nes/game.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 #0
    sta $2000
    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"
example_palette:
.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"
velocities: .res 8

cursor_x: .res 1
cursor_y: .res 1
temp_x:   .res 1
temp_y:   .res 1
var_m:    .res 1
var_n:    .res 1
var_o:    .res 1
var_p:    .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:
@draw:
	jsr draw
	jsr ppu_update

    lda frame_counter
    clc
    adc #1
    sta frame_counter
	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

        tay
        lda #0
        sta velocities, Y

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

    rts

draw:
    lda frame_counter
    and #1
    cmp #0
    bne @end

    ldx #0
    ldy #0
    :
        lda velocities, Y
        clc
        adc #1    ; Add 2 to velocity
        sta velocities, Y

        lda oam+0, X
        clc
        adc velocities, Y
        sta oam+0, X

        cmp #200
        bmi :+
            lda #245
            sta velocities, Y
        :

        inx
        inx
        inx
        inx

        iny
        cpy #8
        bne :--
@end:
	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 :--
	; set all attributes to 0
	ldx #64 ; 64 bytes
	:
		sta $2007
		dex
		bne :-
	; fill in an area in the middle with 1/2 checkerboard
	lda #1
	ldy #8 ; 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