[Coco] CoCo 3 display offset question.

[Arthur Flexser]

Phill, in the 40/80 column screen mode, the original CoCo offset registers are
ignored.  You can, however, set the display origin using one of the GIME
registers that affects only the 40/80 column display.  As I recall, there's both
a coarse and fine-tuning register for this;  I think you can set the display
origin to any 8-byte boundary between the two of them.  

Art

On Thu, 14 Sep 2006, Phill Harvey-Smith wrote:

> Hi,
> 
> Can anyone tell me if on the CoCo 3 the display generation in the GIME 
> goes through the mapped memory or if it is limited to a particular block 
> (or blocks). A quick check using mess seems to suggest that it honours 
> the tradition display offset regs at $FFC6-$FFD3, but I just verified 
> this with a quick 4 line basic program, which will have had the mapping 
> regs turned off (well CoCo 1/2 compatible mode anyway).
> 
> I'm asking as one of the things I am working on is to use a 1M 30 pin 
> SIMM, and build a banked system for the CoCo 1/2 & Dragon, using a CPLD, 
>   a small SRAM to do the banking. The current design however takes the 
> Address bus input to the SAM, and modifies this through the bank 
> registers before it goes into the SAM. This should work as far as the 
> CPU is concerned, but a side effect may well be that the CPU and the VDG 
> will see the display memory differently, whilst this probably won't be a 
> problem it's something I'll have to bear in mind :)
> 
> Cheers,
> 
> Phill.
> 
> -- 
> Phill Harvey-Smith, Programmer, Hardware hacker, and general eccentric !
> 
> "You can twist perceptions, but reality won't budge" -- Rush.
> 
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