Christian Bogey <khryssun@xxxxxxxx> wrote:
> > I'd suggest trying a routine using rol/ror, instead
> > of whatever you're doing now ;-)
>
> I'am already using als, ror and rol ;-)
If you want to do playfield scrolling with an asymmetric playfield
in repeated mode, take a look at the INV source that I posted. Look
for the labels MoveEachRow0 and MoveEachRow1. That uses a series
of shifts and rotates to move the playfield bits across the screen
through both copies of all the PF registers. The only tricky
part is moving bit 7 of PF2 into bit 4 of the second copy of PF0,
and here's how my code does that:
lda Invaders+2,X ;
rol ;
sta Invaders+2,X ;We've gotta get the carry into bit 3 of A
lda #0 ;
rol ;
rol ;
rol ;
rol ;
ora Invaders+3,X ;
rol ;
sta Invaders+3,X ;
The four ROLs move the carry (shifted out of PF2) into bit 3 of
the new PF0, then the last ROL moves it into bit 4. Of course,
nowadays I know that could be optimized....
sta Invaders+2,X
lda Invaders+3,X
bcc Label0
ora #%00001000
Label0
rol
sta Invaders+3,X
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