LIST OFF ; *** I N D Y 5 0 0 *** ; Copyright 1977 Atari, Inc. ; Designer: Ed Riddle ; Analyzed, labeled and commented ; by Glenn Saunders ; Last Update: Jan. 5, 2004 ; by Dennis Debro ; Last Update: Sep 14, 2004 processor 6502 ; ; NOTE: You must compile this with vcs.h version 105 or greater. ; TIA_BASE_READ_ADDRESS = $30 ; set the read address base so this runs on ; the real VCS and compiles to the exact ; ROM image include vcs.h LIST ON ;=============================================================================== ; A S S E M B L E R - S W I T C H E S ;=============================================================================== NTSC = 0 PAL = 1 COMPILE_VERSION = NTSC ; change this to compile for different ; regions ;============================================================================ ; T I A - C O N S T A N T S ;============================================================================ HMOVE_L7 = $70 HMOVE_L6 = $60 HMOVE_L5 = $50 HMOVE_L4 = $40 HMOVE_L3 = $30 HMOVE_L2 = $20 HMOVE_L1 = $10 HMOVE_0 = $00 HMOVE_R1 = $F0 HMOVE_R2 = $E0 HMOVE_R3 = $D0 HMOVE_R4 = $C0 HMOVE_R5 = $B0 HMOVE_R6 = $A0 HMOVE_R7 = $90 HMOVE_R8 = $80 ; values for NUSIZx: ONE_COPY = %000 TWO_COPIES = %001 TWO_WIDE_COPIES = %010 THREE_COPIES = %011 DOUBLE_SIZE = %101 THREE_MED_COPIES = %110 QUAD_SIZE = %111 MSBL_SIZE1 = %000000 MSBL_SIZE2 = %010000 MSBL_SIZE4 = %100000 MSBL_SIZE8 = %110000 ; values for REFPx: NO_REFLECT = %0000 REFLECT = %1000 ; SWCHA joystick bits: MOVE_RIGHT = %01111111 MOVE_LEFT = %10111111 MOVE_DOWN = %00100000 MOVE_UP = %00010000 NO_MOVE = %11111111 ; mask for SWCHB BW_MASK = %1000 ; black and white bit SELECT_MASK = %10 RESET_MASK = %01 ;============================================================================ ; U S E R - C O N S T A N T S ;============================================================================ ROM_BASE_ADDRESS = $F000 IF COMPILE_VERSION = NTSC VBLANK_TIME = $24 YELLOW = $16 ELSE VBLANK_TIME = $23 YELLOW = $26 ENDIF ; color constancts BLACK = $00 WHITE = $0E ORANGE = $20 BRICK_RED = $30 RED = $40 PURPLE = $50 BLUE_PURPLE = $60 DK_BLUE = $70 LT_BLUE = $90 GREEN = $D0 BROWN = $E0 MAX_GAME_SELECTION = 14 SELECT_DELAY = 30 ; number of frames to wait to check select ; switch H_KERNEL = 222 H_FONT = 5 H_CAR = 8 MAX_ROTATIONS = 8 MAX_CAR_SPRITES = H_CAR*MAX_ROTATIONS MAX_COLOR_XOR = 64 YMIN = 40 YMAX = 232 ; game state values GAME_RUNNING = #255 GAME_OVER = 0 TIME_TRIAL_SECONDS = $60 ; BCD TIME_TRIAL_REDUCTION = $01 ; BCD MAX_RACE_LAPS = $25 ; BCD MAX_CRASH_AND_SCORE = $50 ; BCD MAX_TAG_SCORE = $99 ; BCD ; game variation flags MASK_TIME_TRIAL = %01000000 MASK_GAME_COLORS = %00111000 MASK_TRACK_NUMBER = %00000111 GRAND_PRIX_TRACK = %000 DEVIL_ELBOW_TRACK = %001 CRASH_AND_SCORE_TRACK1 = %010 CRASH_AND_SCORE_TRACK2 = %011 SPRINT_TRACK = %100 ; game selection values CRASH_AND_SCORE = 4 TAG = 8 ICE_RACE = 10 FINISH_LINE_X = 85 ;init game values PLAYER1_STARTX1 = 134 PLAYER2_STARTX1 = 134 PLAYER1_STARTY1 = 192 PLAYER2_STARTY1 = 210 PLAYER1_START_DIR1 = REFLECT PLAYER2_START_DIR2 = REFLECT BALL_STARTY1 = 128 ;these values used for "Crash and Score" and "Tag" games PLAYER1_STARTX2 = 28 PLAYER2_STARTX2 = 136 PLAYER1_STARTY2 = 202 PLAYER2_STARTY2 = 68 PLAYER1_START_DIR2 = NO_REFLECT PLAYER2_START_DIR2 = REFLECT BALL_STARTY2 = 255 ;values for pixel movement VMOVE_0 = 8 VMOVE_U2 = VMOVE_0+2 VMOVE_U1 = VMOVE_0+1 VMOVE_D1 = VMOVE_0-1 VMOVE_D2 = VMOVE_0-2 ;============================================================================ ; Z P - V A R I A B L E S ;============================================================================ SEG.U variables org $80 gameSelection ds 1 gameVariation ds 1 playerScores ds 2 ;-------------------------------------- player1Score = playerScores player2Score = playerScores+1 scoreOffsets ds 4 ;-------------------------------------- lsbScoreOffsets = scoreOffsets player1LSBOffset = lsbScoreOffsets player2LSBOffset = lsbScoreOffsets+1 ;-------------------------------------- msbScoreOffsets = scoreOffsets+2 player1MSBOffset = msbScoreOffsets player2MSBOffset = msbScoreOffsets+1 scoreGraphics ds 2 ;-------------------------------------- scoreGraphic1 = scoreGraphics scoreGraphic2 = scoreGraphics+1 playerVelocity ds 2 ;-------------------------------------- player1Velocity = playerVelocity player2Velocity = playerVelocity+1 playerMotion ds 2 ;-------------------------------------- player1Motion = playerMotion player2Motion = player1Motion+1 lapCheckPoints ds 2 ;-------------------------------------- player1CheckPoints = lapCheckPoints