[Coco] Coco3 backwards compatibility

[jdaggett at gate.net]

Actually the pointer to the secondary vectors are hard coded into ROM and not in 
the GIME or the SAM Chip. In fact for the SAM chip, any address that is from 
$FFE0 to $FFFF will read from the ROM. I would imagine that the same would be 
true of the GIME chip in this case. 

The interesting thing is that the GIME chip can have the 256 byte page from 
$FE00 to $FEFF as either RAM or ROM. 


james

On 24 Oct 2006 at 23:49, Gene Heskett wrote:

> On Tuesday 24 October 2006 23:32, William Astle wrote:
> >Robert Gault wrote:
> >> The game was made to run on a Coco3 by limiting the data transfer to an
> >> upper value of $FEED; protecting the secondary interrupt vectors.
> >
> >You know, I think they chose that precise location for the secondary
> >vector table because the hex value is such a nice one to remember. I've
> >always been struck by that. Of course, it just works out that way if you
> >  start at the top of the reserved page.
> >
> >Given what I know about programmer types, however, I'd almost wager it
> >was intentional. And if it wasn't, they probably went around telling
> >everyone it was...
> 
> I believe it was intentional, but dictated by the fact that the last few 
> upper bytes of memory are ram regardless of the mapping of the rest of the 
> space above $7FFF.  This is hard coded into the gime, and the rest of the 
> coco and coco2 sam chips.  I've seen references to that, but cannot now 
> quote where I read them.
> 
> >--
> >William Astle
> >finger lost at l-w.ca for further information
> >
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