[Coco] Semigraphics modes examples

[Robert Gault]

Stephen Robinson wrote:
> Hello all,
>
> Long time lurker here. I am a latecomer to the Coco scene. They had
> been out of production for a long time by when I got my start. When I
> was a pre-teen back in 2005-ish, my father gave me a Coco 2, some
> cassette tapes, a stack of old Rainbow magazines, and a pocket
> reference guide. I managed to hook it to our TV by taping some wires
> together. And after typing in:
>
> 10 PRINT "HI"
> 20 GOTO 10
> RUN
>
> I was hooked! End result: I learned to program from Rainbow magazines,
> asked my father for more Coco stuff, and have gone one to study and
> get a job in Computer Science.
>
> I don't really have much time at all for Coco's anymore (sadly). But
> there was one especially large lingering question in my mind. What
> were the semigraphics modes? I heard much about them in the Rainbow
> magazines. How the Coco 3 didn't support them (all). How the size of
> them was odd. Etc.. But never could I find an example to run.
>
> So, if nothing more than to finally solve this childhood mystery, is
> there some reference which I could see to finally explain these
> magical text graphics modes to me?
>
> Thanks,
> Stephen
>

There are several graphics modes that can be called Semi-Graphics. These are 
described in detail in the Motorola document describing the SN74LS783 / MC6883 
Synchronous Address Multiplexer known to the Coco community as the SAM.

The mode with the most detail is the Semigraphics-24 mode. Which you can see an 
example of and download a program to use it from
http://aaronwolfe.com/robert.gault/Coco/History/History_files/Semi24.html
This mode was never supported by Tandy and it was left to hobbyists to create 
programs to use it. Unfortunately, the true S-24 mode exists only on the Coco1 
and Coco2. With a setting undocumented by Tandy in the Coco3 service manual, it 
is possible to partially achieve the S-24 mode on a Coco3 but without text 
characters.
One nice thing about the MESS emulator is that the authors did not know that you 
"can't use" the S-24 mode on a Coco3 so it works with this emulator. :)

If you have a Coco3, I can supply an updated version of my program to at least 
draw in S-24. I'm not sure if the site referenced above has that update. So if 
you find the program of interest, contact me for the update.

Robert