396 lines
8.0 KiB
Plaintext
396 lines
8.0 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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;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;
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seg.u Variables
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org $80
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Temp byte
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PlyrXPos byte ; player X position
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PlyrYPos byte ; player Y position
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TrailFrac byte ; 8-bit fractional trail position
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TrailPos word ; current 16-bit position along trail
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Speed byte ; velocity along trail
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Random word
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PF1Mask byte
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PF2Mask byte
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YP0 byte ; sprite 0 vertical counter
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YP1 byte ; sprite 1 vertical counter
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SpritePtr0 word ; sprite 0 pointer
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SpritePtr1 word ; sprite 1 pointer
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YStop byte ; temp. register
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NextHeight byte ; next sprite height
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ObsIndex byte ; current obstacle index (0-7)
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; Y offset into first obstacle
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TrailObsYOffset byte
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; next 8 bytes of trail data, starting from top of screen
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TrailData ds 8
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; Data for all 8 obstacle sprites
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ObsXpos ds 8
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ObsColor ds 8
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ObsPtrLo ds 8
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ObsPtrHi ds 8
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SpriteHeight equ 20 ; Height of all sprites
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TrailDisplayHeight equ 160 ; Height of trail dispaly area
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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 #70
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sta PlyrXPos
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lda #70
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sta ObsXpos+0
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lda #$7f
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sta ObsColor+0
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lda #<Frame0
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sta ObsPtrLo+0
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lda #>Frame0
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sta ObsPtrHi+0
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lda #40
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sta ObsXpos+1
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lda #$68
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sta ObsColor+1
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lda #<Frame0
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sta ObsPtrLo+1
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lda #>Frame0
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sta ObsPtrHi+1
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lda #60
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sta ObsXpos+2
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lda #$28
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sta ObsColor+2
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lda #<Frame0
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sta ObsPtrLo+2
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lda #>Frame0
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sta ObsPtrHi+2
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lda #$40
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sta Speed
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NextFrame
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VERTICAL_SYNC
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TIMER_SETUP 37
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lda #150
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sta YP0
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lda #$ff
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sta PF1Mask
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lda #$0f
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sta PF2Mask
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lda #<Frame0
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sta SpritePtr0
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lda #>Frame0
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sta SpritePtr0+1
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lda #$ff
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sta COLUP0
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lda PlyrXPos
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ldx #0
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jsr SetHorizPos
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sta WSYNC
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sta HMOVE
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TIMER_WAIT
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TIMER_SETUP 192
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jsr DrawTrail
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TIMER_WAIT
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TIMER_SETUP 30
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jsr MoveAlongTrail
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TIMER_WAIT
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jmp NextFrame
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;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;
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; Move player along the trail
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MoveAlongTrail subroutine
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lda TrailFrac
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sec
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adc Speed
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sta TrailFrac
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php
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lda TrailPos
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adc #0
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sta TrailPos
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lda TrailPos+1
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adc #0
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sta TrailPos+1
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plp
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lda TrailObsYOffset
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adc #0
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cmp #SpriteHeight+4
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bcc .NoNextObstacle
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; Move obstacles up one slot
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ldx #6
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.MoveLoop
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lda ObsXpos,x
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sta ObsXpos+1,x
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lda ObsColor,x
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sta ObsColor+1,x
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lda ObsPtrLo,x
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sta ObsPtrLo+1,x
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lda ObsPtrHi,x
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sta ObsPtrHi+1,x
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dex
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bpl .MoveLoop
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; Generate new obstacle
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jsr NextRandom
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sta ObsColor
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and #$5f
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sta ObsXpos
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; Reset TrailObsYOffset
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lda #0
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.NoNextObstacle
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sta TrailObsYOffset
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rts
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; Put the trail edges into the PF1 and PF2 registers.
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; X = trail position lo byte
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MAC TRAIL_PLAYFIELD
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lda PF1Mask
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and $f000,x
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sta PF1
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lda PF2Mask
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and $f100,x
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sta PF2
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dex
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ENDM
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; Get next bitmap byte of sprite 0
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MAC DRAW_SPRITE0
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sec ; ensure carry set for subtraction
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lda #SpriteHeight
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isb YP0 ; INC yp0, then SBC yp0
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tay
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bcs .DoDraw ; outside bounds?
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lda #0 ; clear bitmap
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.byte $2c ; (BIT aaaa, skips next 2 bytes)
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.DoDraw
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lda (SpritePtr0),y ; lookup sprite bitmap
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sta GRP0 ; A -> GRP0
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ENDM
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; Draw the trail
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DrawTrail subroutine
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lda TrailObsYOffset
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sta NextHeight
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lda #$c4
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sta COLUPF
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lda #$F0
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sta PF0
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lda #%0000001
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sta CTRLPF
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; lda #1
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; sta VDELP0
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; Set up ObsIndex (index of next obstacle to draw)
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ldy #0
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sty ObsIndex
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; Set up X register with trail position lo byte,
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; which displays the scrolling "forest" mask.
