by Achim

Moving sprites can be annoying in terms of msb handling. To avoid the msb issue you usually use tables for moving the sprites and let a small routine update the VIC registers every frame.

spritey:	.byte $00, $00, $00, $00, $00, $00, $00, $00
spritex:	.byte $00, $00, $00, $00, $00, $00, $00, $00
spritemsb:	.byte $00, $00, $00, $00, $00, $00, $00, $00
spritecolor: 	...
spritepointer: 	...

Now simple 16bit additions/subtractions apply to moving sprites. The main program can ignore the msb handling.

Here's a small routine to update VIC registers. Should be called by an irq. Preferably before VIC starts to draw the next frame.

		ldx #$07
		ldy #$0e
			
loop:		lda spritey,x
		sta $d001,y		//write y
		lda spritex,x
		sta $d000,y		//write x
		lda spritemsb,x
		cmp #$01		//no msb=carry clear  / msb=carry set
		rol $d010		//carry -> $d010, repeat 8 times and all bits are set
			
		lda spritecolor,x
		sta $d027,x
		lda spritepointer,x
		sta $07f8,x		//screen at $0400

		dey
		dey
		dex
		bpl loop

This allows to use 8bit calculations for moving the sprite on x axis. No need for a msb table anymore. On the downside sprites can only be moved with 2px/frame.

spritey:	.byte $00, $00, $00, $00, $00, $00, $00, $00
spritex:	.byte $00, $00, $00, $00, $00, $00, $00, $00
spritecolor: 	...
spritepointer: 	...

Update VIC registers:

	ldx #$07
	ldy #$0e
			
loop:	lda spritey,x
	sta $d001,y		//write y
	lda spritex,x
	asl			//x*2>$ff -> carry set=msb, x*2<=$ff -> carry clear=no msb
	sta $d000,y		//write x*2
	rol $d010		//carry -> $d010
			
	lda spritecolor,x
	sta $d027,x
	lda spritepointer,x
	sta $07f8,x		//screen at $0400
	
        dey
	dey
	dex
	bpl loop