dos33fsprogs/xmas_2019/snow.s

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; Display falling snow
; by deater (Vince Weaver) <vince@deater.net>
; Zero Page
CH = $24
CV = $25
GBASL = $26
GBASH = $27
BASL = $28
BASH = $29
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SEEDL = $4E
SEEDH = $4F
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HGR_COLOR = $E4
SNOWX = $F0
HGR = $F3E2
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NUMFLAKES = 10
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.include "hardware.inc"
;==================================
;==================================
jsr HGR
bit FULLGR
display_loop:
; 0 4 8 c 10 14 18 1c
; 0 1 2 3 4 5 6 7
;=========================
; erase old snow
ldx #0
erase_loop:
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lda snow_y,X ; get Y
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lsr
lsr
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lsr ; divide by 8
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tay
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clc
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lda hgr_offsets_l,Y
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adc snow_x,X
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sta GBASL ; point GBASL to right location
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lda snow_y,X
asl
asl
and #$1f
clc
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adc hgr_offsets_h,Y
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sta GBASH
ldy #0
lda #0
sta (GBASL),Y
inx
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cpx #NUMFLAKES
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bne erase_loop
;==========================
; move snow
ldx #0
move_snow:
lda snow_y,X ; inc to next line
cmp #191
bne just_inc
lda #0
sta snow_y,X
jmp done_inc
just_inc:
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jsr random16
lda SEEDL
and #$f
beq snow_left
cmp #$1
beq snow_right
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inc snow_y,X
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jmp snow_no
snow_right:
inc snow_offset,X
jmp snow_no
snow_left:
dec snow_offset,X
snow_no:
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done_inc:
inx
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cpx #NUMFLAKES
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bne move_snow
;=========================
; draw new snow
ldx #0
draw_loop:
lda snow_y,X
lsr
lsr
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lsr
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tay
clc
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lda hgr_offsets_l,Y
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adc snow_x,X
sta GBASL
lda snow_y,X
asl
asl
and #$1f
clc
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adc hgr_offsets_h,Y
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sta GBASH
ldy #0
lda #1
sta (GBASL),Y
inx
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cpx #NUMFLAKES
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bne draw_loop
lda #100
jsr WAIT
jmp display_loop ; 3
snow_x:
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.byte 2,4,6,8,10,12,14,16,18,20
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snow_offset:
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.byte 0,1,2,3,4,5,6,7,0,1
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snow_y:
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.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
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.include "random16.s"