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

[Robert Gault]

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

Thanks, I did not proof read that very well. The lines should end with

FOR T=0TO400: NEXT T,M


Actually that isn't necessary but makes it easier to see when changes 
are made to the screen.


> 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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> 
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