Date   : Fri, 23 Sep 1994 10:50:46 -0600
From   : barnett@... (David Barnett)
Subject: Re: sheila

Chris Lam Wrote:
>
>am i right in saying that the system via is not only mapped into &fe40 to
>&fe4f but also &fe50 to &fe5f? similarly for the user via in &fe70 to &fe7f?
>
>i caught 'castle quest' trying to read &fe78 so there must be something
>there!
>

To save hardware, Acorn did not always decode fully its hardware addresses
with the result that ghosts of the same item can appear in the memory map
(i.e. an item can live at both its official address and at addresses
displaced by certain powers of 2 - sometimes many times).  I don't remember
all the cases but I think that the via's are examples.  I believe the
serial port and video processor also do the same thing.  If you want to be
sure, look at a circuit diagram.

Castle quest was probably using this undocemented feature as a copyright
trap (there is no need to use the ghost so if it shows up in someone else's
code it can support the proposition that the code was copied).

David Barnett