[Coco] Re: Color Computer 3 prototype

[John Kowalski]

At 10:31 PM 21/01/2005 -0500, CoCoList for Color Computer Enthusiasts wrote:
>Further testing of this ($FF99=252) mode shows that every other screen 
>byte is active and that only bit 7 of the even bytes causes color 
>change. The palettes used are $FFB0 and $FFB1 and the pixel is 8x1 
>almost like a semi-graphics mode. Therefore, as is, the effective screen 
>size is 80x255 using two colors with 8x1 pixel size.
>
>How this might relate to a 256 color mode is questionable unless there 
>are some other secret POKEs that can activate more bits for color selection.
>
>The above represents the results of testing with an '87 GIME. John, have 
>you found the same results with either '86 or '87?

I get the same results on my '87 GIME CoCo3.  I could dig out an '86 to see
if it's different, but I vaguely recall that it did the same thing.

"252" in itself doesn't really do anything special, except that it disables
the GIME's normal pixel shifter.  It just shows it can be done in a graphics
mode.  256 color mode operation is rumored to be seperate from the normal
16-out-of-64 color generator, which means there has/had to be some sort of
switch to enable a secondary 256-out-of-512 color mapper.  If there are to
be 256 colors, then the existing 4-bits-per-pixel shifter and palette mapper
cannot be used for this mode.

"255" might be a better hint of a 256 color mode, but since it doesn't
actually display anything on-screen it doesn't make much of an impression...
but if you test the address lines, you would notice that the GIME is still
going through the motions of fetching video RAM in accordance to a 160 bytes
per line video mode.  Why would it have disabled normal video generation in
this mode?  "252" shows that even if the pixel shifter stops, you still see
fetched data being displayed on-screen.  If in "255" you no longer see
fetched data being displayed on-screen, then it may have been deliberately
disabled.  Why?  Wouldn't that take extra logic in the chip?  Maybe it's
part of the set up to disable the regular 16-of-64 color mode before
enabling the 256 color mode.

It's those other secret POKEs that are elusive.  My guess is that they were
never included in the final production of the GIME, but I'm willing to be
optimistic and look forward to whatever can be discovered about the CoCo 3
prototype.

                                         John Kowalski (Sock Master)
                                         http://www.axess.com/twilight/sock/