[Coco] [Color Computer] Hardware specifics regarding GIME , RAM and ROM

[Darren A]

On 4/29/09, Mike Pepe <lamune at doki-doki.net> wrote:
>
>> OK. I understand. but I what I think may be a correction to the
> statement
>> comes from the Unravelled Series I re-presented some years back: "The
> GIME
>> chip (and the
>> SAM chip in the older CoCos) redirect the CPU's address request from
> the
>> $FFF0-
>> $FFFF range to $BFF0-$BFFF so that the interrupt vectors can be stored
> in
>> the Basic
>> ROM." on page 29. Is this correct still?
>
> Not exactly. There is no redirection, it just enables the ROM as if a
> read in the $A000-$BFFF were occurring. In effect the ROM vectors are
> just a mirror of the ROM.
>
> In the CoCo 3, I assume that happens at reset when the thing is running
> in CoCo2 mode. Once things are set up, the vectors except reset point to
> RAM in $FExx which has jump instructions to the code to handle the event
> in the patched ROM which now lives in RAM.
>
> I would guess that at boot time that the vectors are probably also a
> mirror of $BFF0-F
>


No.  The vectors at $BFF0-F are not used in the CoCo 3.  They instead
come from $FFF0-F in the CoCo 3's 32K ROM.

Darren