Hi,
Erik Mooney wrote:
> 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
Hey, Thanks ;-)
I'll try to use that to make a new demo I'll be able
to add to my programming page for newbies.
BTW : for the newbies,
I just added Horizontal scrolling Logic shemas on my
site : http://khryssun.free.fr/programming.html
Take care
Chris,
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