Right now I have a screen with the default CRTC dimensions (mode 0, but that's not relevant). I'm changing the start address to do horizontal scrolling.
I get the basic layout theory: To go to the next character I add #50 to the address, and to go to the next row within the line I add #800 more. The tiles I'm rendering are aligned with character lines, so that makes it easier (once I start with to render it, I can assume the next 8 rows are all part of the same character line).
However, I'm not happy about the wraparound. When I scroll enough, it gets to a point that, on the bottom right of the screen, when I get the next character line, the address value becomes greater than #FFFF. If that's the case, I need to do some extra addition to set the address to the correct wrap-around.
My question is: Is there a way to avoid that and have a simple function that gives the address of the next character line (or row across character lines) without having to check for wraparound?
I found this one which claims to do it, but unless I'm missing something, it also fails: https://cpctech.cpc-live.com/docs/scraddr.html (at the bottom of the page).
It does make sure it's always in the right range, but when it wraps around, you still need to subtract #800 from the address. Am I wrong and somehow I misunderstood that code?
Alternatively, is it possible to slightly tweak the screen dimensions to turn it into a nice multiple so wraparound works without extra work?
Behind...
adc a,&C0
...you have to add...
res 3,a
...to get the wraparound.
Screens with a width of 64 bytes (instead of 80) would never have a wraparound inside a line. Because of this several games choosed this screensize. But if all your objects/sprites only have the size of a character that wouldn't matter in your case.
*EDIT* Sorry for the confusion, this RES 3,A should always work, no matter where the screen starts.
Quote from: Prodatron on 16:40, 06 December 22Behind...
adc a,&C0
...you have to add...
res 3,a
...to get the wraparound.
Screens with a width of 64 bytes (instead of 80) would never have a wraparound inside a line. Because of this several games choosed this screensize. But if all your objects/sprites only have the size of a character that wouldn't matter in your case.
*EDIT* Sorry for the confusion, this RES 3,A should always work, no matter where the screen starts.
Oh wow, you're right! That's a very efficient way to do the same thing. Works like a charm. Thank you!
(Now on to vertical scrolling, which is going to add a whole new bunch of issues I'm sure).
Quote from: Prodatron on 16:40, 06 December 22Screens with a width of 64 bytes (instead of 80) would never have a wraparound inside a line
Isn't that only true for vertical scrolling? Surely if you're scrolling horizontally, the wrap around within a line is inevitable?
Agreed, for horizontal scrolling it has to wrap around regardless of screen width. Not the same for vertical scrolling of course.