[Coco] CoCo 128-column text mode

[Bob Devries]

Hi William,
On my Coco3 (PAL) it scrolls 2 characters at a time. It looks like the 
top 2 lines are normal, i.e. black on green characters, which are being 
scrolled, and the rest of the screen is filled with inverse @ signs, 
with a few apparently random characters scrolling left on the  6th line. 
 From the 8th line, the lines alternate between inverse @ and black 
blocks (32 of each).

Hope that helps

Regards, Bob Devries
Dalby, QLD, Australia


On 8/01/2015 6:28 AM, William Astle wrote:
> It does exactly that.
>
> I just did some testing on the 32 column VDG screen and found an
> interesting behaviour. It seems to behave as described but the system
> actually crashes when it's in use, at the very least when the offset
> gets up to around 64 bytes. It's not clear to me why that is the case.
>
> The test program I used is:
>
> 10 FORX=128TO255
> 20 POKE&HFF9F,X
> 30 FORY=1TO400
> 40 NEXTY,X
>
> The program ran through flawlessly on an 80 column screen, stepping
> about once per second.
>
> On a 32 column screen, it started out stepping two characters at a time
> and then eventually crashed out, either scribbling all over the screen
> or even sending the hardware squirrelly.
>
> I wonder if this is consistent on all Coco3s or if it's an indication
> that mine has issues.
>
> On 15-01-07 01:15 PM, Nick Marentes wrote:
>> On 8/01/2015 6:06 AM, William Astle wrote:
>>> Theoretically, it would allow for 256 columns in a text mode with no
>>> attributes (1 byte per character). Bit 7 of FF9F actually forces the
>>> line width to 256 bytes instead of whatever the actual screen width is
>>> set to (each line starts 256 bytes past the previous one).
>>>
>>
>> In graphics mode, bit 7 is the Enable bit so that's 128 combinations so
>> to extend to 256 bytes wide, the minimum increment is 2 bytes.
>>
>> I other words, the horizontal scrolling is 2 bytes or 4 graphic pixels
>> at a time.
>>
>> I haven't played with the text mode but I think a similar restriction
>> occurs here.
>>
>> It probably allows single character scrolling across 128 columns with
>> text attributes on.
>>
>> Nick
>>
>>
>>
>
>