mirror of
https://github.com/sehugg/8bitworkshop.git
synced 2024-07-05 19:29:03 +00:00
373 lines
7.5 KiB
NASM
373 lines
7.5 KiB
NASM
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include "nesdefs.asm"
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;;;;; ZERO-PAGE VARIABLES
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seg.u ZEROPAGE
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org $0
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TrackFrac .byte ; fractional position along track
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Speed .byte ; speed of car
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TimeOfDay .word ; 16-bit time of day counter
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; Variables for preprocessing step
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XPos .word ; 16-bit X position
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XVel .word ; 16-bit X velocity
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TPos .word ; 16-bit track position
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TrackLookahead .byte ; current fractional track increment
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; Variables for track generation
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Random .byte ; random counter
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GenTarget .byte ; target of current curve
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GenDelta .byte ; curve increment
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GenCur .byte ; current curve value
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ZOfs .byte ; counter to draw striped center line
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Weather .byte ; bitmask for weather
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Heading .word ; sky scroll pos.
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XCenter = 128
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NumRoadSegments = 28
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; Preprocessing result: X positions for all track segments
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RoadX0 ds NumRoadSegments
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; Generated track curve data
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TrackLen equ 5
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TrackData ds TrackLen
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InitialSpeed equ 10 ; starting speed
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;;;;; OTHER VARIABLES
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seg.u RAM
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org $300
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;;;;; NES CARTRIDGE HEADER
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NES_HEADER 0,2,1,0 ; mapper 0, 2 PRGs, 1 CHR, horiz. mirror
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;;;;; START OF CODE
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Start:
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NES_INIT ; set up stack pointer, turn off PPU
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jsr WaitSync ; wait for VSYNC
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jsr ClearRAM ; clear RAM
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jsr WaitSync ; wait for VSYNC (and PPU warmup)
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jsr SetPalette ; set palette colors
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jsr FillVRAM ; set PPU video RAM
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jsr RoadSetup
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lda #0
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sta PPU_ADDR
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sta PPU_ADDR ; PPU addr = $0000
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sta PPU_SCROLL
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sta PPU_SCROLL ; scroll = $0000
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lda #CTRL_NMI
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sta PPU_CTRL ; enable NMI
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lda #MASK_BG|MASK_SPR
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sta PPU_MASK ; enable rendering
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.endless
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jmp .endless ; endless loop
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;;;;; ROAD SUBS
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RoadSetup: subroutine
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lda #1
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sta Random
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lda #InitialSpeed
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sta Speed
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lda #0
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sta TimeOfDay+1
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lda #0
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sta Weather
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rts
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RoadPreSetup: subroutine
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; Set up some values for road curve computation,
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; since we have some scanline left over.
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lda #0
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sta XVel
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sta XVel+1
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sta XPos
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sta XPos+1
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lda TrackFrac
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sta TPos
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lda #0
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sta TPos+1
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lda #10 ; initial lookahead
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sta TrackLookahead
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rts
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RoadPostFrame: subroutine
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; Advance position on track
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; TrackFrac += Speed
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lda TrackFrac
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clc
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adc Speed
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sta TrackFrac
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bcc .NoGenTrack ; addition overflowed?
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jsr GenTrack ; yes, generate new track segment
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.NoGenTrack
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; TimeOfDay += 1
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inc TimeOfDay
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bne .NoTODInc
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inc TimeOfDay+1
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lda TimeOfDay+1
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; See if it's nighttime yet, and if the stars come out
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clc
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adc #8
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and #$3f
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cmp #$35
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ror
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sta Weather
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.NoTODInc
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lda Heading
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sec
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sbc XPos+1
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sta Heading
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bit XPos+1
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bmi .NegHeading
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lda Heading+1
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sbc #0
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sta Heading+1
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rts
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.NegHeading
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lda Heading+1
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sbc #$ff
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sta Heading+1
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rts
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; Compute road curve from bottom of screen to horizon.
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PreprocessCurve subroutine
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ldx #NumRoadSegments-1
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.CurveLoop
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; Modify X position
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; XPos += XVel (16 bit add)
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lda XPos
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clc
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adc XVel
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sta XPos
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lda XPos+1
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adc XVel+1
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sta XPos+1
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sta RoadX0,x ; store in RoadX0 array
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; Modify X velocity (slope)
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; XVel += TrackData[TPos]
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ldy TPos+1
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lda TrackData,y
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clc ; clear carry for ADC
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bmi .CurveLeft ; track slope negative?
