processor 6502
        include "vcs.h"
        include "macro.h"
        include "xmacro.h"

	seg.u   Variables
	org	$80

PFOfs		.byte	; offset into PFData (0-20)
SpritePtr0	.word	; pointer to bitmap for sprite 0
SpritePtr1	.word	; pointer to bitmap for sprite 1
ColorPtr0	.word	; pointer to colors for sprite 0
ColorPtr1	.word	; pointer to colors for sprite 1

YPos0		.byte	; current Y position of sprite 0
YPos1		.byte	; current Y position of sprite 1
XPos0		.byte	; current X position of sprite 0
XPos1		.byte	; current X position of sprite 1
XPosPrev	.byte	; previous X position of sprite 0
YPosPrev	.byte	; previous X position of sprite 1

RoomType	.byte	; current room definition byte

; these are modified line-by-line by the sprite kernel
Colp0		.byte	; temp. colors for player 0
YP0		.byte	; counts y-position for player 0
YP1		.byte	; counts y-position for player 1
tmpPF0		.byte	; temp. PF0
tmpPF1		.byte	; temp. PF1
tmpPF2		.byte	; temp. PF2

Temp		.byte

PFData	equ	$c0	; 21-byte array of playfield bytes

SpriteHeight	equ 16	; hard-coded height of sprites

;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;

	seg Code
	org  $f000

Start
	CLEAN_START
Data0        
        lda #<Frame0
        sta SpritePtr0
        lda #>Frame0
        sta SpritePtr0+1
        lda #<ColorFrame0
        sta ColorPtr0
        lda #>ColorFrame0
        sta ColorPtr0+1
        lda #<Frame0
        sta SpritePtr1
        lda #>Frame0
        sta SpritePtr1+1
        lda #<ColorFrame1
        sta ColorPtr1
        lda #>ColorFrame1
        sta ColorPtr1+1
        lda #242
        sta YPos0
        lda #200
        sta YPos1
        lda #58
        sta XPos0
        sta XPos1
        lda #1
        sta VDELP0	; updates to GRP0 will be delayed
        lda #1
        sta RoomType
        jsr BuildRoom

NextFrame
	VERTICAL_SYNC

; Set up VBLANK timer
; We'll do four fewer lines than usual, so we can
; load the playfield registers in the first four lines.
	TIMER_SETUP 33
        lda #$68
        sta COLUP0	; player color
        lda #1
        sta CTRLPF	; symmetry
        lda #72
        sta PFOfs	; reset playfield offset
; Set temporary Y counter and set horizontal position
        lda YPos0
        sta YP0		; yp0 = temporary counter
        lda YPos1
        sta YP1	; yp0 = temporary counter
        lda XPos0
        ldx #0
        jsr SetHorizPos
        lda XPos1
        ldx #1
        jsr SetHorizPos
        sta WSYNC
        sta HMOVE	; gotta apply HMOVE
        sta CXCLR	; clear collisions
; Wait for end of VBLANK
	TIMER_WAIT
        sta WSYNC

KernelLoop
; Phase 0: Fetch PF0 byte
        jsr DrawSprites
        jsr FetchPlayfield
        sta tmpPF0
; Phase 1: Fetch PF1 byte
        jsr DrawSprites
        jsr FetchPlayfield
        sta tmpPF1
; Phase 2: Fetch PF2 byte
        jsr DrawSprites
        jsr FetchPlayfield
        sta tmpPF2
; Phase 3: Write PF0/PF1/PF2 registers
        jsr DrawSprites
	lda tmpPF0
        sta PF0
        lda tmpPF1
        sta PF1
        lda tmpPF2
        sta PF2
; Go to next scanline?
	lda PFOfs
	bne KernelLoop

NoMoreLines

; Set up overscan timer
; We'll take back those four lines we skipped earlier.
	TIMER_SETUP 34
        lda #0
        sta PF0
        sta PF1
        sta PF2
; Did the player collide with the wall?
	bit CXP0FB
        bpl NoCollision
; Yes, load previous position
        lda YPosPrev
        sta YPos0
        lda XPosPrev
        sta XPos0
        jmp NoMoveJoy
NoCollision
; No collision, update previous position and move player
        lda YPos0
        sta YPosPrev
        lda XPos0
        sta XPosPrev
        jsr MoveJoystick
NoMoveJoy
        TIMER_WAIT
        jmp NextFrame

