Tube Interface
                              ==============

+========+===========================+=============================+
|        |           Write           |           Read              |
+========+===========================+=============================+
| base+0 |          Control          |          Status             |
|        | b7: Clear/Set for b0-b6   | b7: RxReady Register 1      |
|        | b6: Reset Tube Registers  | b6: TxReady Register 1      |
|        |                b5: Reset Parasite                       |
|        |                b4: 1/2-byte FIFO3                       |
|        |                b3: Enable ParasiteNMI3                  |
|        |                b2: Enable ParasiteIRQ4                  |
|        |                b1: Enable ParasiteIRQ1                  |
|        |                b0: Enable HostIRQ4                      |
+--------+---------------------------+-----------------------------+
| base+1 | Transmit Data Register 1  |   Receive Data Register 1   |
|        |               Register 1 used for VDU data              |
+--------+---------------------------+-----------------------------+
| base+2 |                           |          Status             |
|        |                           | b7: RxReady Register 2      |
|        |                           | b6: TxReady Register 2      |
+--------+---------------------------+-----------------------------+
| base+3 | Transmit Data Register 2  |   Receive Data Register 2   |
|        |          Register 2 used for commands/results           |
+--------+---------------------------+-----------------------------+
| base+4 |                           |          Status             |
|        |                           | b7: RxReady Register 3      |
|        |                           | b6: TxReady Register 3      |
+--------+---------------------------+-----------------------------+
| base+5 | Transmit Data Register 3  |   Receive Data Register 3   |
|        |            Register 3 used for data transfer            |
+--------+---------------------------+-----------------------------+
| base+6 |    Select parasite CPU    |          Status             |
|        |                           | b7: RxReady Register 4      |
|        |                           | b6: TxReady Register 4      |
+--------+---------------------------+-----------------------------+
| base+7 | Transmit Data Register 4  |   Receive Data Register 4   |
|        |        Register 4 used for interrupt processing         |
+========+===========================+=============================+

  Base                       Device
------------------------------------------------------
    FEE0 at 1-byte offsets   BBC Host Interface
    FEF0 at 1-byte offsets   6502 Second Processor
    0000 at 1-byte offsets   Z80 Second Processor
  FFFFF0 at 2-byte offsets   32016 Second Processor
    0080 at 2-byte offsets   80186 Second Processor
01000000 at 4-byte offsets   ARM Evaluation System
F0000000 at 2-byte offsets   ARM CoProcessor
    FEE0 at 1-byte offsets   6809 Second Processor
    F800 at 2-byte offsets   Toal 6809 Second Processor
    FEF0 at 1-byte offsets   65186 Second Processor
    FFF0 at 2-byte offsets   PDP-11 CoProcessor