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adc XVel
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sta XVel
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lda XVel+1
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adc #0 ; carry +1
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jmp .NoCurveLeft
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.CurveLeft
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adc XVel
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sta XVel
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lda XVel+1
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sbc #0 ; carry -1
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nop ; make the branch timings are the same
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.NoCurveLeft
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sta XVel+1
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; Advance TPos (TrackData index)
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; TPos += TrackLookahead
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lda TPos
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clc
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adc TrackLookahead
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sta TPos
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lda TPos+1
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adc #0
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sta TPos+1
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; Go to next segment
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inc TrackLookahead ; see further along track
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dex
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bpl .CurveLoop
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rts
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; Generate next track byte
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GenTrack subroutine
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; Shift the existing track data one byte up
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; (a[i] = a[i+1])
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ldx #0
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.ShiftTrackLoop
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lda TrackData+1,x
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sta TrackData,x
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inx
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cpx #TrackLen-1
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bne .ShiftTrackLoop
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; Modify our current track value and
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; see if it intersects the target value
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lda GenCur
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clc
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adc GenDelta
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cmp GenTarget
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beq .ChangeTarget ; target == cur?
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bit GenTarget ; we need the sign flag
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bmi .TargetNeg ; target<0?
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bcs .ChangeTarget ; target>=0 && cur>=target?
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bcc .NoChangeTarget ; branch always taken
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.TargetNeg
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bcs .NoChangeTarget ; target<0 && cur<target?
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; Generate a new target value and increment value,
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; and make sure the increment value is positive if
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; the target is above the current value, and negative
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; otherwise
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.ChangeTarget
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lda Random
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jsr NextRandom ; get a random value
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sta Random
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and #$3f ; range 0..63
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sec
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sbc #$1f ; range -31..32
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sta GenTarget ; -> target
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cmp GenCur
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bmi .TargetBelow ; current > target?
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lda Random
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jsr NextRandom ; get a random value
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sta Random
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and #$f ; mask to 0..15
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jmp .TargetAbove
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.TargetBelow
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lda Random
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jsr NextRandom
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sta Random
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ora #$f0 ; mask to -16..0
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.TargetAbove
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ora #1 ; to avoid 0 values
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sta GenDelta ; -> delta
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lda GenCur
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.NoChangeTarget
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; Store the value in GenCur, and also
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; at the end of the TrackData array
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sta GenCur
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sta TrackData+TrackLen-1
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rts
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; fill video RAM
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FillVRAM: subroutine
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txa
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ldy #$20
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sty PPU_ADDR
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sta PPU_ADDR
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ldy #$10
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.loop:
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sta PPU_DATA
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adc #7
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inx
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bne .loop
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dey
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bne .loop
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rts
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; set palette colors
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SetPalette: subroutine
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ldy #$00
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lda #$3f
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sta PPU_ADDR
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sty PPU_ADDR
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ldx #32
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.loop:
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lda Palette,y
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sta PPU_DATA
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iny
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dex
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bne .loop
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rts
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; set sprite 0
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SetSprite0: subroutine
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sta $200 ;y
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lda #1 ;code
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sta $201
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lda #0 ;flags
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sta $202
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lda #8 ;xpos
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sta $203
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rts
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;;;;; COMMON SUBROUTINES
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include "nesppu.asm"
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;;;;; INTERRUPT HANDLERS
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MAC SLEEP ;usage: SLEEP n (n>1)
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.CYCLES SET {1}
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IF .CYCLES < 2
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ECHO "MACRO ERROR: 'SLEEP': Duration must be > 1"
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ERR
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ENDIF
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IF .CYCLES & 1
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bit $00
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.CYCLES SET .CYCLES - 3
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ENDIF
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REPEAT .CYCLES / 2
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nop
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REPEND
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ENDM
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NMIHandler: subroutine
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SAVE_REGS
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; setup sky scroll
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lda Heading+1
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sta PPU_SCROLL
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lda #0
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sta PPU_SCROLL
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; load sprites
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lda #112
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jsr SetSprite0
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lda #$02
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sta PPU_OAM_DMA
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; do road calc
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jsr RoadPreSetup
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jsr PreprocessCurve
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jsr RoadPostFrame
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; wait for sprite 0
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.wait0 bit PPU_STATUS
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bvs .wait0
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.wait1 bit PPU_STATUS
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bvc .wait1
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; alter horiz. scroll position for each scanline
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ldy #0
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.loop
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tya
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lsr
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lsr
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tax
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lda RoadX0,x
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sta PPU_SCROLL ; horiz byte
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lda #0
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sta PPU_SCROLL ; vert byte
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SLEEP 84
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iny
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cpy #112
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bne .loop
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RESTORE_REGS
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rti
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;;;;; CONSTANT DATA
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align $100
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Palette:
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hex 1f ;background
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hex 09092c00 ;bg0
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hex 09091900 ;bg1
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hex 09091500 ;bg2
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hex 09092500 ;bg3
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;;;;; CPU VECTORS
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NES_VECTORS
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;;;;; TILE SETS
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org $10000
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incbin "jroatch.chr"
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