[Coco] Character set

[William Astle]

On 2020-08-07 2:59 p.m., Andrew wrote:
> Has anyone ever written a program to do like they did for the CoCo 2 - 
> that is, a "mixed" graphics mode and text mode screens? I mean, if OS-9 
> can do it...
> 
> Though thinking about it, I'm not sure you would gain much, since you'd 
> still be in a more-or-less graphics mode...?

I did achieve a mixed graphics/text screen through experimentation a 
while back. I used the GIME timer to trigger an FIRQ where I wanted to 
split the mode. In the FIRQ handler, I switched to text mode and in the 
VSYNC IRQ, I switched back to graphics mode. I never used it for 
anything for a few reasons, though:

1. The GIME timer doesn't behave quite the same between the two extant 
versions so timing the split was problematic.

2. There was instability in the output at the split point between 
graphics and text. As in the output was misaligned horizontally for 
several scanlines.

3. The screen address cannot be reset mid-frame. That means you have to 
work out where in memory the text or graphics actually is after 
activating the split mode. See (1) for one issue with this.

All that said, it *did* work and I had four lines of hardware text below 
a graphics screen.