[Coco] DS-69B Reverse Engineering

poco6809 at hotmail.com poco6809 at hotmail.com
Sat Aug 5 03:19:21 EDT 2006

I did some programming on one of those things years ago. Actually wrote a 
cute little VCR program.
If I remember correctly the 69B scanned in columns, 8 to be exact. You would 
program a comparator register to tell it what luminance level you were 
looking for another register telling it what column you wanted, then wait 
for the scan beam to be in the right spot, read 16 bits from another 
register and wait for the next scan line. that would give you 1 column of 
video, then just do it again for the next column. If you want to send me the 
code I'll sell if it shakes any cobwebs loose.

>From: Joel Ewy <jcewy at swbell.net>
>Reply-To: CoCoList for Color Computer Enthusiasts <coco at maltedmedia.com>
>To: CoCoList for Color Computer Enthusiasts <coco at maltedmedia.com>
>Subject: Re: [Coco] DS-69B Reverse Engineering
>Date: Sat, 05 Aug 2006 00:43:46 -0500
> >
> > L. Curtis Boyle wrote:
> >
> >> ...
> >>     Is there self modifying code later on, that fills in the NOP's? Or
> >> have you disassemlbed from an incorrect offset, and are looking at a
> >> data table?
> >>
> >> --L. Curtis Boyle
> >>
> > I'm fairly sure this doesn't get modified anywhere else though it's
> > possible I'm missing something.  I'm almost positive it's disassembled
> > from the correct offset.  There's quite a bit before and after this that
> > is perfectly sensible code.  It's just this little section that lapses
> > into apparent lunacy.  It would be far too much of a coincidence if all
> > the rest of it just happens to look sensible even though it's
> > disassembled wrong.
> > I've remembered that in addition to the RANDACB.BAS for the DS-69B there
> > is a RANDAC.BAS for the older DS-69 model.  I should do a hex dump of
> > that and see if it has corresponding wackiness.
> >
> > JCE
> >
> >
>     OK, according to my hex dump, RANDAC.BAS also has the mystery code
>in it:  12 12 12 10 12.  Both files can't be corrupted the same way.
>This isn't bit rot.  Either there is an intentional illegal opcode in
>here, or the code gets modified sometime before it runs.  I wouldn't
>swear to it, but I think that 10 12 codes to an instruction on the
>6309.  So these programs might really misbehave on an upgraded CoCo.
>But a stock 6809 should just treat the unrecognized opcode as a NOP,
>no?  I guess I'd better start plowing through the code looking for
>something that might modify the mystery section.  Anybody interested in
>looking at the whole program?
>Coco mailing list
>Coco at maltedmedia.com

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