[BBC-Micro] Place of FDC in memory map

[David Harper]

Can anyone who knows the history explain why the registers for the WD1770 
are mapped into different locations in the memory map between the Master and 
the B+ (and Model B with 1770 interface)?

On the Master these are at four locations from &FE28, and on the B/B+ they 
start at &FE84. In each case the Drive Control Register is 4 bytes earlier 
(and the bits in this register are mapped differently).

This doesn't make for great difficulties. But it does lead to a little more 
complication when trying to write programs to work on all machines, and I 
would be interested to discover the reason for the change.

David Harper