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

[Brendan Donahe]

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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