[Coco] What do you make of this non-approved HSCREEN mode?

[mike delyea]

There's an extra T in lines 30 and 40:  Should they be there or is
there a for/next missing?

On Wed, Jun 3, 2009 at 9:12 PM, Robert Gault
<robert.gault at worldnet.att.net> wrote:
> Anyone who has looked at the middle chart on page 18 of the Coco3 service
> manual will have noticed that there are missing entries. I and other Coco
> enthusiasts have expanded this table to fill in the missing entries.
>
> It seems to me that there are two alternating series of screen widths in
> bytes. Expanding on what is given by Tandy, the chart logically would be:
>
> HRES2 HRES1 HRES0  Width in bytes
>  1     1     1         160
>  1     1     0         128
>  1     0     1          80
>  1     0     0          64
>  0     1     1          40
>  0     1     0          32
>  0     0     1          20
>  0     0     0          16
>
> I've tested the new entries and not all work for all color resolutions. For
> example 160 bytes of 2 colors requires a screen 1280 pixels wide which could
> be beyond the capabilities of the Coco3.
>
> Here is a program in Basic that demonstrates something weird with the "new"
> 20byte width screen. I'd love to hear comments about what might be happening
> with the hardware that could cause what I, and hopefully you, see.
>
> 10 HSCREEN2: POKE&HFF99,5 : You can also use ,4 and ,6
> 20 PALETTE1,25:POKE&HFFB0,7
> 30 FOR M=0TO15:LPOKE &H60000+M,128: NEXT T,M
> 40 FOR M=0TO 3:LPOKE &H60010+M, 64: NEXT T,M
> 50 GOTO50
>
> The critical command is setting the new screen width by sending a value to
> $FF99. Single pixels are turned on with one color in line 30 and a second
> color in line 40. Changing colors make the weird effect easier to see but it
> is not necessary.
>
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