[Coco] Any info on a mystery 256 color GIME mode?

[Roger Taylor]

You're well knowledged on the GIME's hardware and such, so you can apply 
your own logic to the GIME's logic and do a better job than me of guessing 
where this mode is and how it works.  :)

I'll flip over backwards if anybody discovers it.



At 12:33 AM 12/3/2003 -0500, you wrote:
>Roger
>
>Considering the R&D Memo and have 512 capable and pallette ram of 256 colors
>seemed odd as 256 displayable colors can be done with 8 bits. 512 color 
>with 9 bits.
>512 color can be done using RGB333. 256 color is done using RGB332. It is 
>stupid
>to use lookup tables for 8-bit color unless you only plan to display 16 to 
>48 at aany
>given time.
>
>There could be a mode that uses the upper two bits of the pallette 
>registers. The
>chip seems to have three DACs already for the RGB signal.  Internally they 
>may
>already be 3-bit. To use just 2-bit color DACs, the lower bit of the DAC 
>is set to zero
>through a 2:1 mux. In this case the mux is essentially a 2 input and gate. 
>That is
>easy to do in silicon and does not take up much real estate.
>
>My quick guess based on the time frame the chip was designed and 
>fabricated, the
>die uses either 0.75 micron or 1 micron process on either a 6 inch or 8 
>inch warfer.
>Best guess is a 6 inch wafer and 0.75 micron. So if the die is 0.25 inch 
>square the
>real estate is there to put in some extra circuits.
>
>As I stated in my other email, there are some inconsistancies to me that 
>are not
>totally clear. The yyyyyrgb format tends to lead me to believe that is for 
>a composite
>signal. I would think that RGB 332 mode would be used if the CM8 is hooked 
>up.
>Also if the mode ws intended for games, composite video to a TV is more 
>likely the
>intention. 320x200 mode is not to much a stretch for a TV set. 160x200 is 
>just fine.
>The CoCo1 and 2 maxed out at 256x192. Played games using a coco1 and a TV.
>
>If it does exist, the information needed is unknown as to how many steps 
>are in the
>sequence to put the internal state machine in the 256 mode. 2? 3? 4? 5? 
>Also what
>are they? Anything done will be just stabbing around in the dark. My first 
>guess is to
>put the video in 320x200 or 160x200 mode. After that it is a poke in the 
>dark!!!!
>
>james
>
>On 2 Dec 2003 at 15:36, Roger Taylor wrote:
>
> > At 07:13 AM 12/3/2003 +1000, you wrote:
> > >I was just wondering what your opinions are on my personal quest to
> > >resolve the secret 256 color mode...is it real or is it fake? Read my
> > >web page I have created documenting all the information I have
> > >collected to date.
> > >http://www.nickm.launch.net.au/ProjectArchive/256mode.html Nickolas
> > >Marentes
> >
> > My guess at the moment is that there's a hidden or undocumented switch
> > in the GIME that enables the upper 2 palette bits, maybe enabling an
> > intensity field of the usual xxrgbrgb color format.
> >
> > If this secret mode is actually the same palette of 16 colors that can
> > hold iirgbrgb values, then that's 4 intensities of an RGBRGB
> > combination that yields one out of 64 possible colors.
> >
> > Perhaps these upper 2 bits can only be active at a certain time, or by
> > performing a "secret" process.
> >
> > --
> >
> > My big question is: is this 256-color mode a secret mode or a real
> > mode that was being put into the CoCo 4 prototype?  I came across some
> > CoCo write up, I think on one of the museum sites, that mentioned a
> > CoCo 4 was being designed but never made it.
> >
> > If the CoCo 3 guys snuck their photo into the ROM, then my guess is
> > that somebody was either trying to sneak a 256-color mode in, or that
> > there was actually going to be such a mode, since a 16-color mode just
> > wouldn't cut it in a new CoCo model, especially when other computers
> > at the time were doing 256 colors easily.
> >
> > If this mode does exist in the CoCo 3, then we have 2 GIME chips to
> > hack to death in order to discover it since the interviewed developers
> > can't seem to recall anything useful to us.
> >
> > We also have to ask ourselves, what amount of memory is required to
> > support 256 colors at a time.  It seems to me that each pixel would
> > have it's own byte or palette indice.  That's roughly 64,000 bytes for
> > a 320x200 screen.  51,200 bytes for a 256x200 screen, etc.  It's easy
> > to compute by multiplying the width times the height.
> >
> >
> > --
> > Coco mailing list
> > Coco at maltedmedia.com
> > http://five.pairlist.net/mailman/listinfo/coco
>
>
>
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{Roger Taylor}