[Coco] hardware scrolling revisited...

[Arthur Flexser]

Why, then, support SG-4 (SET/RESET graphics) but not the other
semigraphics modes?  Presumably, SG-4 would require the SAM if the
other semigraphics modes do.

Art

On Mon, Dec 2, 2013 at 1:30 PM, Nick Marentes <nickma at optusnet.com.au> wrote:
>> 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
>
>
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