[Coco] hardware scrolling revisited...

[Steve Bjork]

Besides battlestations, there were a number of games that use this trick 
of switching from text to graphics mode.  While those games did not scroll.

The real problem with this technique is the garbage on the high res 
graphics screen.  As the screen scrolls, the data used to display the 
text is used by the VDG to display the graphics. Since the formats of 
the data stream are uncompilable between the two Video modes, you will 
always have garbage somewhere on the screen.

He can get rid of the screen by not changing the Video mode.  That way 
he can use the same data stream format and only change the number of 
time the SAM/VDG reads the same scan line's data for scrolling the 
screen down.  (Like I used in Desert Rider.)

But still he is just pushing a graphics window up and down and that's a 
long way from hardware scrolling.

I can see one way of true scroll game but he is not going to like it 
since you will need 24k of screen memory.  That's twice I used in my 
CoCo 1/2 games.  One hint, he will need to point to an new screen buffer 
after scrolling 16 scan lines or 512 bytes of ram. That's the RAM 
boundary of the SAM Video Memory address.

Steve

On 12/1/2013 5:57 AM, Luis Antoniosi (CoCoDemus) wrote:
> the game battlestations does that. mix text and graphics on transition screens
>
> On Sun, Dec 1, 2013 at 6:39 AM, Simon Jonassen <simon at roust-it.dk> wrote:
>> Hey Luis !
>>
>> Here you go:
>>
>> https://www.youtube.com/watch?v=QhOYqtH-WIE
>>
>> The video is slow because of capture software, the real maching does this
>> much smoother/faster....
>>
>> /Simon :-)
>>
>>
>>