[Coco] hardware scrolling revisited...

[Nick Marentes]

> I think it was about late 1982 or early 1983 when the first signs of the
> SG-8/12/24 semi-graphics modes where no longer supported. That was when
> Tandy ask me to re-write Audio Spectrum Analyzer without using SG modes.
> (Tandy never released the non SG mode version A.S.A.) Around this time
> line was when Tandy stop selling 4K CoCo. 


It was my understanding that semigraphics mode was not an official graphics
mode. It is not a mode created within the 6847 VDG but a "phantom" mode
created with the combination of the 6847 VDG and the SAM chip.

In other words, without the SAM there would be no semigraphics.

For example, the MC-10 computer has a 6847 VDG but no SAM chip and hence no
semigraphic mode.

I can understand that Tandy wanted to lay the rules for software development
to include only the "true" modes since the SAM may not be in part of future
CoCo designs.

Ironically, the CoCo3's GIME chip actually emulates the SAM and semigraphics
mode does still work... at least 95% of it anyway (no text).

Nick