== Technical ==
Port decoding:
A15-A10, A8-A7 are decoded by U11.
A4,A3 from U9B.
A9 is G1, A2 = A, A5 = B, A6 = C into U10
A1,A0 are used when I/O range FBE0-FBE3.
R/W are used.
Data is used by device.
FBE0-FBE3 are decoded as 111110111110000rr
R/W not used, so read and write will access this port. Data is not important.
FBE4 is decoded as 111110111110001xx
Therefore FBE4-FBE7 access the same port.
Dobbertin HD20 is a hard disk that has an interface that appears to implement the SASI protocol (Shugart Associates System Interface).