Re: [stella] If it's not one thing it's another.

Subject: Re: [stella] If it's not one thing it's another.
From: emooney@xxxxxxxxxxxxxxxx (Erik Mooney)
Date: Thu, 04 Sep 1997 06:12:13 GMT
>vertb	LDX  #$00	;vertical blank         
>	LDA  #$02           
>	STA  WSYNC         
>	STA  WSYNC        
>	STA  VSYNC        
>	STA  WSYNC         
>	STA  WSYNC      ;Didn't know I could do an HMOVE here, but I'm  
>	STA  HMOVE	;doing HMOVE here to avoid the black line on screen
>	LDA  #$2C	;is this causing my problems? I dunno. But I don't

Shouldn't be.  You can position and HMOVE your sprite anytime, and it
will stay where it's put till you move it again.

>	STA  TIM64T     ;think so because when I remove the vertical   
>	LDA  #$00	;movement routines everything works fine.
>	STA  CXCLR        
>	LDA  #$C0	;reset the number of vertical lines.
>	LDA  #$1B       ;reset the number of sprite lines.
>	STA  WSYNC        

You're gonna hit yourself ;)  Look at that last STA VSYNC... you're
writing $1B to VSYNC.  $1B has bit 1 set, so you're leaving VSYNC
active while the 2600 tries to draw the screen, and that doesn't work.

>load	LDX  BLOCK
>	LDA  playf0,X      
>	STA  PF0
>	LDA  playf1,X  
>	STA  PF1
>	LDA  playf2,X  
>	STA  PF2
>grfx1	DEC  VERTLIN	;Checking to see if it's time to draw a sprite yet.
>	BMI  sprite1
>grfx2	STA  WSYNC	;If not then draw then scanline
>	DEC  BLKSIZE	;next line of block                 
>	BEQ  newblk     ;or is it time for a new block?          
>	JMP  grfx1      ;get ready for next scanline    
>sprite1	LDX  GP0SIZE
>	BEQ  nosprt	;See if we're finished drawing the sprite.
>	LDA  p0data,x	;If not then load the sprite data.
>	STA  GP0        
>	DEC  GP0SIZE    ;get set up for next sprite line 
>	JMP  grfx2      ;go draw current scanline
>nosprt	LDA  #$00       ;if we're finished with the sprite then
>	STA  GP0	;just fill the sprite with zeros.
>	JMP  grfx2	;then draw the scanline.
>newblk	LDA  BLOCK	;check to see if we've finished drawing the screen               
>	BEQ  clear         
>	DEC  BLOCK	;get ready for the next playfield block              
>	LDA  #$08
>	STA  BLKSIZE          
>	JMP  load           

I count a maximum of 85 cycles between WSYNCs here.  Recall that the
2600 only has 76 cycles.  The screen will end up taking one too many
scanlines in a few spots (anytime it updates both the playfield and
player on the same line.)

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