549 lines
13 KiB
Plaintext
549 lines
13 KiB
Plaintext
processor 6502
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include "vcs.h"
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include "macro.h"
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include "xmacro.h"
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seg.u Variables
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org $80
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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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NumRoadSegments equ 28
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; Preprocessing result: X positions for all track segments
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RoadX0 REPEAT NumRoadSegments
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.byte
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REPEND
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; Generated track curve data
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TrackLen equ 5
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TrackData REPEAT TrackLen
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.byte
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REPEND
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InitialSpeed equ 10 ; starting speed
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;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;
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seg Code
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org $f000
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Start
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CLEAN_START
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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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NextFrame
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VERTICAL_SYNC
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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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lda #70 ; approx. center of screen
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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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; VSYNC+36+198+24 = 4+258 = 262 lines
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TIMER_SETUP 36
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; Initialize array with X road positions
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jsr PreprocessCurve
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TIMER_WAIT
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; Now draw the main frame
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TIMER_SETUP 198
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jsr DrawSky
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jsr SetupRoadComponents
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jsr DrawRoad ; draw the road
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TIMER_WAIT
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TIMER_SETUP 24
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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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TIMER_WAIT
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jmp NextFrame
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;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;
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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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; Set road component X positions and enable registers
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SetupRoadComponents subroutine
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lda RoadX0
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sta HMCLR ; clear HMOVE registers
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sec
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sta WSYNC
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.DivideLoop
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sbc #15 ; subtract 15
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bcs .DivideLoop ; branch while carry still set
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adc #15
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tay
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lda HMoveTable,y ; lookup HMOVE value
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sta HMM0 ; set missile 0 fine pos
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sta HMBL ; set ball fine pos
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sta HMM1 ; set missile 1 fine pos
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sta RESM0 ; set missile 0 position
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sta RESBL ; set ball position
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sta RESM1 ; set missile 1 position
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sta WSYNC
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sta HMOVE
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; Make road components converge at horizon
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; This will require an additional HMOVE
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lda #$90 ; right 7 pixels
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ldy #$70 ; left 7 pixels
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ldx #$00 ; no movement
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sta HMM0
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sty HMM1
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stx HMBL
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sta WSYNC
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sta HMOVE
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rts
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; Draw the sky, adding clouds and time-of-day colors.
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DrawDaytime subroutine
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lda TimeOfDay+1 ; offset into sunset color table
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and #$3f
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tay
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lda #16 ; initial height of sky segment
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.SkyLoop2
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tax ; height -> X
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pha ; push original height
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.SkyLoop
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lda SunsetColors,y ; get sunset color
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sta WSYNC ; start scanline
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sta COLUBK ; set background color
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lda SunsetColors+2,y ; get cloud color
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sta COLUPF ; set foreground color
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lda CloudPFData0,x ; load clouds -> playfield
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sta PF0
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lda CloudPFData1,x
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sta PF1
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lda CloudPFData2,x
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sta PF2
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dex
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bne .SkyLoop ; repeat until sky segment done
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iny ; next sky color
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tya
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and #$3f ; keep sky color in range 0-63
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tay ; sky color -> Y
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pla ; restore original segment height
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sec
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sbc #2 ; segment height - 2
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cmp #2 ; done with segments?
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bcs .SkyLoop2 ; no, repeat
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; Draw mountains
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; First, load mountain color
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lda TimeOfDay+1
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lsr
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lsr ; divide time-of-day by 4
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and #$f ; keep in range 0-15
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tax ; -> Y
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lda MountainColors,x ; load mountain color
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sta COLUPF ; set foreground
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lda GroundColors,x ; load ground color
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pha ; save it for later
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ldx #0
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stx PF0
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stx PF1 ; to avoid artifacts, we have to
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stx PF2 ; clear previous clouds
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.MtnLoop
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lda SunsetColors,y ; get sunset color
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sta WSYNC ; start scanline
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sta COLUBK ; set background color
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lda MtnPFData0,x ; load mountains -> playfield
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sta PF0
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lda MtnPFData1,x
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sta PF1
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lda MtnPFData2,x
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sta PF2
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iny ; next sky color
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tya
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and #$3f ; keep sky color in range 0-63
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tay ; sky color -> Y
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inx
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cpx #7 ; only 7 scanlines for the mountains
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bne .MtnLoop
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; Setup colors and enable road components
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pla ; restore ground color
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sta COLUBK ; set background
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lda #0
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sta PF0
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sta PF1
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sta PF2
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rts
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DrawSky
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bit Weather
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bmi DrawNight
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jmp DrawDaytime
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; Draw the night sky, with stars.
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DrawNight subroutine
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lda #6
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sta ENABL
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sta COLUPF
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ldy #0
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.MoreStars
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sta RESBL ; strobe the ball to display a star
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adc Start,y ; "randomize" the A register
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bmi .Delay1
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.Delay1
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ror
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bcc .Delay2
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.Delay2
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ror
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bcs .Delay3
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.Delay3
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ror
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bcs .Delay4
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.Delay4
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iny
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ldx INTIM
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cpx #$89 ; timer says we're done?