player2CheckPoints = player1CheckPoints+1 colorXOR ds 1 playerRotation ds 2 ;-------------------------------------- player1Rotation = playerRotation player2Rotation = player1Rotation+1 changeDirectionTimer ds 2 ;-------------------------------------- player1ChangeDirTimer = changeDirectionTimer player2ChangeDirTimer = player1ChangeDirTimer+1 tagChasing ds 1 ; car being chased in TAG game playfieldBallCollisions ds 2 ;-------------------------------------- player1PFBallCollisions = playfieldBallCollisions player2PFBallCollisions = playfieldBallCollisions+1 missileVertPos ds 1 playerFrameWait ds 2 ;-------------------------------------- player1FrameWait = playerFrameWait player2FrameWait = player1FrameWait+1 volumeChannelValues ds 2 ;-------------------------------------- volumeChannel1 = volumeChannelValues volumeChannel2 = volumeChannel1+1 temp ds 1 drivingControllerValues ds 2 ;-------------------------------------- player1DCValue = drivingControllerValues player2DCValue = drivingControllerValues+1 tempStackPointer ds 1 previousPlayerDirections ds 2 ;-------------------------------------- player1PrevDirections = previousPlayerDirections player2PrevDirections = player1PrevDirections+1 gameState ds 1 hitPlayfieldFlag ds 2 ;-------------------------------------- player1HitPlayfield = hitPlayfieldFlag player2HitPlayfield = player1HitPlayfield+1 carCollisionFlag ds 1 currentScanline ds 1 accelerationRate ds 2 ;-------------------------------------- player1AccelerationRate = accelerationRate player2AccelerationRate = accelerationRate+1 previousDCValues ds 2 ;-------------------------------------- player1PrevDCValue = previousDCValues player2PrevDCValue = player1PrevDCValue+1 difficultySwitchValue ds 1 lastTagChaser ds 1 playerVerticalMotion ds 1 carGraphics ds H_CAR*2 ;-------------------------------------- player1Graphics = carGraphics player2Graphics = player1Graphics+H_CAR frameCount ds 1 selectDebounce ds 1 accelerationSoundTimer ds 2 ;-------------------------------------- player1AccelerationTimer = accelerationSoundTimer player2AccelerationTimer = player1AccelerationTimer+1 playerGraphicsMask ds 2 ;-------------------------------------- player1GraphicMask = playerGraphicsMask player2GraphicMask = playerGraphicsMask+1 initAccelerationRate ds 1 initDecelerationRate ds 1 maximumVelocity ds 1 rotationDelay ds 1 playerDelay ds 2 ;-------------------------------------- player1Delay = playerDelay player2Delay = playerDelay+1 playerHorizPos ds 2 ;-------------------------------------- player1HorizPos = playerHorizPos player2HorizPos = playerHorizPos+1 playerVertPos ds 2 ;-------------------------------------- player1VertPos = playerVertPos player2VertPos = playerVertPos+1 playerDirections ds 2 ;-------------------------------------- player1Direction = playerDirections player2Direction = playerDirections+1 ballVertPos ds 1 missileHorizPos ds 1 ; don't think this is actually used playfieldPointers ds 6 ;-------------------------------------- pf0Pointer = playfieldPointers pf1Pointer = pf0Pointer+2 pf2Pointer = pf1Pointer+2 ;============================================================================ ; R O M - C O D E ;============================================================================ SEG Bank0 org ROM_BASE_ADDRESS Start ; ; Set up everything so the power up state is known. ; sei cld ; clear decimal mode lda #0 ldx #0 .clearLoop sta VSYNC,x ; clear TIA sta SWCHA,x ; clear RIOT inx bne .clearLoop lda #MSBL_SIZE4 sta NUSIZ0 sta NUSIZ1 ldx #$FF txs ; point the stack to the beginning jsr SetDisplayedGameSelection jsr EndGame jsr InitializeGameVariables MainLoop jsr VerticalSync jsr LoadCarGraphics lda frameCount ; get the frame count and #$3F ; increment colorXOR ~ every 60 seconds bne .doGameCalculations inc colorXOR ; causes colors to cycle when game over lda gameState ; get the current game state beq .doGameCalculations ; do game calculations if game over lda colorXOR cmp #MAX_COLOR_XOR ; if colorXOR reaches it's max then beq .resetColorXOR ; reset it's value bit gameVariation bvs .doGameCalculations ldx #1 ; time trial timer is player 2 score jsr ReduceTimeTrialTimer bne .doGameCalculations .resetColorXOR lda #0 jsr SetVariablesToEndGame .doGameCalculations lda gameState ; get the current game state beq .doneGameCalculations ; skip game logic if game over jsr PlayGameSounds jsr ReadDrivingControllers jsr DeterminePlayerMotion jsr CheckIfPlayerHitTrack lda gameSelection ; get the current game selection cmp #CRASH_AND_SCORE bcc GrandPrixGameLogic cmp #ICE_RACE bcs