nes/plat.s

.segment "HEADER"

INES_MAPPER = 0
INES_MIRROR = 1
INES_SRAM   = 0

.byte 'N', 'E', 'S', $1A
.byte $02
.byte $01
.byte INES_MIRROR
.byte (INES_MAPPER & %11110000)
.byte $0, $0, $0, $0, $0, $0, $0, $0


.segment "TILES"
.incbin "background.chr"
.incbin "sprite.chr"


.segment "VECTORS"
.word nmi
.word reset
.word irq


.segment "CODE"
reset:
    sei
    lda #%00100000 ; 8x16 sprites
    sta $2000
    lda #0
    sta $2001
    sta $4015
    sta $4010
    lda #40
    sta $4017
    cld
    ldx #$FF
    txs

    bit $2002
    :
        bit $2002
        bpl :-

    lda #0
    ldx #0
    :
        sta $0000, X
        sta $0100, X
        sta $0200, X
        sta $0300, X
        sta $0400, X
        sta $0500, X
        sta $0600, X
        sta $0700, X
        inx
        bne :-

    lda #255
    ldx #0
    :
        sta oam, X
        inx
        inx
        inx
        inx
        bne :-

    :
        bit $2002
        bpl :-

    lda #%10001000
    sta $2000
    jmp main


.segment "ZEROPAGE"
nmi_lock:       .res 1
nmi_count:      .res 1
nmi_ready:      .res 1
nmt_update_len: .res 1
scroll_x:       .res 1
scroll_y:       .res 1
scroll_nmt:     .res 1
temp:           .res 1

.segment "BSS"
nmt_update:     .res 256
palette:        .res 32

.segment "OAM"
oam:            .res 256

.segment "CODE"
nmi:
    pha
    txa
    pha
    tya
    pha

    lda nmi_lock
    beq :+
        jmp @nmi_end

    :
    lda #1
    sta nmi_lock

    lda nmi_ready
    bne :+
        jmp @ppu_update_end

    :
    cmp #2
    bne :+
        lda #%00000000
        sta $2001
        ldx #0
        stx nmi_ready
        jmp @ppu_update_end

    :
    ldx #0
    stx $2003
    lda #>oam
    sta $4014

    lda #%10001000
    sta $2000
    lda $2002
    lda #$3F
    sta $2006
    stx $2006
    ldx #0
    :
        lda palette, X
        sta $2007
        inx
        cpx #32
        bcc :-

    ldx #0
    cpx nmt_update_len
    bcs @scroll
    @nmt_update_loop:
        lda nmt_update, X
        sta $2006
        inx
        lda nmt_update, X
        sta $2006
        inx
        lda nmt_update, X
        sta $2007
        cpx nmt_update_len
        bcc @nmt_update_loop

    lda #0
    sta nmt_update_len
@scroll:
    lda scroll_nmt
    and #%00000011
    ora #%10001000
    sta $2000
    lda scroll_x
    sta $2005
    lda scroll_y
    sta $2005

    lda #%00011110
    sta $2001

    ldx #0
    stx nmi_ready
@ppu_update_end:
    lda #0
    sta nmi_lock
@nmi_end:
    pla
    tay
    pla
    tax
    pla
    rti


.segment "CODE"
irq:
    rti


.segment "CODE"
ppu_update:
    lda #1
    sta nmi_ready
    :
        lda nmi_ready
        bne :-

    rts

ppu_skip:
    lda nmi_count
    :
        cmp nmi_count
        beq :-

    rts

ppu_off:
    lda #2
    sta nmi_ready
    :
        lda nmi_ready
        bne :-

    rts

ppu_address_tile:
    lda $2002
    tya
    lsr
    lsr
    lsr
    ora #$20
    sta $2006
    tya
    asl
    asl
    asl
    asl
    asl
    sta temp
    txa
    ora temp
    sta $2006
    rts

ppu_update_tile:
    pha
    txa
    pha
    ldx nmt_update_len
    tya
    lsr
    lsr
    lsr
    ora #$20
    sta nmt_update, X
    inx
    tya
    asl
    asl
    asl
    asl
    asl
    sta temp
    pla
    ora temp
    sta nmt_update, X
    inx
    pla
    sta nmt_update, X
    inx
    stx nmt_update_len
    rts

ppu_update_byte:
    pha
    tya
    pha
    ldy nmt_update_len
    txa
    sta nmt_update, Y
    iny
    pla
    sta nmt_update, Y
    iny
    pla
    sta nmt_update, Y
    iny
    sty nmt_update_len
    rts


PAD_A      = $01
PAD_B      = $02
PAD_SELECT = $04
PAD_START  = $08
PAD_U      = $10
PAD_D      = $20
PAD_L      = $40
PAD_R      = $80

