[Coco] Hidden 256-color mode

[Nickolas Marentes]

Roger Taylor wrote:

>
> Still, it seems that 1 byte per pixel would be required which would 
> throw the RGB monitor into another video mode even if the colors don't 
> appear correctly.
>
> I suspect that the mode invoked by holding down F2 while hitting RESET 
> has something to do with the undocumented/hidden 256-color mode.  I 
> see no useful purpose for this blue shade video.  Maybe a missing 
> color component is mixed in somehow to give a new color range.  Who 
> knows?  Think about it... they put in the 3 Mugateers picture and made 
> IT visible by holding down CTRL-ALT-RESET, so it seems logical that 
> the same method of hiding something was also used to help hide the 
> 256-color mode.
>
>
Nick replies:

I'm afraid it goes far beyond a simple "set of special key sequences" 
for entry. If it was this easy, the mode would have been discovered ages 
ago via the many dissassemblies of the ROM. This mode is, according to 
my contact, a matter of accessing a certain location at a certain 
refresh time during a specific interrupt event. Supposedly, some 
register/s change during this stage to reveal the secret/hidden register 
required. A very short "window of opportunity" you could call it.

256 colors is possible without the traditional 1 byte per pixel mapping. 
If you look at my research page, my contact says " It uses a yyyyyrgb 
format, 5 bits of intensity, 3 bits of color". This may suggest that the 
mode may only be available under the composite video out.

I think the next stage is deciphering Al's prototype.

Nickolas Marentes