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

[Robert Gault]

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.