[Coco] CoCo 128-column text mode

[William Astle]

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
>
>
>