C-128 Smooth Scrolling in 8563/80-column screen
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2020-06-01 20:26:14 UTC

I have been playing around in a VICE x128 app, trying my luck at the 8563 smooth scrolling features.

I know that writing values 1-7 to Register 24 causes the display to scroll the intended number of scan lines, and I see that working (sort of). While the content gracefully slides up and out of view at the top, what appears at the bottom of the screen, the new content to be scrolled in, appears as "gibberish."

I am using the $1000 area of 8563/video RAM as the character display RAM (set via registers 12 and 13) and have written/copied 2 entire screens worth (4096 bytes) of blank screen characters (#102) to fill the RAM before I try scrolling. But no matter what the new data scrolling in at the bottom is garbled. I have advanced Register 13 (low byte of screen data) by 80 bytes before and after the attempted smooth scroll to no avail.

Any clues appreciated!
2020-06-11 06:36:23 UTC
The new content doesn't "slide in". What you are doing is shifting the entire screen up. What you need to do is hide the top and bottom borders.
Then you need to scroll the screen up. Once you've scrolled 8 pixel lines, what you need to do, is reset the scroll and then at the same time COPY the characters on the screen up a row, and add the new row at the bottom.
Then continue to smooth scroll up.