Re: A89: Re: conditional return


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Re: A89: Re: conditional return




>>
>>How can you work a conditional return in 68k?  I tried branching to a label
>>before a rts instruction, but that gave me an invalid instruction error.
>>Does it have something to do with the differences in the way jsr and bsr
>>push?
>>
>>thanks
>>
>
>This is what's going on, if you can help me to get this working, I would
>appreciate it.  Everything I've tried has crashed the calc.
>It clips the top fine, except that it shouldn't display the sprite at all
>when the sprite is completely off the screen instead of resetting to the
>first byte of the screen.  I also needed to clip the left of the screen
>because FindPixel can't handle negative numbers.
>
>PutSprite:
> movem.l d0-d3/a0-a3,-(a7)
> tst.w d1
> bmi clipTop
> tst.w d0
> bmi clipLeft
> jsr FindPixel
> moveq #15,d0
> lea 64(a0),a2
>psLoop:
> clr.l d2
> move.l (a1),d3
> move.l d3,(a2)+
> clr.l d3
> move.w 32(a0),d3
> rol.l d1,d3
> not.l d3
> and.l d3,(a1)
> move.w (a0)+,d2
> lsl.l d1,d2
> or.l d2,(a1)
> add.l #30,a1
> dbra d0,psLoop
> movem.l (a7)+,d0-d3/a0-a3
> rts
>
>clipTop:
> cmp.w #-16,d1

beq Return
                                        ;should return here if <= -16
> neg.w d1
> and.w #15,d1
> move.b d1,d3
> ext.l d1
> lsl.b #1,d1
> add.l d1,a0
> clr.w d1
> jsr FindPixel
> lea 64(a0),a2
> move.b d3,d0
> eor.b #15,d0
> bra psLoop
>
>clipLeft:
> neg.w d0
> move.w d0,d3
> clr.w d0
> jsr FindPixel
> add.w d3,d1
> moveq #15,d0
> lea 64(a0),a2
> bra psLoop

Return:
  rts


I would make a label called Return, which you could do a conditional jump
to with Bcc, such as BRA or BEQ.  The only instruction after the label
would be the rts instruction.  It should not give you any instruction
error, if you did the simple Bcc variant.  The address to return to, as on
most processors, is on top of the stack, so unless you forget to pop off
some pushed values, or popped off to many, there is no reason for that
error.  That is one of the most common assembly errors!  Watch the SP :)

Hope that helps!

-Mark



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