SCSI Interface
                       ==============
+--------+--------------------+------------------------+
|        |      Write         |         Read           |
+========+====================+========================+
| base+0 |                  Data                       |
+--------+--------------------+------------------------+
| base+1 |       -            |        Status          |
|        |                    | b7: 0=Command, 1=Data  |
|        |                    | b6: 0=Input, 1=Output  |
|        |                    | b5: REQ                |
|        |                    | b4: INTs enabled       |
|        |                    | b3: 0                  |
|        |                    | b2: 0                  |           
|        |                    | b1: BUSY               |
|        |                    | b0: MSG                |
+--------+--------------------+------------------------+
| base+2 |   Write Select     |           -            |
|        |  Data irrelevant   |                        |
+--------+--------------------+------------------------+
| base+3 | Interupt Enable    |           -            |
|        | b7: 0=disable INTs |                        |
|        |     1=enable INTs  |                        |
+--------+--------------------+------------------------+

Base  Device
----------------------------------------------
FC40: BBC/Master/Electron/etc