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; We decrement this value every scanline and when it
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; hits "YStop" we are done drawing.
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lda TrailPos
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sta YStop
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clc
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adc #TrailDisplayHeight
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tax
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; Set position of sprite
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.SetSpritePos
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sta HMCLR ; clear fine offsets
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DRAW_SPRITE0
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; Set up more values for sprite 1
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ldy ObsIndex
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lda ObsPtrLo,y
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sta SpritePtr1
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lda ObsPtrHi,y
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sta SpritePtr1+1
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lda ObsColor,y
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sta COLUP1
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; Update the playfield masks
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; lda PFMask1,x
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; sta PFMask1
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; lda PFMask2,x
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; sta PFMask2
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; Set next height
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lda NextHeight
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sta YP1
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lda #SpriteHeight
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sta NextHeight
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; Make sure grass texture is aligned
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dex
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cpx YStop
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beq .NoMoreObjects
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dex
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cpx YStop
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beq .NoMoreObjects
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dex
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cpx YStop
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beq .NoMoreObjects
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; Redraw sprite
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DRAW_SPRITE0
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; Position object horizontally
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sta WSYNC
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ldy ObsIndex
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lda ObsXpos,y
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iny
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sty ObsIndex
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sec
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.DivideLoop
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sbc #15 ; subtract 15
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bcs .DivideLoop ; branch until negative
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eor #7
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asl
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asl
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asl
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asl
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sta HMP1 ; set fine offset
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sta RESP1 ; fix coarse position
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.Loop
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sta WSYNC
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; Draw sprite 0, with skipdraw
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DRAW_SPRITE0
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; Draw the playfield
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TRAIL_PLAYFIELD
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; Should we exit?
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cpx YStop
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beq .NoMoreObjects
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; Draw sprite 1, no skipdraw
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ldy YP1
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lda (SpritePtr1),y
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dey
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sty YP1 ; YP1--
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sta GRP1 ; A -> GRP1
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; repeat until sprite 1 finished drawing (YP1<0)
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bpl .Loop
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; If not out of scanlines, position next sprite and continue
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jmp .SetSpritePos
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; Clean up playfield
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.NoMoreObjects
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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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sta GRP0
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sta GRP1
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rts
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; Random number generation
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NextRandom subroutine
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lda Random
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lsr
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ror Random+1
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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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eor Random+1
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rts
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PrevRandom subroutine
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lda Random
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asl
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rol Random+1
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bcc .NoEor
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eor #$a9
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.NoEor:
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sta Random
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eor Random+1
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rts
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SetHorizPos
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sta WSYNC ; start a new line
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sec ; set carry flag
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DivideLoop
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sbc #15 ; subtract 15
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bcs DivideLoop ; branch until negative
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eor #7 ; calculate fine offset
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asl
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asl
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asl
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asl
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sta HMP0,x ; set fine offset
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sta RESP0,x ; fix coarse position
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rts ; return to caller
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;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;
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align $100
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PFMask1
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byte #%00000000
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byte #%10000000
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byte #%11000000
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byte #%11100000
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byte #%11110000
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byte #%11111000
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byte #%11111100
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byte #%11111110
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byte #%11111111
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byte #%11111111
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byte #%11111111
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byte #%11111111
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byte #%11111111
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byte #%11111111
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byte #%11111111
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byte #%11111111
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byte #%11111111
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PFMask2
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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 #%00000000
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byte #%00000000
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byte #%00000000
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byte #%00000001
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byte #%00000011
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byte #%00000111
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byte #%00001111
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byte #%00011111
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byte #%00111111
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byte #%01111111
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byte #%11111111
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; Bitmap data "standing" position
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Frame0
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.byte #0
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.byte #%01101100;$F6
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.byte #%00101000;$86
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.byte #%00101000;$86
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.byte #%00111000;$86
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.byte #%10111010;$C2
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.byte #%10111010;$C2
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.byte #%01111100;$C2
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.byte #%00111000;$C2
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.byte #%00111000;$16
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.byte #%01000100;$16
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.byte #%01111100;$16
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.byte #%01111100;$18
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.byte #%01010100;$18
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.byte #%01111100;$18
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.byte #%11111110;$F2
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.byte #%00111000;$F4
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.byte 0,0,0,0,0,0,0
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HMoveTable
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hex 7060504030201000f0e0d0c0b0a09080
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;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;
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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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