>vertb LDX #$00 ;vertical blank
> LDA #$02
> STA WSYNC
> 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.
> STA VERTLIN
> LDA #$1B ;reset the number of sprite lines.
> STA GP0SIZE
> STA WSYNC
> STA VSYNC
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.
> LDA VERTLIN
> CMP VERTPOS
> 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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