;
; Example VRC6a (mapper 24) template for asm6f
; Author: Jeremy Chadwick <jdc@koitsu.org>
;

;
; NES header
;
    .db $4E,$45,$53,$1A   ; NES+$1A identifer
    .db 4                 ; PRG-ROM bank count
    .db 0                 ; CHR-ROM bank count (using CHR-RAM)
    .db %10000001         ; Bits 7-4: Mapper number 24 (lower nibble)
                          ; Bit    3: Not 4-screen
                          ; Bit    2: No trainer
                          ; Bit    1: No battery-backed PRG-RAM (SRAM)
                          ; Bit    0: Vertical mirroring
    .db %00010000         ; Bits 7-4: Mapper number 24 (upper nibble)
                          ; Bit    3: Unused
                          ; Bit    2: Unused
                          ; Bit    1: Unused
                          ; Bit    0: Unused
    .db $00,$00,$00,$00   ; Unused
    .db $00,$00,$00,$00   ; Unused

;
; PRG bank #0 -- swappable in/out of $C000-DFFF
;
    .base $C000
    ;
    ; put your code here
    ;
    .pad $E000

;
; PRG bank #1 -- swappable in/out of $C000-DFFF
;
    .base $C000
    ;
    ; put your code here
    ;
    .pad $E000

;
; PRG bank #2 -- swappable in/out of $C000-DFFF
;
    .base $C000
    ;
    ; put your code here
    ;
    .pad $E000

;
; PRG bank #3 -- swappable in/out of $C000-DFFF
;
    .base $C000

    ;
    ; Just fooling around here with code...
    ;
MAIN:
    lda #$DE
    sta $00
    lda #$AD
    sta $01
    lda #$BE
    sta $02
    lda #$EF
    sta $03
    jmp OTHER

    .pad $E000


;
; PRG bank #4 and #5 (16KB) -- we choose put this at $8000-BFFF
;
    .base $8000

   ;
   ; Just an infinite loop to ensure that assembled/calculated addresses
   ; and the like work correctly.
   ;
OTHER:
   lda #$aa
   ldx #$bb
   ldy #$cc
   jmp OTHER

    .pad $C000


;
; PRG bank #6 -- swappable in/out of $C000-DFFF
;
    .base $C000
    ;
    ; put your code here
    ;
    .pad $E000


;
; PRG bank #7 -- fixed to $E000-FFFF
;
    .base $E000

RESET:         ; Code taken from https://wiki.nesdev.com/w/index.php/Init_code
    sei        ; ignore IRQs
    cld        ; disable decimal mode
    ldx #$40
    stx $4017  ; disable APU frame IRQ
    ldx #$FF
    txs        ; Set up stack
    inx        ; now X = 0
    stx $2000  ; disable NMI
    stx $2001  ; disable rendering
    stx $4010  ; disable DMC IRQs

    ;
    ; VRC6a mapper setup
    ;
    lda #%00101000  ; Bit    7: PRG-RAM disabled
                    ; Bit    6: unused
                    ; Bit    5: CIRAM connected to CHR A10
                    ; Bit    4: Nametables come from CIRAM
                    ; Bits 3-0: Vertical mirroring
                    ; NOTE: I sure hope I got these right... a wonderfully confusing mapper
    sta $B003       ; Set VRC6a PPU banking style
    lda #(4 >> 1)   ; Select PRG banks 4+5 (16KB).  The bitshift acts effectively /2, which
                    ; is needed since "internally" the mapper considers PRG banks 8KB.
                    ; This also means for 16KB selection, the bank number needs to be evenly
                    ; divisible by 2 (i.e. you couldn't pick PRG banks 5+6).
    sta $8000       ; Set 16KB PRG-ROM @ $8000-BFFF
    lda #3          ; Select PRG bank 3
    sta $C000       ; Set 8KB PRG-ROM @ $C000

    ; If the user presses Reset during vblank, the PPU may reset
    ; with the vblank flag still true.  This has about a 1 in 13
    ; chance of happening on NTSC or 2 in 9 on PAL.  Clear the
    ; flag now so the @vblankwait1 loop sees an actual vblank.
    bit $2002

    ; First of two waits for vertical blank to make sure that the
    ; PPU has stabilized
@vblankwait1:
    bit $2002
    bpl @vblankwait1

    ; We now have about 30,000 cycles to burn before the PPU stabilizes.
    ; One thing we can do with this time is put RAM in a known state.
    ; Here we fill it with $00, which matches what (say) a C compiler
    ; expects for BSS.  Conveniently, X is still 0.
    txa
@clrmem:
    sta $000,x
    sta $100,x
    sta $300,x
    sta $400,x
    sta $500,x
    sta $600,x
    sta $700,x  ; Remove this if you're storing reset-persistent data

    ; We skipped $200,x on purpose.  Usually, RAM page 2 is used for the
    ; display list to be copied to OAM.  OAM needs to be initialized to
    ; $EF-$FF, not 0, or you'll get a bunch of garbage sprites at (0, 0).

    inx
    bne @clrmem

    ; Other things you can do between vblank waits are set up audio
    ; or set up other mapper registers.

@vblankwait2:
    bit $2002
    bpl @vblankwait2

    jmp MAIN

NMI:
    rti

IRQ:
    rti

    .pad $FFFA
;
; 6502 vectors
;
    .dw NMI
    .dw RESET
    .dw IRQ