8bitworkshop/presets/vcs/examples/complexscene2.a

	processor 6502
        include "vcs.h"
        include "macro.h"
        include "xmacro.h"

        seg.u Variables
	org $80

PFPtr	word	; pointer to playfield data
PFIndex	byte	; offset into playfield array
PFCount byte	; lines left in this playfield segment
YPos0	byte	; Y position of player 0 sprite
XPos0	byte	; X position of player 0 sprite
YPos1	byte	; Y position of player 1 sprite
XPos1	byte	; X position of player 1 sprite

; pointers to bitmap and color tables
SpritePtr0	word
ColorPtr0	word
SpritePtr1	word
ColorPtr1	word

; temporary values for kernel
YP0	byte	; Y counter for player 0
YP1	byte	; Y counter for player 1
Temp	byte
Bitp0	byte
Bitp1	byte
Colp0	byte
Colp1	byte
tmpPF0	byte
tmpPF1	byte
tmpPF2	byte

SpriteHeight	equ 16

;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;

        seg Code
	org $f000

Start
	CLEAN_START
Data0        
        lda #<Data0
        sta PFPtr
        lda #>Data0
        sta PFPtr+1
        lda #<Frame0
        sta SpritePtr0
        lda #>Frame0
        sta SpritePtr0+1
        lda #<ColorFrame0
        sta ColorPtr0
        lda #>ColorFrame0
        sta ColorPtr0+1
        lda #<Frame0
        sta SpritePtr1
        lda #>Frame0
        sta SpritePtr1+1
        lda #<ColorFrame0
        sta ColorPtr1
        lda #>ColorFrame0
        sta ColorPtr1+1
        lda #242
        sta YPos0
        lda #200
        sta YPos1
        lda #38
        sta XPos0
        sta XPos1
        lda #1
        sta VDELP0	; updates to GRP0 will be delayed

NextFrame
	VERTICAL_SYNC

; Set up VBLANK timer
	TIMER_SETUP 37
        lda #$88
        sta COLUBK	; bg color
        lda #$5b
        sta COLUPF	; fg color
        lda #$68
        sta COLUP0	; player color
        lda #1
        sta CTRLPF	; symmetry
        lda #72
        sta PFIndex	; reset playfield offset
; Set temporary Y counter and set horizontal position
        lda YPos0
        sta YP0		; yp0 = temporary counter
        lda YPos1
        sta YP1		; yp0 = temporary counter
        lda XPos0
        ldx #0
        jsr SetHorizPos
        lda XPos1
        ldx #1
        jsr SetHorizPos
        sta WSYNC
        sta HMOVE	; gotta apply HMOVE
; Wait for end of VBLANK
	TIMER_WAIT
        sta WSYNC
        lda #0
        sta VBLANK
        SLEEP 31	; so timing analysis works out

KernelLoop
; Phase 0: Fetch PF0 byte
        jsr DrawSprites
	ldy PFIndex	; no more playfield?
        beq NoMoreLines	; exit loop
        dey
        lda (PFPtr),y	; load PF0
        sty PFIndex
        sta tmpPF0
; Phase 1: Fetch PF1 byte
        jsr DrawSprites
	ldy PFIndex
        dey
        lda (PFPtr),y	; load PF1
        sty PFIndex
        sta tmpPF1
; Phase 2: Fetch PF2 byte
        jsr DrawSprites
	ldy PFIndex
        dey
        lda (PFPtr),y	; load PF2
        sty PFIndex
        sta tmpPF2
; Phase 3: Write PF0/PF1/PF2 registers
        jsr DrawSprites
	lda tmpPF0
        sta PF0
        lda tmpPF1
        sta PF1
        lda tmpPF2
        sta PF2
; Go to next scanline
	jmp KernelLoop

NoMoreLines
; Set up overscan timer
	TIMER_SETUP 30
	lda #2
        sta VBLANK
        jsr MoveJoystick
        TIMER_WAIT
        jmp NextFrame

; DrawSprite subroutine called by kernel
DrawSprites
; Fetch sprite 0 values
        lda #SpriteHeight	; height in 2xlines
        sec
        isb YP0			; INC yp0, then SBC yp0
        bcs DoDraw0		; inside bounds?
        lda #0			; no, load the padding offset (0)
DoDraw0
        tay			; -> Y
        lda (ColorPtr0),y	; color for both lines
        sta Colp0		; -> colp0
	lda (SpritePtr0),y	; bitmap for first line
        sta GRP0		; -> [GRP0] (delayed due to VDEL)
; Fetch sprite 1 values
        lda #SpriteHeight	; height in 2xlines
        sec
        isb YP1			; INC yp0, then SBC yp0
        bcs DoDraw1		; inside bounds?
        lda #0			; no, load the padding offset (0)
DoDraw1
        tay			; -> Y
        lda (ColorPtr1),y	; color for both lines
        tax
	lda (SpritePtr1),y	; bitmap for first line
        tay
; WSYNC and store sprite values
        lda Colp0
	sta WSYNC
        sty GRP1		; GRP0 is also updated due to VDELP0 flag
        stx COLUP1
        sta COLUP0
; Return to caller
	rts

SetHorizPos
	sta WSYNC	; start a new line
        bit 0		; waste 3 cycles
	sec		; set carry flag
DivideLoop
	sbc #15		; subtract 15
	bcs DivideLoop	; branch until negative
	eor #7		; calculate fine offset
	asl
	asl
	asl
	asl
	sta RESP0,x	; fix coarse position
	sta HMP0,x	; set fine offset
	rts		; return to caller

; Read joystick movement and apply to object 0
MoveJoystick
; Move vertically
; (up and down are actually reversed since ypos starts at bottom)
	ldx YPos0
	lda #%00100000	;Up?
	bit SWCHA
	bne SkipMoveUp
        cpx #175
        bcc SkipMoveUp
        dex
SkipMoveUp
	lda #%00010000	;Down?
	bit SWCHA 
	bne SkipMoveDown
        cpx #254
        bcs SkipMoveDown
        inx
SkipMoveDown
	stx YPos0
; Move horizontally
        ldx XPos0
	lda #%01000000	;Left?
	bit SWCHA
	bne SkipMoveLeft
        cpx #1
        bcc SkipMoveLeft
        dex
SkipMoveLeft
	lda #%10000000	;Right?
	bit SWCHA 
	bne SkipMoveRight
        cpx #153
        bcs SkipMoveRight
        inx
SkipMoveRight
	stx XPos0
	rts

; Bitmap data "standing" position
Frame0
	.byte #0
        .byte #%01101100;$F6
        .byte #%00101000;$86
        .byte #%00101000;$86
        .byte #%00111000;$86
        .byte #%10111010;$C2
        .byte #%10111010;$C2
        .byte #%01111100;$C2
        .byte #%00111000;$C2
        .byte #%00111000;$16
        .byte #%01000100;$16
        .byte #%01111100;$16
        .byte #%01111100;$18
        .byte #%01010100;$18
        .byte #%01111100;$18
        .byte #%11111110;$F2
        .byte #%00111000;$F4

; Color data for each line of sprite
ColorFrame0
	.byte #$FF;
	.byte #$F6;
	.byte #$86;
	.byte #$86;
	.byte #$86;
	.byte #$C2;
	.byte #$C2;
	.byte #$C2;
	.byte #$C2;
	.byte #$16;
	.byte #$16;
	.byte #$16;
	.byte #$18;
	.byte #$18;
	.byte #$18;
	.byte #$F2;
	.byte #$F4;

; Epilogue
	org $fffc
        .word Start
        .word Start