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

[jdaggett at gate.net]

On 7 Jun 2009 at 23:20, Robert Gault wrote:

> Robert Gault wrote:
> 
> > So, ignoring the comments that don't apply to the program in
> > question, you are saying there are bugs in the GIME design?
> > 
> > Do you think that in Coco3 graphics mode an HRES=000 is 20 bytes
> > wide or just too buggy to be anything?
> > 
> 
> Sorry that should have been HRES=001 is 20 bytes per row.
> 


And HRES 000 yields 16 bytes per row. 

WHile the register will hold the encoded bits, the actual decoder for the 
horizontal resolution bits may not fully be implemented. Having the register 
bits is small real estate on the die. The actual decode and implementation 
takes more area and thus cost. 

The simplest usage of the HRES bits is to feed them into a 3 of 8 decoder 
and then use those as an address lines to ROM that is 8 by 10 bits wide in 
size.  The output of the ROM then feeds a comparator that takes the row 
bytes per row counter output as the other input. When the two are equal 
then the vertical line counter is incremented and the row counter reset. So 
what ever gets programmed into the small ROM is what is displayed. 

james