[Coco] Higher graphics mode (256 color mode?)

[William Astle]

I wonder if this isn't something similar to the "64 column VDG" screen I
detailed some time back, which works by setting the coco2 text screen
and fiddling with the GIME registers.

If my findings from then hold for the "VDG" graphics modes, then setting
a particular bit in one of the GIME registers (I can't remember which
one off the top of my head) should double the horizontal resolution.
That would give a 512 pixel wide "pmode 4" screen. It could also involve
a mismatch between the "VDG" graphics mode and the "SAM" graphics mode
settings which only makes it look like 4 areas by virtue of running the
data fetch counter at the wrong rate so it fetches the same data
multiple times to make up a full pixel row.

The only thing I doubt is that it is actually 1024 pixels across, based
simply on the characteristics of the NTSC signal. If someone could
demonstrate the mode, it would be interesting. I'd try some experiments
but my coco3 is in a storage locker on the other side of the city and I
have nowhere to set it up.

On 2015-07-15 15:25, Arthur Flexser wrote:
> P.S  I didn't mean for my last message to seem negative.  This could be an
> interesting contribution, if in fact the mode is truly 1024 pixels wide,
> even if there is no way to uncouple the 4 areas.  Maybe could be used for
> special effects in a game in situations where it doesn't matter if each
> quarter of the screen is identical to the others, and assuming that the
> 1024 resolution is appreciably sharper than 512 pixels on any type of
> monitor usable with a CoCo.
> 
> Art
> 
> On Wed, Jul 15, 2015 at 5:08 PM, Arthur Flexser <flexser at fiu.edu> wrote:
> 
>> A couple of questions occur to me:
>>
>> 1.  You say the graphics are repeated 4 times across the screen.  Could
>> there be any way in this mode to change one of the 4 areas without
>> similarly affecting the other three?  (I tend to doubt it.)
>>
>> 2.  Are we sure that this is really 1024 pixel resolution?  It could be,
>> for example, that the 4 areas are in fact only reproducing every 2nd or
>> every 4th horizontal pixel of the original screen.  Indeed, maybe the 4
>> areas are not entirely identical, but 1/4 resolution and each offset by 1
>> pixel horizontally from the adjacent area to yield the same number of
>> pixels as were there in the original screen.
>>
>> 3.  Why exactly might this have anything to do with a 256-color mode?
>>
>> Art
>>
>> On Wed, Jul 15, 2015 at 4:49 PM, Paul Wittkoski via Coco <
>> coco at maltedmedia.com> wrote:
>>
>>> Hi everyone,
>>>
>>> At one time, I found a higher graphics mode that is not documented with
>>> the
>>> CoCo 3. It was 1024 pixels across and the pictures in pmode graphics could
>>> be seen 4 times across the screen. I am unable to pull the exact method
>>> from
>>> my computer since my 5.25" floppy drive blew a chip some time back. But,
>>> someone may be able to find it again from what I remember what I did. It
>>> may
>>> have been odd thinking at the time but I wondered if pmode and hscreen
>>> could
>>> be used at the same time. Following the machine code for pmode and hscreen
>>> in the unravelled series I found a specific poke to turn on pmode and
>>> another one for hscreen. When I used them together on a cm-8 monitor it
>>> produce a screen with 1024 pixel resolution across. I am not sure if this
>>> would also lead to the 256 color mode if someone messed around with it.
>>> Interesting enough, I have not ever heard anyone else describe what I have
>>> done. Maybe that's how Tandy hid the 256 color mode so well; right under
>>> our
>>> noses with pmode. I hope this information is useful to someone in some way
>>> after all of these years.
>>>
>>>
>