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

[Darren A]

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