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bcs .MoreStars ; nope, make more stars
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lda #0
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sta ENABL ; disable ball
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rts
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DrawRoad subroutine
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lda #2
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sta ENAM0
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sta ENAM1 ; enable missiles
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sta COLUPF
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sta COLUP0
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sta COLUP1 ; set their colors too
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; Draw road
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lda TrackFrac
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asl
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sta WSYNC ; WSYNC so scanline starts at same place each time
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asl ; TrackFrac * 4
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sta ZOfs ; -> counter for animated stripe
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ldx #0 ; 0 is farthest segment
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.RoadLoop
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lda RoadColors,x ; color of sides and center line
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sta COLUP0
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sta COLUP1
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sta COLUPF
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lda RoadX0+1,x ; get next X coordinate
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sec
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sbc RoadX0,x ; subtract this X coordinate
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clc
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adc #7 ; add 7
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tay ; -> Y
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lda HMoveTable-3,y ; left side biased left
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sta HMM0
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lda HMoveTable,y ; center line
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sta HMBL
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lda HMoveTable+3,y ; right side biased right
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sta HMM1
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sta WSYNC
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sta HMOVE
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sta WSYNC
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; Make dashed road stripe by using a counter
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; initialized to the fractional track position,
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; then subracting the PIA timer as an approximation
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; to Z value.
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lda ZOfs
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sec
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sbc INTIM
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sta ZOfs ; ZOfs -= timer
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rol
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rol
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rol ; shift left by 3
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sta ENABL ; enable ball (bit 2)
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sta WSYNC
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lda RoadWidths,x ; lookup register for missile size
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sta NUSIZ0 ; store missile 0 size
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sta NUSIZ1 ; store missile 1 size
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sta WSYNC
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inx
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cpx #NumRoadSegments-1
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bne .RoadLoop ; repeat until all segments done
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; Clean up road objects
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lda #0
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sta ENAM0
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sta ENAM1
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sta ENABL
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sta COLUBK
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sta NUSIZ0
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sta NUSIZ1
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rts
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; Get next random number
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NextRandom subroutine
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lda Random
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lsr
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bcc .NoEor
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eor #$d4
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.NoEor:
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sta Random
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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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jsr NextRandom ; get a random value
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and #$3f ; range 0..63
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sec
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sbc #$1f ; range -31..32
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sta GenTarget ; -> target
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cmp GenCur
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bmi .TargetBelow ; current > target?
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jsr NextRandom ; get a random value
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and #$f ; mask to 0..15
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jmp .TargetAbove
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.TargetBelow
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jsr NextRandom
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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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;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;
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; HMOVE table from -7 to +8
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HMoveTable
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hex 7060504030201000f0e0d0c0b0a09080
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; Sunset table
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SunsetColors
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hex 00000000a07060504446383efc0c9f9f
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hex aeaeaeaeaeaeaeaeaeaeaeaeaeaeaeae
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hex 9f9f9f0cfc3e384644506070a0000000
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hex 00000000000000000000000000000000
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hex 0000 ; for overflow
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MountainColors
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hex 00021012f4f6e6264604020000000000
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GroundColors
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hex 00c0c2c4c6c8c8e6e4e2e00000000000
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RoadColors
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hex 020202
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hex 040404
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hex 060606
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hex 08080808
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hex 0a0a0a0a0a
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hex 0c0c0c0c0c0c
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hex 0e0e0e0e0e0e0e0e0e
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RoadWidths
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hex 000000000000
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hex 10101010
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hex 1010101010
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hex 202020202020
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hex 202020202020202020
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; Cloud data
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CloudPFData0
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.byte #%10000000
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.byte #%11100000
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.byte #%10000000
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.byte #%00000000
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.byte #%00000000
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.byte #%10000000
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.byte #%11100000
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.byte #%00000000
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CloudPFData1
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.byte #%11000000
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.byte #%11110000
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.byte #%11100001
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.byte #%00000011
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.byte #%00000111
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.byte #%11000000
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.byte #%00000000
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.byte #%00011000
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CloudPFData2
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.byte #%00000000
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.byte #%00001110
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.byte #%00011111
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.byte #%00000111
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.byte #%01100000
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.byte #%11000000
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.byte #%00000110
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.byte #%00000000
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; Mountain data
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MtnPFData0
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.byte #%00000000
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.byte #%00000000
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.byte #%00000000
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.byte #%00000000
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.byte #%00000000
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.byte #%10000000
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.byte #%11100000
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MtnPFData1
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.byte #%00000000
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.byte #%00010000
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.byte #%00110000
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.byte #%01111000
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.byte #%11111100
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.byte #%11111110
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.byte #%11111110
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MtnPFData2
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.byte #%00000000
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.byte #%00000000
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.byte #%00000000
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.byte #%00000100
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.byte #%00101110
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.byte #%01111111
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.byte #%11111111
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CarSprite
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.byte 0
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.byte #%10000001;--
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.byte #%10111101;--
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.byte #%11111111;--
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.byte #%10000001;--
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.byte #%10111101;--
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.byte #%01011010;--
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.byte #%01011010;--
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.byte #%01011010;--
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.byte #%00100100;--
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.byte #%11111111;--
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.byte #%10111101;--
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.byte #%00111100;--
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; Epilogue
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org $fffc
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.word Start
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.word Start
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