.segment "HEADER"
    .byte $4E, $45, $53, $1A ;Nes header identifier
    .byte 2 ;2x 16KB PRG code
    .byte 1 ; 1x 8KB CHR Data
    .byte $01, $00 ; mapper 0, vertical mirroring

.segment "VECTORS"
    .addr nmi ; nmi interrupt
    .addr reset ; reset interrupt
    .addr 0 ; external interrupt

.segment "STARTUP"

.segment "CHARS"
.incbin "./CHR-ROM.bin"

.segment "CODE"

bg_pallets:
    .byte 0, 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15

check:
    cmp #5
    bcc skip
    clc
    adc #3
skip:
    rts

reset:
    ldx #15
    lda #$3F
    sta $2006
    lda #$00
    sta $2006
loop:
    lda bg_pallets, x
    sta $2007
    dex
    bpl loop
    lda #173
    sta $00
    lda #0
    sta $01
    sta $02
dabble:
    ; check first digit
    lda $01
    and #$F
    jsr check
    sta $03
    lda #$F0
    and $01
    ora $03
    sta $01
    ; check second digit
    lda $01
    and #$F0
    lsr
    lsr
    lsr
    lsr
    jsr check
    asl
    asl
    asl
    asl
    sta $03
    lda #$F
    and $01
    ora $03
    sta $01
    ; check third digit
    lda $02
    and #$F
    jsr check
    sta $03
    lda #$F0
    and $02
    ora $03
    sta $02
    ; Shift
    clc
    rol $00
    rol $01
    rol $02
    lda $00
    cmp #0
    bne dabble
    rts

nmi:
    rti