1770 Floppy Disk Controller
                       ===========================

+--------+---------------------------+-----------------------------+
|        |      Write                |         Read                |
+========+===========================+=============================+
| base-4 |   Drive control register  |                             |
|        |Master:                    |                             |
|        |   b5: Density 0=MFM, 1=FM |                             |
|        |   b4: Side select         |                             |
|        |   b3: Drive select 2      |                             |
|        |   b2: Reset               |                             |
|        |   b1: Drive select 1      |                             |
|        |   b0: Drive select 0      |                             |
|        |                           |                             |
|        |BBC B/B+/Electron:         |                             |
|        |   b5: Reset               |                             |
|        |   b4: Send DRQ to NMI     |                             |
|        |   b3: Density 0=MFM, 1=FM |                             |
|        |   b2: Side select         |                             |
|        |   b1: Drive select 1      |                             |
|        |   b0: Drive select 0      |                             |
+========+===========================+=============================+
| base+0 |       Command             |     Status                  |
|        |                           | b7: Motor On                |
|        |                           | b6: Write protect           |
|        |                           | b5: Deleted data/Spin up    |
|        |                           | b4: Sector not found        |
|        |                           | b3: CRC error               |
|        |                           | b2: Lost Data/Trk not 0     |           
|        |                           | b1: Data request/Index      |
|        |                           | b0: Command busy            |
+--------+---------------------------+-----------------------------+
| base+1 |                     Track Register                      |
+--------+---------------------------+-----------------------------+
| base+2 |                     Sector Register                     |
+--------+---------------------------+-----------------------------+
| base+3 |                      Data Register                      |
+--------+---------------------------+-----------------------------+

Base  Device
----------------------------------------------
FCC4  Electron floppy disk controller
FE28  Master floppy disk controller
FE84  BBC B/B+ floppy disk controller