GrandPrixGameLogic cmp #TAG bcs TagGameLogic CrashAndScoreGameLogic jsr CheckForCarCollision jsr CheckIfPlayerCaughtBox jmp .doneGameCalculations GrandPrixGameLogic jsr CheckForCarCollision jsr CheckForCompletedLaps jmp .doneGameCalculations TagGameLogic jsr CheckTagCollisions ldx tagChasing ; get the car number being chased lda frameCount and #2 beq .doneGameCalculations lda #$FF ; turn car graphic on every other frame sta playerGraphicsMask,x .doneGameCalculations jsr DoneGameCalculations jsr DisplayKernel jsr ReadConsoleSwitches jsr SetGameColors jmp MainLoop VerticalSync .waitTime lda INTIM bne .waitTime sta WSYNC IF COMPILE_VERSION = PAL lda #%11010010 sta VBLANK ; disable TIA (D1 = 1) sta WSYNC sta WSYNC sta WSYNC ENDIF lda #22 sta VSYNC ; start vertical sync (D1 = 1) sta TIM8T ; set vertical sync wait time inc frameCount .vsyncWaitTime lda INTIM bne .vsyncWaitTime sta WSYNC sta VSYNC ; end vertical sync (D1 = 0) lda #VBLANK_TIME sta TIM64T ; set vertical blank wait time rts DisplayKernel SUBROUTINE lda #0 sta CXCLR ; clear all collisions sta currentScanline ; set initial scan line count sta scoreGraphic1 ; clear score graphic data to avoid sta scoreGraphic2 ; bleeding from previous frame lda #%00000010 ; set playfield to SCORE mode (i.e. sta CTRLPF ; player colors same as score) tsx stx tempStackPointer ; save stack pointer .waitTime lda INTIM bne .waitTime sta WSYNC sta VBLANK ; enable TIA (D1 = 0) IF COMPILE_VERSION = PAL ldy #24 .skip25Scanlines sta WSYNC dey bpl .skip25Scanlines ENDIF .scoreKernelWait inc currentScanline ; 5 sta WSYNC ;-------------------------------------- lda currentScanline ; 3 cmp #2 ; 2 bcc .scoreKernelWait ; 2³ ScoreKernel sta WSYNC ;-------------------------------------- lda scoreGraphic1 ; 3 get the score graphic for display sta PF1 ; 3 = @06 ldy player1MSBOffset ; 3 lda NumberFonts,y ; 4 read the number fonts and #$F0 ; 2 mask the lower nybble sta scoreGraphic1 ; 3 save it in the score graphic ldy player1LSBOffset ; 3 lda NumberFonts,y ; 4 read the number fonts and #$0F ; 2 mask the upper nybble ora scoreGraphic1 ; 3 or with score graphic to get LSB sta scoreGraphic1 ; 3 value lda scoreGraphic2 ; 3 get the score graphic for display sta PF1 ; 3 = @39 ldy player2MSBOffset ; 3 lda NumberFonts,y ; 4 read the number fonts and #$F0 ; 2 mask the lower nybble sta scoreGraphic2 ; 3 save it in the score graphic ldy player2LSBOffset ; 3 lda NumberFonts,y ; 4 read the number fonts and #$0F ; 2 mask the upper nybble ora scoreGraphic2 ; 3 or with score graphic to get LSB sta scoreGraphic2 ; 3 value nop ; 2 waste 8 cycles to push to the next nop ; 2 scan line nop ; 2 nop ; 2 ;-------------------------------------- inc currentScanline ; 5 = @03 increment scan line count lda currentScanline ; 3 cmp #H_FONT+2+1 ; 2 bcs GameKernel ; 2³ lda scoreGraphic1 ; 3 sta PF1 ; 3 = @16 inc player1LSBOffset ; 5 inc player1MSBOffset ; 5 inc player2LSBOffset ; 5 inc player2MSBOffset ; 5 lda scoreGraphic2 ; 3 sta PF1 ; 3 = @42 jmp ScoreKernel ; 3 GameKernel lda #0 ; 2 sta PF1 ; 3 = @16 clear PF1 register lda (VSYNC,x) ; 6 waste 6 cycles :-) lda #32 ; 2 sta currentScanline ; 3 lda #%00000001 ; 2 sta CTRLPF ; 3 = @32 reflect the playfield .gameKernelLoop ldx #ENAM1 ; 2 txs ; 2 stack trick to enable missiles sec ; 2 lda player1VertPos ; 3 get player1's vertical position sbc currentScanline ; 3 subtract current scan line and #$FE ; 2 graphics drawn every other line tax ; 2 and player1GraphicMask ; 3 used to make player blink in Tag beq .loadPlayer1Graphics ; 2 lda #0 ; 2 beq .drawPlayer1Car ; 3 .loadPlayer1Graphics lda carGraphics,x ; 4 .drawPlayer1Car sta WSYNC ;-------------------------------------- sta GRP0 ; 3 = @03 clc ; 2 lda missileVertPos ; 3 get missile's vertical position sbc currentScanline ; 3 subtract current scan line and #$F8 ; 2 mask D2 - D0 php ; 3 push Z status to stack to php ; 3 enable/disable M0 and M1 lda currentScanline ; 3 get the current scan line bpl .determinePFOffset ; 2³ eor #$F8 ; 2 .determinePFOffset lsr ; 2 divide the value by 8 (each PF byte lsr ; 2 uses 8 scan lines) lsr ; 2 tay ; 2 save for PF lookup inc player1LSBOffset ; 5 probably here to waste 5 cycles nop ; 2 lda currentScanline ; 3 get the current scan line cmp ballVertPos ; 3 compare with ball vertical position bcc .setupToDrawPlayer2 ; 2³ skip drawing finish line if less sta ENABL ; 3 = @52 draw finish line every other line .setupToDrawPlayer2 lda player2VertPos ; 3 get player2's vertical position sec ; 2 sbc currentScanline ; 3 subtract current scan line ora #1 ; 2 graphics drawn