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136 lines
1.5 KiB
ArmAsm
136 lines
1.5 KiB
ArmAsm
; Display falling snow
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; by deater (Vince Weaver) <vince@deater.net>
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; Zero Page
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CH = $24
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CV = $25
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GBASL = $26
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GBASH = $27
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BASL = $28
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BASH = $29
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HGR_COLOR = $E4
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SNOWX = $F0
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HGR = $F3E2
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.include "hardware.inc"
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;==================================
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;==================================
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jsr HGR
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bit FULLGR
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display_loop:
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; 0 4 8 c 10 14 18 1c
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; 0 1 2 3 4 5 6 7
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;=========================
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; erase old snow
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ldx #0
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erase_loop:
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lda snow_y,X
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lsr
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lsr
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and #$fe
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; lsr
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tay
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clc
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lda hgr_offsets,Y
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adc snow_x,X
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sta GBASL
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lda snow_y,X
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asl
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asl
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and #$1f
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clc
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adc hgr_offsets+1,Y
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sta GBASH
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ldy #0
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lda #0
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sta (GBASL),Y
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inx
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cpx #8
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bne erase_loop
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;==========================
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; move snow
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ldx #0
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move_snow:
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lda snow_y,X ; inc to next line
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cmp #191
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bne just_inc
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lda #0
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sta snow_y,X
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jmp done_inc
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just_inc:
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inc snow_y,X
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done_inc:
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inx
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cpx #8
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bne move_snow
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;=========================
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; draw new snow
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ldx #0
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draw_loop:
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lda snow_y,X
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lsr
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lsr
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; lsr
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and #$fe
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tay
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clc
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lda hgr_offsets,Y
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adc snow_x,X
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sta GBASL
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lda snow_y,X
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asl
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asl
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and #$1f
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clc
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adc hgr_offsets+1,Y
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sta GBASH
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ldy #0
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lda #1
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sta (GBASL),Y
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inx
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cpx #8
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bne draw_loop
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lda #100
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jsr WAIT
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jmp display_loop ; 3
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snow_x:
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.byte 2,4,6,8,10,12,14,16
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snow_offset:
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.byte 0,1,2,3,4,5,6,7
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snow_y:
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.byte 0,0,0,0,0,0,0,0
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hgr_offsets:
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.word $2000,$2080,$2100,$2180,$2200,$2280,$2300,$2380
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.word $2028,$20A8,$2128,$21A8,$2228,$22A8,$2328,$23A8
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.word $2050,$20D0,$2150,$21D0,$2250,$22D0,$2350,$23D0
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