0:RxIRQ=On, 1:RxIRQ=Off   If RxIRQ=On then DSR/DCD  |
|        | b0:   0:DTR=Off,  1:DTR=On         changes causes IRQ     |
|        | Note: Hardware error requires Parity=Off and TxIRQ=Off    |
+--------+-----------------------------+-----------------------------+
| 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 : 115,200 (16x clock)  1000 : 1200             |
|        |       0001 : 50                   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
5000  BenEater serial port