Subject: Re: [stella] Horizontal scrolling demo From: "Erik Mooney" <erik@xxxxxxxxxx> Date: Fri, 14 May 2004 13:51:05 -0500 |
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 ---------------------------------------------------------------------------------------------- Archives (includes files) at http://www.biglist.com/lists/stella/archives/ Unsub & more at http://www.biglist.com/lists/stella/
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