[Coco] hardware scrolling revisited...

[John W. Linville]

On Mon, Dec 02, 2013 at 06:30:35PM +0000, Nick Marentes wrote:
> > I think it was about late 1982 or early 1983 when the first signs of the
> > SG-8/12/24 semi-graphics modes where no longer supported. That was when
> > Tandy ask me to re-write Audio Spectrum Analyzer without using SG modes.
> > (Tandy never released the non SG mode version A.S.A.) Around this time
> > line was when Tandy stop selling 4K CoCo. 
> 
> 
> It was my understanding that semigraphics mode was not an official graphics
> mode. It is not a mode created within the 6847 VDG but a "phantom" mode
> created with the combination of the 6847 VDG and the SAM chip.
> 
> In other words, without the SAM there would be no semigraphics.
> 
> For example, the MC-10 computer has a 6847 VDG but no SAM chip and hence no
> semigraphic mode.

That is somewhat true.  But without the SAM there is also no way
to relocate the video buffer.  Both of those features are enabled
by the fact that the VDGs address lines are completely ignored.
The SAM controls the video RAM addressing, enabling it to fetch from
different parts of memory and to step the addresses on different line
boundaries than the VDG expects.  (You, Nick, probably know that --
I'm just stating this bit for those watching at home...)

> I can understand that Tandy wanted to lay the rules for software development
> to include only the "true" modes since the SAM may not be in part of future
> CoCo designs.

I guess this seems as plausible as any other reasons I've seen offered.
It just seems a bit odd that whomever concocted those rules would
choose this one feature to deprecate.  I wonder if they figured that
the non-standard color/pixel encodings would be difficult to replicate
with off-the-shelf hardware options?  If so, it is ironic that the
only follow-up device used an ASIC...

Was there any prohibition on using the SG-4 (or SG-6) modes in
machine code programs?  BASIC could be augmented to emulate them
on any likely follow-up hardware, of course.  But if those modes
(which shared the weird color/pixel encodings) were still allowed,
then that would suggest that most of the work to support the other SG
modes in hardware would have to be included by any follow-on design...?

Of course, probably nobody wanted to use SG-4 or SG-6 anyway... :-)

> Ironically, the CoCo3's GIME chip actually emulates the SAM and semigraphics
> mode does still work... at least 95% of it anyway (no text).

This, of course, illustrates that in all likelihood the GIME designers
had at least some interest in supporting the SG modes.  It is a shame
that they ran out of time or design space or whatever.

John
-- 
John W. Linville		Someday the world will need a hero, and you
linville at tuxdriver.com			might be all we have.  Be ready.