.segment "ZEROPAGE"
gamepad: .res 1

.segment "CODE"
; gamepad_poll: this reads the gamepad state into the variable labelled "gamepad"
;   This only reads the first gamepad, and also if DPCM samples are played they can
;   conflict with gamepad reading, which may give incorrect results.
gamepad_poll:
	; strobe the gamepad to latch current button state
	lda #1
	sta $4016
	lda #0
	sta $4016
	; read 8 bytes from the interface at $4016
	ldx #8
	:
		pha
		lda $4016
		; combine low two bits and store in carry bit
		and #%00000011
		cmp #%00000001
		pla
		; rotate carry into gamepad variable
		ror
		dex
		bne :-
	sta gamepad
	rts


.segment "RODATA"

level:
.byte 0,0,0,0,0,0,0,0,0,0,0,0,1,1,1,1
.byte 0,1,0,0,1,0,0,0,0,0,0,0,1,1,1,1
.byte 0,0,0,0,0,0,0,0,0,0,0,0,1,1,1,1
.byte 0,1,0,0,1,0,0,0,0,0,0,0,1,1,1,1
.byte 0,0,1,1,0,0,0,0,0,0,0,0,0,0,0,0
.byte 0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0
.byte 0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0
.byte 0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0
.byte 0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0
.byte 0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0
.byte 0,0,0,0,0,0,0,0,1,0,1,0,1,0,0,0
.byte 0,0,0,0,0,0,0,0,0,1,0,1,0,0,0,0
.byte 0,0,0,0,0,0,0,0,1,0,1,0,1,0,0,0
.byte 0,0,0,0,0,0,0,0,0,1,0,1,0,0,0,0
.byte 0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0

metatiles:
.byte 0,0,0,0,0
.byte 3,4,5,6,1

example_palette:
.byte $0F,$00,$3D,$20 ; greyscale
.byte $0F,$09,$1A,$16 ; grass
.byte $0F,$15,$26,$37 ; bg0 purple/pink
.byte $0F,$09,$19,$29 ; bg1 green
.byte $0F,$01,$11,$21 ; bg2 blue
.byte $0F,$00,$10,$30 ; bg3 greyscale
.byte $0F,$18,$28,$38 ; sp0 yellow
.byte $0F,$14,$24,$34 ; sp1 purple
.byte $0F,$1B,$2B,$3B ; sp2 teal
.byte $0F,$12,$22,$32 ; sp3 marine

.segment "ZEROPAGE"
buttons:    .res 1

vel_x: .res 1
vel_y: .res 1

cursor_x: .res 1
cursor_y: .res 1
temp_x:   .res 1
temp_y:   .res 1
temp_mul: .res 1
var_m:    .res 1
var_n:    .res 1
var_o:    .res 1
var_p:    .res 1

jump_pressed_last_frame: .res 1

frame_counter: .res 1

.segment "CODE"
main:
	ldx #0
	:
		lda example_palette, X
		sta palette, X
		inx
		cpx #32
		bcc :-

	jsr setup_background
	jsr ppu_update

    lda #$40
    sta cursor_x
    sta cursor_y

    lda #0
    sta frame_counter

    jsr init_objects
@loop:
    lda frame_counter
    clc
    adc #1
    sta frame_counter

    jsr controller
    jsr movement

@draw:
    jsr update_background
	jsr ppu_update

	jmp @loop

init_objects:
    lda #0
    sta var_n
    ldx #0
    :
        lda var_n
        clc
        adc #8
        sta var_n

        sta oam+0, X ; Set Y position
        sta oam+3, X ; Set X position

        txa
        lsr
        lsr
        sta oam+1, X

        inx
        inx
        inx
        inx
        cpx #(8*4)
        bne :-

    lda #0
    sta vel_x
    sta vel_y

    rts

draw:
@end:
	rts

movement:
    ; Horizontal velocity
    ldx #0
    jsr btn_right
    cmp #0
    beq :+
        ldx #1
        lda #3
        sta vel_x
    :

    jsr btn_left
    cmp #0
    beq :+
        ldx #1
        lda #253
        sta vel_x
    :

    cpx #1
    beq @end
        lda frame_counter
        and #1
        cmp #0
        bne @end

        lda #0
        cmp vel_x
        beq @end ; If vel_x = 0, skip decay

        ; If vel_x > 0, decrement
        bpl :+
            ldx vel_x
            dex
            stx vel_x
            jmp @end
        :
        ; Else if vel_x < 0, increment
        ldx vel_x
        inx
        stx vel_x
    @end:

    ; Jump
    jsr btn_a
    cmp #0
    beq @fail_jump ; If jump not pressed, forget it
    lda jump_pressed_last_frame
    cmp #1
    beq @fail_jump ; If jump last frame, forget it

    ; Jump newly pressed this frame
    lda #250
    sta vel_y

    @fail_jump:
    
    jsr btn_a
    sta jump_pressed_last_frame


    ; Gravity
    lda frame_counter
    and #1
    cmp #0
    bne :+
        lda vel_y
        clc
        adc #1    ; Add 2 to velocity
        sta vel_y
    :

    ; Apply Y velocity
    lda oam+0
    clc
    adc vel_y
    sta oam+0

    ; Keep grounded
    cmp #200
    bcc :+
        lda #0
        sta vel_y
        lda #200
        sta oam+0
    :

    ; Bonk
    lda #8
    cmp oam+0
    bcc :+
        lda #0
        sta vel_y
        lda #8
        sta oam+0
    :

