6850 Interface
                             ==============

+--------+---------------------------+-----------------------------+
|        |      Write                |         Read                |
+========+===========================+=============================+
| base+0 |       Control             |     Status                  |
|        | b7:   Enable receive ints | b7: Interupt Request        |
|        | b6-5: Transmit control    | b6: Receive Parity Error    |
|        | b4:   0=7bit, 1=8bit data | b5: Receive data overrun    |
|        | b3-2: parity & stop bits  | b4: Framing Error           |
|        | b1-0: Clock divide        | b3: Clear To Send           |
|        |                           | b2: DCD not present         |           
|        |                           | b1: Transmit Register Empty |
|        |                           | b0: Receive Register Full   |
+--------+---------------------------+-----------------------------+
| base+1 |  Transmit Data Register   |    Receive Data Register    |
+--------+---------------------------+-----------------------------+

Base  Device
----------------------------------------------
FC08  M2000 MIDI Port 1
FC0A  M2000 MIDI Port 2
FC0C  M2000 MIDI Port 3
FCF0  JGH MIDI Port 1, ETI MIDI Port
FCF2  JGH MIDI Port 2
FCF4  JGH MIDI Port 3
FE08  Internal serial port

MIDI requires 8-bit data, clock divide by 16, which can be set with control
set to &11.