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

[Ciaran Anscomb]

Darren A 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.
> 
> 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.

If you set the SAM divisor to give you 1 row lines you can get some
pseudo-hires stuff onto the screen along with 4-colour semigraphics
(except on the later CoCos, where a buffer was put in place so the
data lines stopped being treated as the "external" character set).

You end up able to manipulate 7 out of 8 "pixels": bit 6 always ends up
the same colour because setting it asserts INV which then inverts the
whole pattern :)

There was a BASIC+M/C game published in Dragon User called "Flee!" that
made use of this if you want to see it in action (XRoar supports this
use): http://www.6809.org.uk/tmp/da/dragon_originals/

Something else you might want to implement: RP occurs on the switch from
graphics to alphanumeric, which (with some cleverness updating memory
at the right time) can give a smooth scroll effect in (normal SG4)
text mode as it resets the row index into the font.

I guess you'll already have the data/CSS latch timing in mind, as lots
of things use that (DragonFire being the obvious example).

Probably less important is that while data/CSS is latched ahead of time
and populates the bit pattern/palette stuff, switching G/¬A happens
pretty immediately and due to system timing always partway through a
byte, and that leads to some non-obvious effects as the different buffers
are selected.  I think the only time I've seen this used in anger is in
the PAL hack of DragonFire, and even then only used to change the border
colour for the bouncing message.

..ciaran