    ; Apply X velocity
    lda oam+3
    clc
    adc vel_x
    sta oam+3


    rts

update_background:
    rts
    ;lda $2002
    ;lda #$20
    ;sta $2006
    ;lda #$00
    ;sta $2006

    ldx #1
    ldy #1
    jsr ppu_update_tile

    lda #1
    sta $2007

    rts

setup_background:
	lda $2002 ; reset latch
	lda #$20
	sta $2006
	lda #$00
	sta $2006
	; empty nametable
	lda #0
	ldy #30 ; 30 rows
	:
		ldx #32 ; 32 columns
		:
			sta $2007
			dex
			bne :-
		dey
		bne :--

    lda $2002
    lda #$20
    sta $2006
    lda #$00
    sta $2006
	ldy #0
	@row_start:
        ldx #0
        :
            txa
            pha

            ; index is (Y * 16) + X
            stx temp_x
            lda #16
            jsr mul_y
            clc
            adc temp_x
            tax

            lda level, X
            ldx #5
            jsr mul_x
            tax
            
            lda metatiles+0, X
            sta $2007
            lda metatiles+1, X
            sta $2007

            pla
            tax
            inx
            cpx #16
            bne :-

        ; again
        ldx #0
        :
            txa
            pha

            ; index is (Y * 16) + X
            stx temp_x
            lda #16
            jsr mul_y
            clc
            adc temp_x
            tax

            lda level, X
            ldx #5
            jsr mul_x
            tax

            lda metatiles+2, X
            sta $2007
            lda metatiles+3, X
            sta $2007

            pla
            tax
            inx
            cpx #16
            bne :-
		iny
        cpy #15
		bne @row_start


	; clear attributes
    lda #0
	ldx #64 ; 64 bytes
	:
        lda #%11100100

		sta $2007
		dex
		bne :-

    rts



	lda $2002 ; reset latch
	lda #$20
	sta $2006
	lda #$00
	sta $2006
	; empty nametable
	lda #0
	ldy #30 ; 30 rows
	:
		ldx #32 ; 32 columns
		:
			sta $2007
			dex
			bne :-
		dey
		bne :--
	; set all attributes to 0
    lda #0
	ldx #64 ; 64 bytes
	:
        txa
        and #%00000011
        asl
        asl
        asl
        asl
        sta temp
        txa
        and #%00000011
        ora temp
        lda #0

		sta $2007
		dex
		bne :-





	; fill in an area in the middle with 1/2 checkerboard
	lda #1
	ldy #0 ; start at row 8
	:
		pha ; temporarily store A, it will be clobbered by ppu_address_tile routine
		ldx #8 ; start at column 8
		jsr ppu_address_tile
		pla ; recover A
        
		ldx #8
		:
			sta $2007
			eor #$3
			inx
			cpx #(32-8)
			bcc :-
		eor #$3
		iny
		cpy #(30-8)
		bcc :--

	lda #$24
	sta $2006
	lda #$00
	sta $2006
	lda #$00
	ldy #30
	:
		ldx #32
		:
			sta $2007
			clc
			adc #1
			and #3
			dex
			bne :-
		clc
		adc #1
		and #3
		dey
		bne :--

	lda #0
	ldy #4
	:
		ldx #16
		:
			sta $2007
			dex
			bne :-

		clc
		adc #%01010101
		dey
		bne :--

	rts

mul_x:
    cmp #0
    beq @zero
    cpx #0
    beq @zero

    sta temp_mul

    txa
    pha
    lda #0
    :
        clc
        adc temp_mul
        dex
        bne :-
    sta temp_mul
    pla
    tax
    lda temp_mul
    rts

    @zero:
    lda #0
    rts

mul_y:
    cmp #0
    beq @zero
    cpy #0
    beq @zero

    sta temp_mul

    tya
    pha
    lda #0
    :
        clc
        adc temp_mul
        dey
        bne :-
    sta temp_mul
    pla
    tay
    lda temp_mul
    rts

    @zero:
    lda #0
    rts

controller:
    lda #1
    sta $4016
    sta buttons
    lda #0
    sta $4016
    :
        lda $4016
        lsr
        rol buttons
        bcc :-

    rts

btn_right:
    lda buttons
    and #%00000001
    rts

btn_left:
    lda buttons
    and #%00000010
    lsr
    rts

btn_down:
    lda buttons
    and #%00000100
    lsr
    lsr
    rts

btn_up:
    lda buttons
    and #%00001000
    lsr
    lsr
    lsr
    rts

btn_start:
    lda buttons
    and #%00010000
    lsr
    lsr
    lsr
    lsr
    rts

btn_select:
    lda buttons
    and #%00100000
    lsr
    lsr
    lsr
    lsr
    lsr
    rts

btn_b:
    lda buttons
    and #%01000000
    lsr
    lsr
    lsr
    lsr
    lsr
    lsr
    rts

btn_a:
    lda buttons
    and #%10000000
    lsr
    lsr
    lsr
    lsr
    lsr
    lsr
    lsr
    rts