[Coco] MC6847 VGA (FPGA) Project underway...

[Arthur Flexser]

What Darren says corresponds with my recollection as well, although I did
not have an explanation for it.  You get vertical colored stripes but there
are black gaps between them.  Also, no ability to mix in letters, a useful
feature of other semigraphics modes.  When the CoCo 2B with the real
lowercase came out, with SG6 eliminated, I heard of zero complaints of
software that wouldn't work.  (I was selling ADOS at the time, which could
be configured to default to the real lowercase, so I was in a position to
hear such complaints.)  SG6 is a completely useless mode.  If you are going
to put in lowercase characters, I think you should substitute this mode for
SG6 for compatibility with CoCo 2B and CoCo 3 software that uses the real
lowercase.  That is, have the real lowercase mode turned on by what would
previously have turned on SG6, as is the case with the CoCo 2B and the CoCo
3.

Art

On Thu, May 26, 2016 at 12:22 PM, Brendan Donahe <brendan at polylith.com>
wrote:

> Thanks, Darren - that explains some of the things I have seen in SG6 mode.
>
> The Mini-MPI looks great, Ed!
>
> Brendan
>
>
>
> On Thu, May 26, 2016 at 11:18 AM, Darren A <mechacoco at gmail.com> wrote:
>
> > On Thu, May 26, 2016 at 7:41 AM, Brendan Donahe wrote:
> >
> > >
> > > I had the understanding that SG6 didn't work right on the CoCo, too,
> > > however, it looks to me like it can on some systems, and I don't want
> to
> > > exclude the Dragon or MC-10.  (Also, I could swear that I saw 6 pixels
> > per
> > > character block in one of Steve Spiller's tests of SG6 on my CoCo 2.
> Can
> > > someone describe clearly what is broken?  Is it, perhaps, a limited
> > number
> > > of colors, caused by what pins are tied together on the board?  I
> > obviously
> > > have not spent much time investigating this yet...)
> > >
> >
> >
> > The problem with SG6 is that all 256 byte values are used to produce
> > graphics characters in that mode. But because the CoCo, Dragon and MC-10
> > all have the A/S pin hard-wired to data line D7, you can only use values
> > 128-255, eliminating half of the available colors in each color set. Byte
> > values 0-127 will give you the External Character ROM mode. Since there
> is
> > no external character ROM you end up with vertically striped characters
> > corresponding to the byte's bit pattern.
> >
> > - Darren
> >
> > --
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> >
>
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