;=============================
;DEFINES
;=============================

PPUCTRL         = $2000
PPUMASK         = $2001
PPUSTATUS       = $2002
OAMADDR         = $2003
OAMDATA         = $2004
PPUSCROLL       = $2005
PPUADDR         = $2006
PPUDATA         = $2007
OAMDMA          = $4014


;=============================
;ZP RESERVATIONS
;=============================
.segment "ZEROPAGE"
    ptr:    .res 2      ;general purp pointer
    cbank:  .res 1      ;keeps current bank in memory so NMI can restore it.

;=============================
;ROM START
;=============================
.segment "HEADER"
    .byte "NES", $1A    ;magic number
    .byte 8             ;8 banks, range 0-7
    .byte 0             ;CHR RAM instead of ROM
    .byte $20, $08      ;Mapper 2 - horz mirroring
    .byte $00           ;
    .byte $00           ;
    .byte $00           ;PRG RAM: no.
    .byte $07           ;8kB CHR RAM
    .byte $00           ;NTSC
    .byte $00           ;

;=============================
;ROM ADDR $8000
;=============================
.segment "BANK0_TITLE"


;=============================
;ROM ADDR $C000
;=============================


.segment "BANKTABLE"
banktable:   ;used for bus conflict avoidance. Bank $07 is fixed.
  .byte $00, $01, $02, $03, $04, $05, $06


.segment "CODE_MAIN"    ;all program code in fixed bank goes here.

reset:
;makeshift without proper initialization
ldy #0
    sty PPUCTRL
    sty PPUMASK

jsr bankswitch ;expects a 0 at y for bank 0.
jsr w_vblank

;set up sprite pattern
lda #>title_spr
ldy #<title_spr
sta ptr+1
sty ptr+0

ldy #0
lda #$00   ;0 is table 0, $10 is table 1
jsr set_ppuaddr_ay

ldx #16      ; + 16 pages' worth for spr chr.
jsr st_ppudata_y_x_ptr

;set up background
lda #>title_bg
ldy #<title_bg
sta ptr+1
sty ptr+0

ldy #0
lda #$10   ;0 is table 0, $10 is table 1
jsr set_ppuaddr_ay

ldx #20      ; + 20 pages' worth - 16 for bg chr and 4 for nt
jsr st_ppudata_y_x_ptr


Load_palette:
;should be made a subroutine
.scope
    lda PPUSTATUS
    lda #$3F
    sta PPUADDR
    lda #0
    sta PPUADDR
    ldx #0
    lda title_pal ;QUESTION: why do i have to load title_pal here to make colours load correctly?
loop:
    lda title_pal, x
    sta PPUDATA
    inx
    cpx #32
    bne loop ; iterate
.endscope

ldx #0

curtain_up:
;enable viewing. NMI is currently disabled.
    stx PPUSCROLL
    stx PPUSCROLL

    lda #%00010000
    sta PPUCTRL

    lda #%00011110
    sta PPUMASK

    clc ;set a known condition
loop_forever:
    bcc loop_forever

;=============================
;SUBROUTINES
;=============================

bankswitch:
  sty cbank       ; save the current bank in RAM so the NMI handler can restore it
  lda banktable, y      ; |
  sta banktable, y      ; |<-- read and write to switch bank.
  rts

w_vblank:
.scope
    bit PPUSTATUS  ;spinlock starts here
spinlock:
    bit PPUSTATUS
    bpl spinlock
    rts
.endscope

set_ppuaddr_ay:
    sta PPUADDR
    sty PPUADDR
    rts

st_ppudata_y_x_ptr:
.scope
loop:
   lda (ptr), y
   sta PPUDATA
   iny
   bne loop    ; done a page yet?  (y reg)
   inc ptr+1
   dex
   bne loop    ; done all pages yet? (x reg)
rts
.endscope


;=============================
;INTERRUPTS
;=============================

nmi:
   pha
   txa
   pha
   tya
   pha

;Place new stuff here and reenable NMI

   pla
   tay
   pla
   tax
   pla
rti


.segment "RODATA"
;place data here that should always be available regardless bank.

;QUESTION: Why does nametable etc break when i move this to the bank0 segment at line 41?
title_spr:  .incbin "dd_sprite.chr" ;4k
title_bg:   .incbin "dd.chr" ;4k
title_nam:  .incbin "dd.nam" ;1k
title_pal:
    .incbin "dd.pal"
    .incbin "dd.pal"

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