IDE Interface
                                   =============
+--------+--------------------------------+---------------------------------+
|        |      Write                     |         Read                    |
+========+================================+=================================+
| base+0 |                           Data Low Byte                          |
+--------+--------------------+-----------+---------------------------------+
| base+1 |  Special Features  |               Error                         |
|        |                    | b7: BBLK Bad block                          |
|        |                    | b6: UNC  Uncorrectable data error           |
|        |                    | b5: MC   Media changed                      |
|        |                    | b4: IDNF Sector Not Found/ID mark not found |
|        |                    | b3: MCR  Media Change Requested             |
|        |                    | b2: ABRT Abort                              |
|        |                    | b1: NM   No media present/Track 0 not found |
|        |                    | b0: AMNF Address mark not found             |
+--------+--------------------+-----------+---------------------------------+
| base+2 |                           Sector Count                           |
+--------+--------------------------------+---------------------------------+
| base+3 |                    Sector Number or LBA b0-b7                    |
+--------+--------------------------------+---------------------------------+
| base+4 |                    Cylinder Low or LBA b8-b15                    |
+---------+--------------------------------+---------------------------------+
| base+5 |                   Cylinder High or LBA b16-b23                   |
+--------+--------------------------------+---------------------------------+
| base+6 |                          LBA+Drive+Head                          |
|        |                 b7:    unused                                    |
|        |                 b6:    0=CHS access, 1=LBA access                |
|        |                 b5:    unused                                    |
|        |                 b4:    Drive                                     |
|        |                 b3-b0: Head or LBA b24-b27                       |
+--------+--------------------------------+---------------------------------+
| base+7 |             Command            |            Status               |
|        |                                | b7: BSY  Busy                   |
|        |                                | b6: DRDY Drive Ready            |
|        |                                | b5: DF   Drive Write Fault      |
|        |                                | b4: DSC  Drive Seek Complete    |
|        |                                | b3: DRQ  Data Request           |
|        |                                | b2: CORR Error Corrected        |
|        |                                | b1: IDX  Index pulse            |
|        |                                | b0: ERR  Error in reg 1         |
+--------+--------------------------------+---------------------------------+
| base+8 |                          Data High Byte                          |
+--------+--------------------------------+---------------------------------+
| base+9-|                             Undefined                            |
| base+13|                                                                  |
+--------+--------------------------------+---------------------------------+
| base+14|         Digital Output         |     Alternative Status          |
|        |                                |     Doesn't reset IRQs          |
+--------+--------------------------------+---------------------------------+
| base+15|               -                |         Drive Data              |
+--------+--------------------------------+---------------------------------+

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