[Color Computer] [Coco] Hidden 256-color mode

[James Diffendaffer]

If you look at the design document you will find discrepancies between
it and what the engineer said.  For that reason I don't think there
was a shortage of pins but of internal registers.  An additional
register requires additional address decoding and takes up memory for
hardware.  However... it looks to me like there were unused bits and
bit combinations so that doesn't make much sense either.

The engineer said 320x200 mode.  320x200 = 64000 bytes and a lot of
bandwidth for one scan of the screen at that clock freqency.
The design document says 160x200 which sounds much more realistic and
requires half the memory bandwidth.  To increase the colors you have
to decrease the resolution.  With 16 colors maxing out at 320x200 and
using 4 bits per pixel it just makes sense that it would have to be
half that resolution to use 1 byte/pixel.

The design document lists a 512 color palette with a max of 256
displayed.  So it may require setting up palette registers with the
yyrrggbb or yyyyyrgb values.  I'm not sure how that would add up to a
possible 512 color palette though.  Perhaps it combines palette values
with the bytes to expand the range but it sounds kinda strange to me
and never seems to add up to 512 possible colors unless some are
duplicates.

If I were to try this mode I'd place the screen on a 32K or 64K
boundary, set the display to 160x200, enable the MMU, enable CoCo3
mode and place the mode setting code in a timer interrupt handler in
the fixed memory region.

VMODE and VRES are obviously what would enable it so I'd try setting
CRES to 11.  From there I'd try placing the video at different
addresses since the MMU setting may have something to do with it.
The video setting should make at least some sense and I doubt it would
require more than a couple settings during the interrupt or it would
complicate the hardware design.

As for the D/A converter... 
External D/A converters weren't cheap.  That's why the Orchestra 90c
uses stacked resistors as a D/A converter.  However, added bits to
internal D/A conversion on a gate array may not have been so pricy.  


> Since the DACs are internal, I'm curious if anyone has any idea
> what function these unavailable pins might have needed to perform?
>
>	Also, wouldn't leaving the mode in have required the DACs to be
> 3-bit instead of 2? This wasn't a negligible thing at the time, was it?
>
>					kevin
>





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