[Coco] [Spam] Re: hardware scrolling revisited...

[Simon Jonassen]

My "feeble" understanding would be that SG is supported by the 6847 as
is....

Agreed sg6/12/24 are adaptions provided by the SAM.....

So why would anyone "ban" the sg4/8 /broken modes.... ?????

I have plenty of games on tape that use SG as loading screens......

/Simon :-)
 

-----Oprindelig meddelelse-----
Fra: coco-bounces at maltedmedia.com [mailto:coco-bounces at maltedmedia.com] På
vegne af John W. Linville
Sendt: 2. december 2013 21:09
Til: CoCoList for Color Computer Enthusiasts
Emne: [Spam] Re: [Coco] hardware scrolling revisited...

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.

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