Date   : Tue, 14 Nov 2006 10:11:22 -0000
From   : dl.harper@... (David Harper)
Subject: Place of FDC in memory map

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