[Coco] CC-Five (was Re: Pseudo CoCo4???) (LONG)

[John Kowalski]

At 02:28 PM 24/01/2007 -0500, James Daggett wrote:

>Actually  to improve color resolution would be to have an extra bit set in
>say the INIT1 register that would be say for COCO 4. This would then allow
>bit 6 and bit 7 of the color pallette to be written. Therefore you now have
>a pallette of 16 colors of 256. There could be another trick to swap the
>pallette register bank  with writes to a register to map in banks of 
>16  colors. This would keep some backward compatibility.

I think a neat way of extending the color resolution to 8 bits without
causing compatibility issues would be to treat bits 6 and 7 of the palette
registers as  low order bits for all three R,G and B values:

Palette register:
bit7 bit6 bit5 bit4 bit3 bit2 bit1 bit0
 I1   I0   R3   G3   B3   R2   G2   B2

This way red, green or blue can have 4 bit values to create 16 shades
instead of just 4.

   bits  3  2  1  0
Red   = R3 R2 I1 I0
Green = G3 G2 I1 I0
Blue  = B3 B2 I1 I0

Even if existing software writes values to bits 6 or 7, the resulting
displayed colors will still be "correct" except for a slight difference in
brightness.


>As for increase resolution there is an unused bit in video resolution register
>at $FF99 Bit 7 in combination with bits 6 and 5 to double the lines per frame.

This would fit very nicely with the existing video resolution structure.

Bits 1,0 set to 11 could enable 256 colors (1 pixel per byte).
Bits 7,6,5 set to 010 could enable 480(450?) scan lines.
Bits 7,6,5 set to 110 could enable 480 scan lines + double horizontal
resolution.

It might be tempting to treat bit 7 alone as horizontal resolution * 2, but
it's probable that this could break a lot of existing software.

Bits 6,5 almost *never* get set to 10 because that makes the display go
wonky (Boink bouncing ball demo is one of the only exceptions that does.)

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