dos33fsprogs/demos/xmas_2019/snow.s

; Display falling snow

; by deater (Vince Weaver) <vince@deater.net>

; Zero Page
CH		= $24
CV		= $25
GBASL		= $26
GBASH		= $27
BASL		= $28
BASH		= $29

SEEDL		= $4E
SEEDH		= $4F

HGR_COLOR	= $E4
SNOWX		= $F0
TEMPY		= $F1

HGR		= $F3E2


NUMFLAKES	= 10


.include	"hardware.inc"


	;==================================
	;==================================

	jsr	HGR
	bit	FULLGR


	;==================================
	; init snow
	;==================================

	ldx	#NUMFLAKES-1
snow_init_loop:

	jsr	random16

	lda	SEEDL
	and	#$3f
	sta	snow_x,X

	lda	SEEDH
	and	#$7f
	sta	snow_y,X

	dex
	bpl	snow_init_loop


display_loop:

	; 0 4 8 c 10 14 18 1c
	; 0 1 2 3 4  5  6  7

	;=========================
	; erase old snow
	;=========================
	; 2 + (40+38+7)*NUMFLAKES - 1
	; 1 + 85*NUMFLAKES = 851

	ldx	#0					; 2
erase_loop:
	lda	snow_y,X	; get Y			; 4+
	lsr						; 2
	lsr						; 2
	lsr			; divide by 8		; 2
	sta	TEMPY					; 3
	lda	snow_x,X				; 4+
	tay						; 2
	lda	div_7_q,Y				; 4+

	ldy	TEMPY					; 3
	clc						; 2
	adc	hgr_offsets_l,Y				; 4+
	sta	GBASL					; 3
	adc	#30					; 2
	sta	BASL					; 3
						;=============
						;         40

	lda	snow_y,X				; 4+
	asl						; 2
	asl						; 2
	and	#$1f					; 2
	clc						; 2
	adc	hgr_offsets_h,Y				; 4
	sta	GBASH					; 3
	sta	BASH					; 3
	lda	#0					; 2
	tay						; 2
	sta	(GBASL),Y				; 6
	sta	(BASL),Y				; 6

						;============
						;        38

	inx						; 2
	cpx	#NUMFLAKES				; 2
	bne	erase_loop				; 3
						;============
						;	  7

							; -1

	;==========================
	; move snow
	;
	; 2 + NUM_FLAKES*(9+17+56+16+11+7) -1
	; 1 + NUM_FLAKES*116      = 1161

	ldx	#0					; 2
move_snow:

	; Check if off edge of screen

	lda	snow_y,X				; 4+
	cmp	#160					; 2
	beq	snow_new_y				; 3
						;==========
						;	  9


no_new_y:
							; -1
	lda	SEEDH					; 3
	lda	SEEDH					; 3
	lda	SEEDH					; 3
	lda	SEEDH					; 3
	lda	SEEDH					; 3
	jmp	just_inc				; 3
						;============
						;        17

snow_new_y:
	; out of bounds, get new
	lda	#32					; 2
	sta	snow_y,X				; 5
	lda	SEEDH					; 3
	and	#$3f					; 2
	sta	snow_x,X				; 5
						;============
						;	 17
just_inc:

	jsr	random16				; 6+42
	lda	SEEDL					; 3
	and	#$f					; 2
	beq	snow_left				; 3
						;===============
						;        56

; if left  = 7    = 6  + (9) = 16
; if right = 4+10 = 13 + (2) = 16
; else     = 4+9  = 12 + (3) = 16
							; -1
	cmp	#$1					; 2
	beq	snow_right				; 3
						;===============
						;         4

snow_else:
	lda	SEEDL	; nop				; 3
							; -1
	inc	snow_y,X				; 7
	jmp	snow_no					; 3
						;===============
						;         8+3

snow_right:
	nop						; 2
	inc	snow_x,X				; 7
	jmp	snow_no					; 3
						;============
						; 	9+2

snow_left:
	dec	snow_x,X				; 7
	lda	SEEDL		; nop
	lda	SEEDL		; nop
	lda	SEEDL		; nop




snow_no:
	lda	snow_x,X				; 4+
	and	#$3f					; 2
	sta	snow_x,X				; 5
							;====
							; 11

done_inc:
	inx						; 2
	cpx	#NUMFLAKES				; 2
	bne	move_snow				; 3
						;===========
						;	  7

