processor 6502 include "vcs.h" include "macro.h" include "xmacro.h" seg.u Variables org $80 PFPtr word ; pointer to playfield data PFIndex byte ; offset into playfield array PFCount byte ; lines left in this playfield segment Temp byte ; temporary YPos byte ; Y position of player sprite XPos byte ; X position of player sprite SpritePtr word ; pointer to sprite bitmap table ColorPtr word ; pointer to sprite color table ; Temporary slots used during kernel Bit2p0 byte Colp0 byte YP0 byte ; Height of sprite in scanlines SpriteHeight equ 9 ;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;; seg Code org $f000 Start CLEAN_START ; Set up initial pointers and player position lda #PlayfieldData sta PFPtr+1 lda #Frame0 sta SpritePtr+1 lda #ColorFrame0 sta ColorPtr+1 lda #242 sta YPos lda #38 sta XPos NextFrame VERTICAL_SYNC ; Set up VBLANK timer TIMER_SETUP 37 lda #$88 sta COLUBK ; bg color lda #$5b sta COLUPF ; fg color lda #$68 sta COLUP0 ; player color lda #1 sta CTRLPF ; symmetry lda #0 sta PFIndex ; reset playfield offset ; Set temporary Y counter and set horizontal position lda YPos sta YP0 ; yp0 = temporary counter lda XPos ldx #0 jsr SetHorizPos sta WSYNC sta HMOVE ; gotta apply HMOVE ; Wait for end of VBLANK TIMER_WAIT sta WSYNC lda #0 sta VBLANK ; Set up timer (in case of bugs where we don't hit exactly) TIMER_SETUP 192 SLEEP 10 ; to make timing analysis work out NewPFSegment ; Load a new playfield segment. ; Defined by length and then the 3 PF registers. ; Length = 0 means stop ldy PFIndex ; load index into PF array lda (PFPtr),y ; load length of next segment beq NoMoreSegs ; == 0, we're done sta PFCount ; save for later ; Preload the PF0/PF1/PF2 registers for after WSYNC iny lda (PFPtr),y ; load PF0 tax ; PF0 -> X iny lda (PFPtr),y ; load PF1 sta Temp ; PF1 -> Temp iny lda (PFPtr),y ; load PF2 iny sty PFIndex tay ; PF2 -> Y ; WSYNC, then store playfield registers ; and also the player 0 bitmap for line 2 sta WSYNC stx PF0 ; X -> PF0 lda Temp sta PF1 ; Temp -> PF1 lda Bit2p0 ; player bitmap sta GRP0 ; Bit2p0 -> GRP0 sty PF2 ; Y -> PF2 ; Load playfield length, we'll keep this in X for the loop ldx PFCount KernelLoop ; Does this scanline intersect our sprite? lda #SpriteHeight ; height in 2xlines isb YP0 ; INC yp0, then SBC yp0 bcs .DoDraw ; inside bounds? lda #0 ; no, load the padding offset (0) .DoDraw ; Load color value for both lines, store in temp var pha ; save original offset tay ; -> Y lda (ColorPtr),y ; color for both lines sta Colp0 ; -> colp0 ; Load bitmap value for each line, store in temp var pla asl ; offset * 2 tay ; -> Y lda (SpritePtr),y ; bitmap for first line sta Bit2p0 ; -> bit2p0 iny lda (SpritePtr),y ; bitmap for second line ; WSYNC and store values for first line sta WSYNC sta GRP0 ; Bit1p0 -> GRP0 lda Colp0 sta COLUP0 ; Colp0 -> COLUP0 dex beq NewPFSegment ; end of this playfield segment? ; WSYNC and store values for second line sta WSYNC lda Bit2p0 sta GRP0 ; Bit2p0 -> GRP0 jmp KernelLoop NoMoreSegs ; Change colors so we can see when our loop ends lda #0 sta COLUBK ; Wait for timer to finish TIMER_WAIT ; Set up overscan timer TIMER_SETUP 30 lda #2 sta VBLANK jsr MoveJoystick TIMER_WAIT jmp NextFrame SetHorizPos sta WSYNC ; start a new line bit 0 ; waste 3 cycles sec ; set carry flag DivideLoop sbc #15 ; subtract 15 bcs DivideLoop ; branch until negative eor #7 ; calculate fine offset asl asl asl asl sta RESP0,x ; fix coarse position sta HMP0,x ; set fine offset rts ; return to caller ; Read joystick movement and apply to object 0 MoveJoystick ; Move vertically ; (up and down are actually reversed since ypos starts at bottom) ldx YPos lda #%00100000 ;Up? bit SWCHA bne SkipMoveUp cpx #175 bcc SkipMoveUp dex SkipMoveUp lda #%00010000 ;Down? bit SWCHA bne SkipMoveDown cpx #254 bcs SkipMoveDown inx SkipMoveDown stx YPos ; Move horizontally ldx XPos lda #%01000000 ;Left? bit SWCHA bne SkipMoveLeft cpx #1 bcc SkipMoveLeft dex SkipMoveLeft lda #%10000000 ;Right? bit SWCHA bne SkipMoveRight cpx #153 bcs SkipMoveRight inx SkipMoveRight stx XPos rts ;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;; align $100; make sure data doesn't cross page boundary PlayfieldData .byte 4,#%00000000,#%11111110,#%00110000 .byte 8,#%11000000,#%00000001,#%01001000 .byte 15,#%00100000,#%01111110,#%10000100 .byte 20,#%00010000,#%10000000,#%00010000 .byte 20,#%00010000,#%01100011,#%10011000 .byte 15,#%00100000,#%00001100,#%01000100 .byte 8,#%11000000,#%00110000,#%00110010 .byte 4,#%00000000,#%11000000,#%00001100 .byte 0 ; Bitmap data "standing" position Frame0 .byte #0 .byte #0 .byte #%01101100;$F6 .byte #%00101000;$86 .byte #%00101000;$86 .byte #%00111000;$86 .byte #%10111010;$C2 .byte #%10111010;$C2 .byte #%01111100;$C2 .byte #%00111000;$C2 .byte #%00111000;$16 .byte #%01000100;$16 .byte #%01111100;$16 .byte #%01111100;$18 .byte #%01010100;$18 .byte #%01111100;$18 .byte #%11111110;$F2 .byte #%00111000;$F4 ; Color data for each line of sprite ColorFrame0 .byte #$FF; .byte #$86; .byte #$86; .byte #$C2; .byte #$C2; .byte #$16; .byte #$16; .byte #$18; .byte #$F4; ; Epilogue org $fffc .word Start .word Start