; DrawSprite subroutine called by kernel
DrawSprites subroutine
; Fetch sprite 0 values
        lda #SpriteHeight	; height in 2xlines
        sec
        isb YP0			; INC yp0, then SBC yp0
        bcs DoDraw0		; inside bounds?
        lda #0			; no, load the padding offset (0)
DoDraw0
        tay			; -> Y
        lda (ColorPtr0),y	; color for both lines
        sta Colp0		; -> colp0
	lda (SpritePtr0),y	; bitmap for first line
        sta GRP0		; -> [GRP0] (delayed due to VDEL)
; Fetch sprite 1 values
        lda #SpriteHeight	; height in 2xlines
        sec
        isb YP1			; INC yp0, then SBC yp0
        bcs DoDraw1		; inside bounds?
        lda #0			; no, load the padding offset (0)
DoDraw1
        tay			; -> Y
        lda (ColorPtr1),y	; color for both lines
        tax
	lda (SpritePtr1),y	; bitmap for first line
        tay
; WSYNC and store sprite values
        lda Colp0
	sta WSYNC
        sty GRP1		; GRP0 is also updated due to VDELP0 flag
        stx COLUP1
        sta COLUP0
; Return to caller
	rts

; Fetch the next playfield byte.
FetchPlayfield subroutine
        dec PFOfs
        ldx PFOfs
        ldy PFOffsets,x	; get index into PFData array
        lda PFData,y	; load playfield byte
        rts

SetHorizPos subroutine
	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 subroutine
; Move vertically
	ldx YPos0
	lda #%00100000	;Down?
	bit SWCHA
	bne SkipMoveDown
        dex
        cpx #175
        bcs SkipMoveDown
; If we move off the top of the screen,
; go to the next room in the sequence
        ldy #1
        jsr MoveNextRoom
        jsr BuildRoom
        ldx #254
        bne SkipMoveUp
SkipMoveDown
	lda #%00010000	;Up?
	bit SWCHA 
	bne SkipMoveUp
        inx
        cpx #254
        bcc SkipMoveUp
; If we move off the top of the screen,
; go to the previous room in the sequence
        ldy #1
        jsr MovePrevRoom
        jsr BuildRoom
        ldx #174
        bne SkipMoveUp
SkipMoveUp
	stx YPos0
; Move horizontally
        ldx XPos0
	lda #%01000000	;Left?
	bit SWCHA
	bne SkipMoveLeft
        dex
        cpx #1
        bcs SkipMoveLeft
; If we move off the left of the screen,
; go back 7 rooms
	ldy #7
        jsr MovePrevRoom
        jsr BuildRoom
        ldx #152
SkipMoveLeft
	lda #%10000000	;Right?
	bit SWCHA 
	bne SkipMoveRight
        inx
        cpx #153
        bcc SkipMoveRight
; If we move off the right of the screen,
; go forward 7 rooms
	ldy #7
        jsr MoveNextRoom
        jsr BuildRoom
        ldx #1
SkipMoveRight
	stx XPos0
	rts

; Build a room based on the RoomType byte.
; Bits 0-1 = top walls
; Bits 2-3 = middle walls
BuildRoom subroutine
; First fill in the top section.
	lda RoomType
        and #3
        jsr MulBy3ToX
        lda PFRoomTop0+0,x
        sta PFData+0
        lda PFRoomTop0+1,x
        sta PFData+1
        lda PFRoomTop0+2,x
        sta PFData+2
        lda PFRoomTop1+0,x
        sta PFData+3
        lda PFRoomTop1+1,x
        sta PFData+4
        lda PFRoomTop1+2,x
        sta PFData+5
; Now the middle section.
	lda RoomType
        ror
        ror
        and #3
        jsr MulBy3ToX
        lda PFRoomMid0+0,x
        sta PFData+6
        lda PFRoomMid0+1,x
        sta PFData+7
        lda PFRoomMid0+2,x
        sta PFData+8
        lda PFRoomMid1+0,x
        sta PFData+9
        lda PFRoomMid1+1,x
        sta PFData+10
        lda PFRoomMid1+2,x
        sta PFData+11
        lda PFRoomMid2+0,x
        sta PFData+12
        lda PFRoomMid2+1,x
        sta PFData+13
        lda PFRoomMid2+2,x
        sta PFData+14
; And finally, the bottom.
	lda RoomType
        jsr NextRandom
        pha
        and #3
        jsr MulBy3ToX
        lda PFRoomTop1+0,x
        sta PFData+15
        lda PFRoomTop1+1,x
        sta PFData+16
        lda PFRoomTop1+2,x
        sta PFData+17
        lda PFRoomTop0+0,x
        sta PFData+18
        lda PFRoomTop0+1,x
        sta PFData+19
        lda PFRoomTop0+2,x
        sta PFData+20
; Set the room colors and position the Green Man
	lda RoomType
        and #$f0
        sta COLUBK
        and #$7f
        sta XPos1
        pla		; next random value, stored
        ora #$08
        sta COLUPF	; fg color
        ora #$80
        sta YPos1
        rts
; Helper function that computes X <- A*3
MulBy3ToX
	sta Temp
	asl
        clc
        adc Temp
        tax
        rts