							; -1

	;=========================
	; draw new snow
	;=========================
	; 2+ (40+22+28+7)*NUMFLAKES -1
	; 1+97*NUMFLAKES = 971

	ldx	#0					; 2
draw_loop:
	lda	snow_y,X				; 4+
	lsr						; 2
	lsr						; 2
	lsr						; 2
	sta	TEMPY					; 3
	lda	snow_x,X				; 4+
	tay						; 2
	lda	div_7_q,Y				; 4+
	ldy	TEMPY					; 3
	clc						; 2
	adc	hgr_offsets_l,Y				; 4+
	sta	GBASL					; 3
	adc	#30					; 2
	sta	BASL					; 3
						;===========
						;	 40

	lda	snow_y,X				; 4+
	asl						; 2
	asl						; 2
	and	#$1f					; 2
	clc						; 2
	adc	hgr_offsets_h,Y				; 4+
	sta	GBASH					; 3
	sta	BASH					; 3
						;=============
						;	 22

	ldy	snow_x,X				; 4+
	lda	div_7_r,Y				; 4+
	tay						; 2
	lda	pixel_lookup,Y				; 4+

	ldy	#0					; 2
	sta	(GBASL),Y				; 6
	sta	(BASL),Y				; 6
						;=============
						;	 28

	inx						; 2
	cpx	#NUMFLAKES				; 2
	bne	draw_loop				; 3
						;=============
						;	  7

							; -1
	lda	#100
	jsr	WAIT

	jmp	display_loop				; 3


snow_x:
	.byte 0,0,0,0,0,0,0,0,0,0

snow_y:
	.byte 0,0,0,0,0,0,0,0,0,0

hgr_offsets_h:
.byte	>$2000,>$2080,>$2100,>$2180,>$2200,>$2280,>$2300,>$2380
.byte	>$2028,>$20A8,>$2128,>$21A8,>$2228,>$22A8,>$2328,>$23A8
.byte	>$2050,>$20D0,>$2150,>$21D0,>$2250,>$22D0,>$2350,>$23D0

hgr_offsets_l:
.byte	<$2000,<$2080,<$2100,<$2180,<$2200,<$2280,<$2300,<$2380
.byte	<$2028,<$20A8,<$2128,<$21A8,<$2228,<$22A8,<$2328,<$23A8
.byte	<$2050,<$20D0,<$2150,<$21D0,<$2250,<$22D0,<$2350,<$23D0


div_7_q:
	.byte 0,0,0,0,0,0,0		; 0..6
	.byte 1,1,1,1,1,1,1		; 7..13
	.byte 2,2,2,2,2,2,2		; 14..20
	.byte 3,3,3,3,3,3,3		; 21..27
	.byte 4,4,4,4,4,4,4		; 28..34
	.byte 5,5,5,5,5,5,5		; 35..41
	.byte 6,6,6,6,6,6,6		; 42..48
	.byte 7,7,7,7,7,7,7		; 49..55
	.byte 8,8,8,8,8,8,8		; 56..62
	.byte 9				; 63

div_7_r:
	.byte 0,1,2,3,4,5,6		; 0..6
	.byte 0,1,2,3,4,5,6		; 7..13
	.byte 0,1,2,3,4,5,6		; 14..20
	.byte 0,1,2,3,4,5,6		; 21..27
	.byte 0,1,2,3,4,5,6		; 28..34
	.byte 0,1,2,3,4,5,6		; 35..41
	.byte 0,1,2,3,4,5,6		; 42..48
	.byte 0,1,2,3,4,5,6		; 49..55
	.byte 0,1,2,3,4,5,6		; 56..62
	.byte 0				; 63

pixel_lookup:
	.byte $01,$02,$04,$08,$10,$20,$40

.include "random16.s"