[Coco] tetris question

[Robert Gault]

Jeremy Michea wrote:
> I'm trying to play my tetris rom pack on my 512k coco 3 and on 2 different composite tv's its coming out black and white. Can't find a way to change it. Anyone know why and how to fix this?
> Thanks
>
> --
> Coco mailing list
> Coco at maltedmedia.com
> http://five.pairlist.net/mailman/listinfo/coco
>

I don't have the pak so can't try it on a real Coco3.

I mounted an image in MESS and looked at the initial code for the pak. 
It starts with a test that seems problematic to me.
  lda #63
a@ sta $ffbf
  cmpa $ffbf
  bne $c01a	branches to PMODE code Coco1 or 2, artifact colors
  deca
  bne a@
  goes to HSCREEN code Coco3, real colors

When I run this test on my Coco3
10 M=&HFFBF
20 FORI=63TO0STEP-1
30 POKE M,I:PRINT PEEK(M);
40 NEXT
I find that bit6 at $FFBF is locked high so the value at $FFBF will 
never equal that in regA. In fact this true for all the palette 
registers. Makes me wonder if the pak can correctly tell what type of 
Coco is in use.

Anyway, assuming the pak recognizes a Coco3 is present, it then starts a 
320x192x4 HSCREEN. Unless the game has a specific instruction for 
holding down a key when starting this pak, the colors used will be grays 
on a composite monitor; black, white, red, and blue on an RGB monitor. 
There might be some artifact colors with an NTSC composite monitor which 
I can't tell, not having the pak.