====== Waving sprite scroll ======

The sprites 1 through 8 are positioned on the screen next to each other with a fixed spacing. The scrolling takes place by moving them all a bit to the left, 4 pixels at a time, until sprite 1 reaches X≤0. At that moment sprite 2 is 4 pixels to the right of where sprite 1 started.
The sprites are then all positioned back to their starting position and the content is shifted, so that the value of sprites 8 is in 7, 7 in 6, etc. A new letter is fetched from the scroll text and the corresponding pointer value is plotted in the pointer of sprite 8. So basically sprites 1 moves from X=43 to X=255, 4 pixels at a time, after which it resets to value 43 again.

In addition to that all, the Y-coords of the sprites are changed based on sine data.

This codes assumes a letter-per-sprite font, in the same order as the screencodes, located at $2000.

The code is a stripped down and somewhat simplified version of what is used in "This Is Halloweed" (https://csdb.dk/release/?id=197114).

<code>
// Made in Kick Assembler

	.const SPRITESPACING	= 43	// minimal possible spacing
	.const SINLEAP		= 20	// choose anything here to change sine wave
	.const SCROLLSPEED	= 4	// lower value is slower
	.const SPRITEPOINTER	= $07f8
	.label spriteMemory	= $2000	// assumes sprite font from $2000 to $3000
	
*=$0801
	BasicUpstart($0810)
*=$0810
	sei
	jsr $e544			// KERNAL: clear screen
	lda #$00			// reset sine wave pointers
	sta sinreset+1
	sta sinwave+1

	ldx #<scrolltext		// reset scroll text pointer
	ldy #>scrolltext
	stx textpointer+1
	sty textpointer+2
	        		
	lda #$ff	
	sta $d015			// turn on all sprites
	
	ldx #$00	
!:	lda #$a0			// init with spaces in all sprite pointers
	sta SPRITEPOINTER,x
	lda #$0e			// light blue
	sta $d027,x			// set sprite colours
	inx
	cpx #$08
	bne !-

	lda #$01			// init IRQ
	sta $d01a
	lda #$7f
	sta $dc0d
	sta $dd0d
	lda $dc0d
	lda $dd0d

	lda #$32			// arbitrary value
	sta $d012
	lda #$1b
	sta $d011

	ldx #<irq			// set pointers to FLD IRQ routine
	ldy #>irq
	stx $0314
	sty $0315

	cli
	
	jmp *
	
irq:
	inc $d019

	lda scrollpos+1			// X-position of 1st sprite
	sec
	sbc #SCROLLSPEED		// decrease with SCROLLSPEED
	bpl notext			// skip text fetch if sprite 1 can still move left

		ldx #$00		// shift content of sprites pointers
	scrollpointers:
		lda SPRITEPOINTER+1,x
		sta SPRITEPOINTER,x
		inx
		cpx #$07	
		bne scrollpointers
		lda sinreset+1		// shift sine wave
		clc
		adc SINLEAP		// this fixes sine offset
		adc #$03		//  when resetting sprite position
		sta sinreset+1
	textpointer:
		lda scrolltext		// get next letter from scroll text
		bne noreset		// if not #$00 (end indicator)
	
		lda #<scrolltext	// reset scroll text pointer when letter is $00
		sta textpointer+1
		lda #>scrolltext
		sta textpointer+2
		jmp textpointer		// read new letter
	
	noreset:
		clc
		adc #(>spriteMemory<<2)	// correct for location of sprite font
		sta SPRITEPOINTER+7	// store new letter in sprite 8 pointer

		inc textpointer+1	// increase scroll text pointer
		bne !+
		inc textpointer+2
	!:	 
		lda #SPRITESPACING	// move sprite 1 to right most position
notext:
	sta scrollpos+1

	ldx #$00			// position other sprites relative to sprite 1
scrollpos:
	lda #$18			// set new X-coord for all sprites
	sta $d000
	clc
	adc #SPRITESPACING
	sta $d002
	clc
	adc #SPRITESPACING
	sta $d004
	clc
	adc #SPRITESPACING
	sta $d006
	clc
	adc #SPRITESPACING
	sta $d008
	clc
	adc #SPRITESPACING
	sta $d00a
	bcc !+
	ldx #%11100000			// take care of MSB
	clc
!:	adc #SPRITESPACING
	sta $d00c
	bcc !+
	ldx #%11000000
	clc
!:	adc #SPRITESPACING
	sta $d00e
	bcc !+
	ldx #%10000000
!:	stx $d010			// set proper sprite MSB

sinreset:
	ldx #$00			// sine wave counter
	stx sinwave+1			// store in sine wave pointer
	inc sinreset+1
	ldy #$00
sinwave:
	lda sindata			// read sine wave data
	sta $d001,y			// store in Y-coords sprites
	lda sinwave+1
	clc
	adc #SINLEAP			// to make wave more interesting
	sta sinwave+1			// increase sine wave pointer by SINLEAP
	iny
	iny
	cpy #$10			// next sprites
	bne sinwave	
	jmp $ea31			// end of IRQ1
	
.align $100
sindata:
	.fill 256, 120 + 15.5*sin(toRadians(i*(3*360)/256))

scrolltext:
	.text "this is a sprite scroller for codebase64.org        "
	.byte $00
</code>