every other line tax ; 2 and player2GraphicMask ; 3 used to make player blink in Tag beq .loadPlayer2Graphics ; 2³ lda #0 ; 2 beq .drawPlayer2Car ; 3 .loadPlayer2Graphics lda carGraphics,x ; 4 .drawPlayer2Car ;-------------------------------------- sta GRP1 ; 3 = @01 lda (pf0Pointer),y ; 5 draw the race track playfield sta PF0 ; 3 = @09 lda (pf1Pointer),y ; 5 sta PF1 ; 3 = @17 lda (pf2Pointer),y ; 5 sta PF2 ; 3 = @25 clc ; 2 lda currentScanline ; 3 adc #2 ; 2 2LK sta currentScanline ; 3 cmp #H_KERNEL ; 2 bcc .gameKernelLoop ; 2³ ldx tempStackPointer ; 3 = @42 txs ; 2 restore stack pointer lda #$F0 ; 2 sta player1GraphicMask ; 3 reset player graphic mask values sta player2GraphicMask ; 3 lda #0 ; 2 sta ENAM0 ; 3 = @57 sta ENAM1 ; 3 = @60 sta ENABL ; 3 = @63 sta GRP0 ; 3 = @66 sta GRP1 ; 3 = @69 sta GRP0 ; 3 = @72 VDEL'd sta PF2 ; 3 = @75 ;-------------------------------------- sta PF1 ; 3 = @02 sta PF0 ; 3 = @05 IF COMPILE_VERSION = NTSC lda #%11010010 ; 2 sta TIM8T ; 4 = @11 sta VBLANK ; 3 = @14 disable TIA (D1 = 1) ELSE ldy #23 .skip24Scanlines sta WSYNC dey bpl .skip24Scanlines lda #%11010010 sta TIM8T sta WSYNC ENDIF rts ReadConsoleSwitches lda SWCHB ; read console switches lsr ; RESET now in carry bcs .checkForSelectSwitch lda #GAME_RUNNING sta gameState lda #0 ldx #32 .clearGameRAM sta playerScores,x dex bpl .clearGameRAM lda frameCount ; get the current frame count and #1 ; make the frame count between 0 and 1 sta frameCount lda #TIME_TRIAL_SECONDS bit gameVariation ; if this a one player game (Time Trial) bvs .skipTimeTrialSecondsSet ; then don't set time trial seconds sta player2Score ; set time trial seconds .skipTimeTrialSecondsSet lda #0 ; not needed must have been left over rts .checkForSelectSwitch lsr ; SELECT now in carry bcs .clearSelectDebounce lda selectDebounce ; get select debounce value bne .reduceSelectDebounce ; skip select if not 0 lda #SELECT_DELAY sta selectDebounce ; reset select debounce value lda gameSelection ; get the current game selection cmp #MAX_GAME_SELECTION-1 bcc .incrementGameSelection lda #-1 ; set gameSelection to -1 so increment sta gameSelection ; below sets it to 0 .incrementGameSelection inc gameSelection jsr EndGame sta player2Score ; clear player 2 score (a = 0) SetDisplayedGameSelection sed clc lda gameSelection ; get the current game selection tax ; move to x to look up game variation adc #1 ; increase the value by 1 (BCD) to show sta player1Score ; on the screen cld lda GameVariationTable,x ; set the game variation based on game sta gameVariation ; selection rts .clearSelectDebounce lda #0 sta selectDebounce rts .reduceSelectDebounce dec selectDebounce rts SetGameColors ldy #24 sec ; set carry to assume B/W mode lda SWCHB ; read the console switches and #BW_MASK ; mask to get B/W switch value beq .skipColorMode lda gameVariation ; get the current game variation and #MASK_GAME_COLORS ; mask to get the variation colors lsr ; divide the value by 2 tay ; for the table offset .skipColorMode ldx #3 .setColorsLoop lda GameColorTable,y bit gameState bvs .skipColorCycling eor colorXOR .skipColorCycling bcc .setGameColors ; set colors if in COLOR mode and #$0F ; mask the hue .setGameColors sta COLUP0,x ; color the players, playfield, and dey ; background dex bpl .setColorsLoop lda SWCHB ; read console switches and #SELECT_MASK | RESET_MASK ; keep the SELECT and RESET values eor #SELECT_MASK | RESET_MASK bne InitializeGameVariables ; init game vars if either are pressed rts InitializeGameVariables lda RaceTrackPF0LSBTable sta pf0Pointer lda #>RaceTrackGraphics ; set the MSB of the race track pointers sta pf0Pointer+1 sta pf1Pointer+1 sta pf2Pointer+1 lda gameVariation ; get the game variation and #MASK_TRACK_NUMBER ; mask values to get track number tay cmp #DEVIL_ELBOW_TRACK ; set the PF1 pointers if this is not the bne .setTrackPF1Pointer ; Devil's Elbow track lda RaceTrackPF0LSBTable+1 ; set the PF0 pointer for the Devil's sta pf0Pointer ; Elbow track .setTrackPF1Pointer lda RaceTrackPF1OffsetTable,y tax lda RaceTrackPF1LSBTable,x sta pf1Pointer lda RaceTrackPF2LSBTable,y sta pf2Pointer ldx #6 ldy #6 lda gameVariation ; get current game variation and #CRASH_AND_SCORE_TRACK1 ; if this is not a "Crash and Score" or beq .initVariablesLoop ; "Tag" game then set game variables ldy #13 .initVariablesLoop lda InitializationTable,y sta playerHorizPos,x dey dex bpl .initVariablesLoop inx ; x = 0 jsr CalculateHorizPosition ; move player 1 horizontally inx lda player2HorizPos jsr CalculateHorizPosition ; move player 2 horizontally ldx #4 lda #FINISH_LINE_X jsr CalculateHorizPosition ; move ball (finish line) horizontally sta WSYNC ; wait for next scan line sta HMOVE lda gameSelection ; get the current game selection tay lda MomentumRateOffsetTable,y tay ldx #3 .setMomentumRateLoop lda MomentumRateTable,y sta initAccelerationRate,x dey dex bpl .setMomentumRateLoop rts ReadDrivingControllers ldx #1 ; start with player 2 lda SWCHB ; read the console switches sta difficultySwitchValue ; save to manipulate difficulty settings lda SWCHA ; read driving controller bit gameVariation ; check for one player (time trial) game bvc .readPlayer1Controller .setDrivingControllerValue and #3 ; keep valid driving controller values sta drivingControllerValues,x lda INPT4,x ; read the joystick button bpl .accelerateCar ; branch if fire button pressed lda accelerationRate,x ; get the player's acceleration rate bne .reduceAccelerationRate ; if not 0 then don't move lda playerVelocity,x ; get the player's velocity beq .setPlayerRotation ; branch if at rest dec playerVelocity,x ; reduce the player's velocity lda initDecelerationRate sta accelerationRate,x ; set to show car slowing down jsr InitPlayerMotion jmp .setPlayerRotation .accelerateCar lda accelerationRate,x ; don't change player's velocity until bne .reduceAccelerationRate ; acceleration rate reaches 0 lda maximumVelocity bit difficultySwitchValue ; check the difficulty setting bmi .skipSpeedReduction ; skip speed reduction if EXPERT setting sec sbc #2 ; reduce init speed for NOVICE mode .skipSpeedReduction cmp playerVelocity,x bcc .setPlayerRotation inc playerVelocity,x ; increase player's velocity lda initAccelerationRate ; reset the acceleration rate sta accelerationRate,x jsr InitPlayerMotion .setPlayerRotation lda drivingControllerValues,x ; get driving controller values asl ; multiply the value by 4 (i.e. shift asl ; values to D3 and D2) ora previousDCValues,x ; add in the previous DC values tay ; set for RotationValueTable offset lda playerDirections,x ; get the player's direction cmp previousPlayerDirections,x bne .readNewDirectionValues lda RotationValueTable,y sta playerRotation,x .setPlayerDirections clc adc playerDirections,x ; add in direction to manipulate REFLECT and #$0F ; only keep lower nybbles sta playerDirections,x tya lsr ; shift driving controller value back lsr ; into D1 and D0 sta previousDCValues,x ; to save for next frame jsr CheckToChangeDirectionPath .readPlayer1Controller asl difficultySwitchValue ; shift player 1 difficulty setting to D7 lda SWCHA ; read driving controller lsr ; shift player 1 values to lower nybbles lsr lsr lsr dex beq .setDrivingControllerValue rts .reduceAccelerationRate dec accelerationRate,x jmp .setPlayerRotation .readNewDirectionValues lda RotationValueTable,y jmp .setPlayerDirections InitPlayerMotion lda playerVelocity,x ; get the player's velocity and #7 tay lda PlayerMotionTable,y sta playerMotion,x rts DeterminePlayerMotion ldx #1 .determineMotionLoop lda playerVelocity,x ; get the player's velocity and #8 beq .determineMovement jsr MoveCars .determineMovement lda playerMotion,x ; get current motion of player sec ; set carry for rotation bmi .skipCarryClear clc .skipCarryClear rol ; rotate value left (i.e. D0 <- C <- D7) sta playerMotion,x ; set new motion value bcc .nextPlayer ; don't move if playerMotion was positive jsr MoveCars .nextPlayer dex beq .determineMotionLoop rts MoveCars inc playerDelay,x sta HMCLR ; clear all horizontal motion lda previousPlayerDirections,x sec sbc #2 and #3 bne LF364 lda playerDelay,x and #3 beq .leaveSubroutine LF364: lda playerDelay,x and #1 beq LF36C lda #$10 LF36C: ora previousPlayerDirections,x tay lda PlayerPixelMovementTable,y sta HMP0,x ; set player's fine horizontal motion and #$0F ; mask fine horizontal motion to get sec ; vertical motion values sbc #VMOVE_0 sta playerVerticalMotion ; D7 means the player is moving down clc adc playerVertPos,x ; add value to vertical position sta playerVertPos,x ; negative value will move player down bit playerVerticalMotion ; check how player is moving vertically bmi .playerMovingDown cmp #YMAX bcc .setPlayerVerticalPos lda #YMIN+6 bne .setPlayerVerticalPos .playerMovingDown cmp #YMIN bcs .setPlayerVerticalPos lda #YMAX-11 .setPlayerVerticalPos sta playerVertPos,x sta VDELP0,x lda PlayerPixelMovementTable,y lsr ; move fine horizontal motion to lower lsr ; nybbles lsr lsr cmp #8 bcc .setPlayerHorizontalPos ora #$F0 ; make the value negative clc ; values are -1 <= a <= 1 .setPlayerHorizontalPos adc playerHorizPos,x ; add value to horizontal