[Coco] 64 column "low res" text sceen on coco3

[Kip Koon]

William,
This is very cool!  Fantastic work!  This will enable a lot of the old games
to be updated for the Coco 3 and new ones created.  I can hardly wait to see
what everyone comes up with using this new knowledge.
Could you create a master video modes file with a table detailing all the
codes for each memory location and the modes they produce please.  I would
love to experiment with these also.  Thanks a bunch.
Kip

-----Original Message-----
From: coco-bounces at maltedmedia.com [mailto:coco-bounces at maltedmedia.com] On
Behalf Of William Astle
Sent: Thursday, April 11, 2013 7:26 PM
To: coco at maltedmedia.com
Subject: Re: [Coco] 64 column "low res" text sceen on coco3

On 13-04-11 04:16 PM, Arthur Flexser wrote:
> So, all these years we've all believed that the CoCo 3 didn't support 
> any of the higher-resolution semigraphics modes...
>
> If I'm following you correctly, the CoCo 3 could be used for 128 x 32 
> semigraphics drawing, intermixed with half-width (but fully drawn) 
> VDG-style text.  Weird that one one seems to have noticed before.  
> Nice work!

I've done some further experimentation and it turns out you don't get
128x32 - you actually get 64x32. See below.

I stumbled onto this as a result of discussions on #coco_chat (on
freenode) about semigraphics modes. I was bored so I started setting
registers in the FF98..FF9F range and observing the results. This was the
most interesting result.

It turns out that FF9F also works as expected. (The horizontal scroll/offset
register), including the "256 bytes per line" mode. I can see that being
useful.

FF9C does (as has been documented elsewhere) sets up for higher semigraphics
modes except that text rendering doesn't function correctly. It seems to
function by reducing the character row height depending on the value put in
FF9C. Text characters seem to be aligned with the bottom of the cell instead
of the top so the top of the characters gets clipped as the lines per
character row are reduced. In any mode that is not 12 lines per character
row, the graphics blocks are rendered differently. The values for FF9C work
as follows. Unless otherwise noted, the bottom two elements of each graphic
block are shown for all lines of the character row. These have all been
confirmed experimentally.

0: 11 lines per row
1: 10 lines per row
2: 9 lines per row
3: 8 lines per row
4: 7 lines per row
5: 6 lines per row
6: 5 lines per row
7: 4 lines per row
8: 3 lines per row
9 : 2 lines per row
10: 1 line per row
11: 4 lines per row (1)
12: 3 lines per row (2)
13: 2 lines per row
14: 1 line per row (4)
15: 12 lines per row (3)

(1) All four elements of the graphic block are rendered as expected, at
2 lines per element.
(2) All four elements of the graphic block are rendered as expected with the
top elements 1 line high and the bottom elements 2 lines high.
(3) Regular rendering is enabled as usual for the 32 column screen.
(4) This one uses the top two elements in the graphic block rather than the
bottom two.

Further careful experimentation reveals that the graphics blocks behave
differently when combined with the 64 column mode. It turns out that only
the left hand elements in the graphics block are shown for whichever row is
displayed. So what we actually have is 64x32 where every horizontal pixel
can have a different colour. Combined with the semigraphics bits in FF9C, we
can have 64x192 with 9 colours with all elements of the display having
independent colours at the expense of twice the memory usage. (1 byte per
pixel). This could also be 64x32, 64x64, 64x48, etc., using the FF9C bits.
It's too bad the character generator doesn't do anything useful in these
modes.


>
> Art
> On Thu, Apr 11, 2013 at 5:42 PM, William Astle <lost at l-w.ca> wrote:
>
>> I don't know if this is known or not. I can't believe I'm the first 
>> person to try poking values into $FF99 while in the 32 column "low res"
text mode.
>> Anyway, I found it interesting.
>>
>> Anyway, with the GIME set up for the 32x16 "coco2" text screen, if 
>> you set
>> $FF99  to $10, you will end up with a 64x16 "coco2" text screen which 
>> uses
>> 1024 bytes of memory. This screen still supports all the "SG4" 
>> graphics blocks.
>>
>> The "lines per field" setting seems to work, too. Setting it to 200 
>> or 225 lines seems to work as expected. Setting $FF99 to $70 yields a 
>> 64x19 text screen though the bottom 3 rows of the last line are 
>> missing. ($60 would give 32x19.)
>>
>> If you're messing with this from basic, you'll need to use a program 
>> like the following to see the result because basic resets all the 
>> video registers.
>>
>> 10 WIDTH32
>> 20 POKE&HFF99,&H10
>> 30 GOTO30
>>
>> Note that this is totally distinct from the high resolution 64 column 
>> text mode.
>>
>> --
>> Coco mailing list
>> Coco at maltedmedia.com
>> http://five.pairlist.net/**mailman/listinfo/coco<http://five.pairlist
>> .net/mailman/listinfo/coco>
>>
>
> --
> Coco mailing list
> Coco at maltedmedia.com
> http://five.pairlist.net/mailman/listinfo/coco
>


--
Coco mailing list
Coco at maltedmedia.com
http://five.pairlist.net/mailman/listinfo/coco