; Get next random value
NextRandom subroutine
	lsr
	bcc .NoEor
	eor #$d4
.NoEor:
        rts
; Get previous random value
PrevRandom subroutine
	asl
	bcc .NoEor
	eor #$a9
.NoEor:
        rts
; Move to next room(s)
; Y = number of iterations
MoveNextRoom subroutine
	lda RoomType
	jsr NextRandom
        dey
        sta RoomType
        bne MoveNextRoom
        rts
; Move to previous room(s)
; Y = number of iterations
MovePrevRoom subroutine
	lda RoomType
	jsr PrevRandom
        dey
        sta RoomType
        bne MovePrevRoom
        rts

; Table used to get offsets to playfield bytes in memory
; 24 3-byte entries, one for each 8-pixel section
; in reverse order
PFOffsets
	.byte 20,19,18
	.byte 20,19,18
	.byte 17,16,15
	.byte 17,16,15
	.byte 17,16,15
	.byte 17,16,15
	.byte 17,16,15
	.byte 17,16,15
        .byte 14,13,12
	.byte 11,10,9
	.byte 11,10,9
	.byte 11,10,9
	.byte 11,10,9
	.byte 11,10,9
	.byte 11,10,9
	.byte 11,10,9
	.byte 8,7,6
	.byte 5,4,3
	.byte 5,4,3
	.byte 5,4,3
	.byte 5,4,3
	.byte 5,4,3
	.byte 5,4,3
	.byte 2,1,0

; Playfield components for rooms
PFRoomTop0
	.byte #%11110000,#%11111111,#%00000111
	.byte #%00110000,#%00001111,#%11111111
	.byte #%00110000,#%00001111,#%11111111
	.byte #%00110000,#%00000000,#%10000000
PFRoomTop1
	.byte #%00110000,#%00000000,#%00000000
	.byte #%00110000,#%00000000,#%00000000
	.byte #%00110000,#%00000000,#%10000000
	.byte #%00110000,#%00001111,#%11111111
PFRoomMid0
	.byte #%00110000,#%00000000,#%00000000
	.byte #%11110000,#%11111111,#%00000000
	.byte #%11110000,#%11111111,#%00000000
	.byte #%11110000,#%11111111,#%00000000
PFRoomMid1
	.byte #%00000000,#%00000001,#%00000000
	.byte #%00000000,#%00000001,#%00000000
	.byte #%00000000,#%00000000,#%00000000
	.byte #%00000000,#%00000000,#%00000000
PFRoomMid2
	.byte #%11110000,#%11111111,#%00000000
	.byte #%00110000,#%00000000,#%00000000
	.byte #%00110000,#%00000000,#%00000000
	.byte #%00110000,#%00000000,#%00000000

; Bitmap data "standing" position
Frame0
	.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 #$FF;
	.byte #$F6;
	.byte #$86;
	.byte #$86;
	.byte #$86;
	.byte #$C2;
	.byte #$C2;
	.byte #$C2;
	.byte #$C2;
	.byte #$16;
	.byte #$16;
	.byte #$16;
	.byte #$18;
	.byte #$18;
	.byte #$18;
	.byte #$F2;
	.byte #$F4;

; Color data for each line of sprite
ColorFrame1
	.byte #$5F;
	.byte #$56;
	.byte #$36;
	.byte #$36;
	.byte #$36;
	.byte #$32;
	.byte #$32;
	.byte #$32;
	.byte #$32;
	.byte #$c6;
	.byte #$c6;
	.byte #$c6;
	.byte #$c8;
	.byte #$c8;
	.byte #$c8;
	.byte #$02;
	.byte #$02;

; Epilogue
	org $fffc
        .word Start
        .word Start