position sta playerHorizPos,x ; negative value will move player left sta WSYNC ; wait for next scan line sta HMOVE ; move all objects horizontally .leaveSubroutine rts CheckToChangeDirectionPath SUBROUTINE lda playerVelocity,x ; get the player's velocity cmp rotationDelay ; if less than rotationDelay then bcc .resetPreviousDirections ; reset previous directions lda changeDirectionTimer,x ; if not 0 then player not allowed bne .reduceChangeDirectionTimer ; change direction path lda playerDirections,x ; if the player's directions didn't cmp previousPlayerDirections,x ; change then leave beq .leaveSubroutine lda playerRotation,x bmi .reducePreviousDirections inc previousPlayerDirections,x .setPreviousDirections lda previousPlayerDirections,x and #$0F ; only keep lower nybbles sta previousPlayerDirections,x lda gameSelection ; get the current game selection cmp #ICE_RACE lda playerVelocity,x ; get the player's velocity bcs .moveCarAcrossIce lsr ; divide the value by 2 .setChangeDirectionTimer sta changeDirectionTimer,x .leaveSubroutine rts .moveCarAcrossIce asl ; multiply player's velocity by 2 bne .setChangeDirectionTimer .reducePreviousDirections dec previousPlayerDirections,x jmp .setPreviousDirections .resetPreviousDirections lda playerDirections,x sta previousPlayerDirections,x rts .reduceChangeDirectionTimer dec changeDirectionTimer,x rts LoadCarGraphics lda frameCount ; get the current frame count and #1 ; alternate between player 1 and 2 each tax ; frame lda playerDirections,x ; get the player's direction sta REFP0,x ; set the player's reflect state asl ; multiply the value by 8 (i.e number asl ; of rotations) asl cmp #MAX_CAR_SPRITES-1 clc bmi .setCarGraphicsOffset sec eor #$47 .setCarGraphicsOffset tay stx temp ; not needed as x is never restored txa eor #$0E tax .loadGraphicsLoop txa and #1 beq .readCarGraphics bit gameVariation ; if this is not a time trial game then bvs .readCarGraphics ; read the graphics from ROM lda #0 ; clear the second player car beq .setCarGraphics .readCarGraphics lda CarGraphics,y .setCarGraphics sta carGraphics,x bcc .readNextByte dey ; compensate for offset being out of dey ; range .readNextByte iny dex dex bpl .loadGraphicsLoop rts CheckForCompletedLaps SUBROUTINE lda frameCount ; get the current frame count and #1 ; check player collisions on alternating tax ; frames lda playfieldBallCollisions,x ; read saved collisions asl ; shift player/ball collisions to D7 bpl SavePlayerBallCollisions ; if ball not save current collision lda playerVertPos,x ; get the player's vertical position cmp #128 lda #0 bcs .setVerticalCheckPoint lda #1 .setVerticalCheckPoint sta temp lda playerHorizPos,x ; get the player's horizontal position cmp #205 lda temp bcc .setLapCheckPoint ora #2 .setLapCheckPoint tay lda LapCheckPointTable,y ora lapCheckPoints,x ; add in the check point value sta lapCheckPoints,x cmp #15 ; if player didn't make full lap around bne .leaveSubroutine ; then leave lda CXP0FB,x ; read collision register and #$40 ; get player ball collision beq .setBallCollision ; set ball collision if ball not hit lda playfieldBallCollisions,x bmi .leaveSubroutine ; leave if the player hit playfield lda #%11000110 sta playfieldBallCollisions,x sta playerHorizPos,x lda #0 sta lapCheckPoints,x ; clear the check point flags jsr IncrementScore cmp #MAX_RACE_LAPS beq EndGame .leaveSubroutine rts SavePlayerBallCollisions lda CXP0FB,x ; read collision register and #$40 ; get player ball collision sta playfieldBallCollisions,x rts .setBallCollision lda #$40 sta playfieldBallCollisions,x rts EndGame lda #0 sta missileVertPos SetVariablesToEndGame sta gameState ; set game state to GAME_OVER sta AUDV0 ; turn off all sounds sta AUDV1 sta colorXOR ; clear colorXOR sta frameCount ; clear frameCount rts IncrementScore lda #0 sta colorXOR ; clear colorXOR sed clc lda playerScores,x ; get the player's score adc #$01 ; increment the score by 1 .setPlayerScore sta playerScores,x cld ; clear decimal mode rts ReduceTimeTrialTimer sed ; set to decimal mode lda player2Score ; get the time trial timer sec sbc #TIME_TRIAL_REDUCTION ; reduce the timer jmp .setPlayerScore CheckIfPlayerCaughtBox SUBROUTINE sta HMCLR ; clear all horizontal movements lda lapCheckPoints beq .setMissileInitPosition lda frameCount ; get the current frame count and #1 ; alternate between player 1 and 2 each tax ; frame lda CXM0FB,x ; read the missile/playfield collisions bmi .calculateNewRandomPosition ; branch if the missile hit the playfield lda playerFrameWait,x bne .reducePlayersWaitTime lda CXM0P,x ; read the player/missile collisions asl ; move player/missile collision to D7 bpl .leaveSubroutine jsr PlayerCaughtBox .leaveSubroutine rts PlayerCaughtBox lda #8 sta playerFrameWait,x jsr IncrementScore cmp #MAX_CRASH_AND_SCORE beq .endGame lda frameCount CalculateNewMissilePosition and #$7F ; make the value stay lower than 127 adc #64 ; increment the value by 64 (a <= 192) sta missileVertPos ; set the missile's new vertical position eor carGraphics,x ; XOR with player graphics for randomness and #$7F ; make the value stay loser than 127 adc #16 ; increment the value by 16 (a <= 143) MoveMissilesHorizontally sta missileHorizPos sta lapCheckPoints ldx #2 ; set missile 0 offset pha ; save value to stack jsr CalculateHorizPosition ; position missile 0 horizontally inx pla ; pull value off the stack jsr CalculateHorizPosition ; position missile 1 horizontally sta WSYNC ; wait for next scan line sta HMOVE rts .calculateNewRandomPosition lda frameCount ; get the current frame count adc #10 ; increment by 10 jmp CalculateNewMissilePosition .reducePlayersWaitTime dec playerFrameWait,x rts .endGame lda #0 jsr SetVariablesToEndGame rts .setMissileInitPosition lda #132 sta missileVertPos lda #86 bne MoveMissilesHorizontally PlayGameSounds lda frameCount ; get the current frame count and #1 ; alternate between player 1 and 2 each tax ; frame stx temp lda volumeChannelValues,x ; get the volume values bne LF533 ; branch if not reached 0 lda playerFrameWait,x bne LF52F lda playerVelocity,x ; get the player's velocity lsr ; divide the value by 2 tay bne .resetAccelerationSoundTimer lda accelerationSoundTimer,x beq .setIdleEngineSound dec accelerationSoundTimer,x jmp .setVolume .setIdleEngineSound ldy #6 bne .setVolume .resetAccelerationSoundTimer lda #63 sta accelerationSoundTimer,x .setVolume lda AudioVolumeTable,y sta AUDV0,x .setFrequencyAndChannels lda AudioFrequencyTable,y ora temp sta AUDF0,x lda AudioChannelTable,y sta AUDC0,x rts LF52F: ldy #7 bne .setVolume LF533: ldy #8 bne .setFrequencyAndChannels CalculateHorizPosition clc adc #49 pha ; push value to stack for later lsr ; shift top nybble to lower nybble lsr lsr lsr tay ; save the value pla ; get the object's x position and #$0F ; mask upper nybble sty temp ; save coarse value for later clc adc temp ; add in coarse value (A = C + F) cmp #14 bcc .skipSubtractions sec sbc #14 ; subtract 14 iny ; and increment coarse value .skipSubtractions cmp #8 ; make sure hasn't gone pass min x value eor #$0F bcs .skipFineIncrement adc #1 ; increment fine motion value dey ; reduce coarse value .skipFineIncrement iny ; increment coarse value asl ; move fine motion value to upper nybble asl asl asl sty WSYNC ; wait for next scan line .coarseMoveLoop dey bne .coarseMoveLoop sta RESP0,x ; set object's coarse position sta HMP0,x ; set object's fine motion rts CheckIfPlayerHitTrack SUBROUTINE lda frameCount ; get the current frame count and #1 ; alternate between player 1 and 2 each tax ; frame lda CXP0FB,x ; read collision register bpl .clearPlayfieldHitFlag ; branch if didn't hit playfield lda gameVariation ; get current game variation and #CRASH_AND_SCORE_TRACK1 ; if this is a "Crash and Score" or bne .slowCarDown ; "Tag" game then branch lda hitPlayfieldFlag,x ; get the playfield hit state bne .slowCarDown inc hitPlayfieldFlag,x ; increase playfield hit state lda #0 sta playerVelocity,x ; stop player's car jsr InitPlayerMotion lda #15 sta volumeChannelValues,x ; set audio volume to the max sta AUDV0,x .leaveSubroutine rts .clearPlayfieldHitFlag lda #0 sta hitPlayfieldFlag,x rts .slowCarDown lda #2 cmp playerVelocity,x bcs .leaveSubroutine lsr playerVelocity,x ; divide player velocity by 2 jsr InitPlayerMotion rts CheckForCarCollision SUBROUTINE lda CXPPMM ; read collision register bpl .clearCarCollisionFlag ; branch if cars didn't collide lda carCollisionFlag ; collsion recorded last frame bne .leaveSubroutine inc carCollisionFlag lda frameCount ; get the current frame count and #1 ; alternate between player 1 and 2 each tax ; frame inc playerDirections,x ; change the player's direction inc previousPlayerDirections,x lsr playerVelocity,x ; divide player's velocity by 2 jsr InitPlayerMotion txa ; move player index to a eor #1 ; EOR with 1 to change to other player tax dec playerDirections,x ; change player's direction dec previousPlayerDirections,x lsr playerVelocity,x ; divide player's velocity by 2 jsr InitPlayerMotion lda #15 sta volumeChannel1 ; set volume registers to the max sta volumeChannel2 sta AUDV0 sta AUDV1 .leaveSubroutine rts .clearCarCollisionFlag lda #0 sta carCollisionFlag rts CheckTagCollisions SUBROUTINE lda player1FrameWait ora player2FrameWait bne .waitTimeNotDone lda CXPPMM ; read collision register bmi .playerTagged ; branch if cars collided lda frameCount ; increment the player's score ~ every 60 and #$3F ; seconds bne .leaveSubroutine ldx tagChasing ; get the car number being chased jsr IncrementScore ; increment score until car is tagged cmp #MAX_TAG_SCORE beq .endGame .leaveSubroutine rts .endGame jsr EndGame rts .waitTimeNotDone lda #0 ldx lastTagChaser ; get the last chaser sta playerVelocity,x ; stop player's car jsr InitPlayerMotion dec playerFrameWait,x ; reduce the player's wait time rts .playerTagged lda tagChasing ; get the player number being chased sta lastTagChaser ; save as last chaser tax eor #1 ; make other player the chased sta tagChasing lda #$3F ; set tagged player to be able move sta playerFrameWait,x ; again in ~60 seconds rts DoneGameCalculations lda frameCount ; get the current frame count and #1 ; alternate between player 1 and 2 each tax ; frame lda volumeChannelValues,x beq CalculateScoreOffsets ; skip volume reduction if already 0 sec sbc #1 ; reduce volumn sta AUDV0,x sta volumeChannelValues,x CalculateScoreOffsets ldx #1 .scoreOffsetLoop lda playerScores,x ; get the player's score and #$0F ; mask off the upper nybbles sta temp ; save the value for later asl ; shift the value left to multiply by 4 asl clc ; add in original so it's multiplied by 5 adc temp ; [i.e. x * 5 = (x * 4) + x] sta lsbScoreOffsets,x lda playerScores,x and #$F0 ; mask off the lower nybbles lsr ; divide the value by 4 lsr sta temp ; save the value for later lsr ; divide the value by 16 lsr clc ; add in original so it's multiplied by adc temp ; 5/16 [i.e. 5x/16 = (x / 16) + (x / 4)] sta msbScoreOffsets,x dex beq .scoreOffsetLoop rts NumberFonts zero .byte $0E ; |....XXX.| .byte $0A ; |....X.X.| .byte $0A ; |....X.X.| .byte $0A ; |....X.X.| .byte $0E ; |....XXX.| one .byte $22 ; |..X...X.| .byte $22 ; |..X...X.| .byte $22 ; |..X...X.| .byte $22 ; |..X...X.| .byte $22 ; |..X...X.| two .byte $EE ; |XXX.XXX.| .byte $22 ; |..X...X.| .byte $EE ; |XXX.XXX.| .byte $88 ; |X...X...| .byte $EE ; |XXX.XXX.| three .byte $EE ; |XXX.XXX.| .byte $22 ; |..X...X.| .byte $66 ; |.XX..XX.| .byte $22 ; |..X...X.| .byte $EE ; |XXX.XXX.| four .byte $AA ; |X.X.X.X.| .byte $AA ; |X.X.X.X.| .byte $EE ; |XXX.XXX.| .byte $22 ; |..X...X.| .byte $22 ; |..X...X.| five .byte $EE ; |XXX.XXX.| .byte $88 ; |X...X...| .byte $EE ; |XXX.XXX.| .byte $22 ; |..X...X.| .byte $EE ; |XXX.XXX.| six .byte $EE ; |XXX.XXX.| .byte $88 ; |X...X...| .byte $EE ; |XXX XXX.| .byte $AA ; |X.X.X.X.| .byte $EE ; |XXX.XXX.| seven .byte $EE ; |XXX.XXX.| .byte $22 ; |..X...X.| .byte $22 ; |..X...X.| .byte $22 ; |..X...X.| .byte $22 ; |..X...X.| eight .byte $EE ; |XXX.XXX.| .byte $AA ; |X.X.X.X.| .byte $EE ; |XXX.XXX.| .byte $AA ; |X.X.X.X.| .byte $EE ; |XXX.XXX.| nine .byte $EE ; |XXX.XXX.| .byte $AA ; |X.X.X.X.| .byte $EE ; |XXX.XXX.| .byte $22 ; |..X...X.| .byte $EE ; |XXX.XXX.| GameVariationTable .byte %01001000 ;Grand Prix Track .byte %00001000 ;Grand Prix Track (time trial) .byte %11010001 ;Devil's Elbow Track .byte %10010001 ;Devil's Elbow Track (time trial) ; crash and score .byte %01011010 ;I Track .byte %00011010 ;I Track .byte %11011011 ;II Track .byte %10011011 ;II Track ; tag .byte %01100010 ;Barrier Chase Track .byte %11100011 ;Motor Hunt Track ; ice race .byte %01101100 ;Sprint Race Track .byte %00101100 ;Sprint Race Track (time trial) .byte %11101000 ;Rally Track .byte %10101000 ;Rally Track (time trial) PlayerPixelMovementTable .byte HMOVE_R1|VMOVE_0 .byte HMOVE_R1|VMOVE_D1 .byte HMOVE_R1|VMOVE_D2 .byte HMOVE_0 |VMOVE_D2 .byte HMOVE_0 |VMOVE_D2 .byte HMOVE_0 |VMOVE_D2 .byte HMOVE_L1|VMOVE_D2 .byte HMOVE_L1|VMOVE_D1 .byte HMOVE_L1|VMOVE_0 .byte HMOVE_L1|VMOVE_U1 .byte HMOVE_L1|VMOVE_U2 .byte HMOVE_0 |VMOVE_U2 .byte HMOVE_0 |VMOVE_U2 .byte HMOVE_0 |VMOVE_U2 .byte HMOVE_R1|VMOVE_U2 .byte HMOVE_R1|VMOVE_U1 .byte HMOVE_R1|VMOVE_0 .byte HMOVE_R1|VMOVE_D1 .byte HMOVE_R1|VMOVE_D2 .byte HMOVE_R1|VMOVE_D2 .byte HMOVE_0 |VMOVE_D2 .byte HMOVE_L1|VMOVE_D2 .byte HMOVE_L1|VMOVE_D2 .byte HMOVE_L1|VMOVE_D1 .byte HMOVE_L1|VMOVE_0 .byte HMOVE_L1|VMOVE_U1 .byte HMOVE_L1|VMOVE_U2 .byte HMOVE_L1|VMOVE_U2 .byte HMOVE_0 |VMOVE_U2 .byte HMOVE_R1|VMOVE_U2 .byte HMOVE_R1|VMOVE_U2 .byte HMOVE_R1|VMOVE_U1 PlayerMotionTable .byte $00,$01,$11,$25,$55,$DA,$EE,$EF RaceTrackPF1OffsetTable .byte 0,3,0,2,1 LapCheckPointTable .byte 8,1,4,2 RaceTrackPF0LSBTable .byte