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|
;;; From https://github.com/mssola/code.nes which will be open sourced soon
;;; (pinky promess!).
.segment "HEADER"
.byte 'N', 'E', 'S', $1A
.byte $08 ; 128KB of PRG-ROM (8 x 16KB)
.byte $00 ; No CHR-ROM.
.byte $20, $08 ; Mapper 2, horizontal mirroring, NES 2.0
.byte $00
.byte $00
.byte $00
.byte $07 ; 8192 (64 * 2^7) bytes CHR RAM, no battery
.segment "VECTORS"
.addr nmi, reset, irq
.segment "BANK0"
chr: .incbin "../assets/basic.chr"
;;;
;; We use the fixed bank for the core functionality.
.segment "FIXED"
;;;
;; Bank switching. Not actually used here :D
banktable:
.byte $00, $01, $02, $03, $04, $05, $06
m_current_bank = $00
bankswitch:
sty m_current_bank
bankswitch_nosave:
tya
sta banktable, y
rts
;;;
;; From here on the code is basically the same as `sprite.s`, but with a special
;; twist that will be commented in.
reset:
sei
cld
ldx #$40
stx $4017
ldx #$ff
txs
inx
stx $2000
stx $2001
stx $4010
@vblankwait1:
bit $2002
bpl @vblankwait1
ldx #0
lda #0
@ram_reset_loop:
sta $000, x
sta $100, x
sta $300, x
sta $400, x
sta $500, x
sta $600, x
sta $700, x
inx
bne @ram_reset_loop
lda #$ef
@sprite_reset_loop:
sta $200, x
inx
bne @sprite_reset_loop
;; NOTE: after sprites have been reset we can transfer data from PRG-ROM
;; into RAM so the rest of the code can assume that the data is there. This
;; is the main difference with the `sprite.s` example.
jsr transfer_to_chr_ram
lda #$00
sta $2003
lda #$02
sta $4014
@vblankwait2:
bit $2002
bpl @vblankwait2
lda #$3F
sta $2006
lda #$00
sta $2006
lda #$0F
ldx #$20
@palettes_reset_loop:
sta $2007
dex
bne @palettes_reset_loop
jmp main
;;;
;; Transfer the CHR data from PRG-ROM into RAM.
.proc transfer_to_chr_ram
;;;
;; The code is pretty much taken from the NESDev wiki (see:
;; https://www.nesdev.org/wiki/CHR_ROM_vs._CHR_RAM).
;; First we set a 16-bit pointer that points `chr`, which contains the
;; actual data. This pointer will be stored at $00-$01, which is probably
;; not ideal, but this is just an example.
lda #<chr
sta $00
lda #>chr
sta $01
;; Load the destination address into the PPU. We have to initialize both
;; pattern tables, one starting at $0000, and the other at $1000. Hence we
;; start by simply pointing at PPU address $0000, and the loop will simply
;; fill both tables from here.
ldy #0
sty $2006
sty $2006
;; - x contains the number of 256-byte pages to copy.
;; - y will index within the page ($00-$FF).
ldx #32
loop:
;; First part of the loop: copy each byte of the current page.
lda ($00), y
sta $2007
iny
bne loop
;; Go to the next page and repeat the first part of the loop.
inc $01
dex
bne loop
rts
.endproc
;;;
;; And from here on it's pretty much copied from `sprite.s`.
.proc main
jsr init_palettes
jsr init_nametable
jsr init_sprites
cli
lda #%10110000
sta $2000
lda #%00011110
sta $2001
@main_game_loop:
lda #%10000000
ora $20
sta $20
@wait_for_render:
bit $20
bmi @wait_for_render
jmp @main_game_loop
.endproc
.proc init_palettes
lda #$3F
sta $2006
lda #$00
sta $2006
ldx #0
@load_palettes_loop:
lda palettes, x
sta $2007
inx
cpx #$20
bne @load_palettes_loop
rts
palettes:
.byte $0F, $12, $22, $32
.byte $0F, $00, $28, $30
.byte $0F, $28, $16, $2D
.byte $0F, $28, $16, $2D
.byte $0F, $00, $05, $30
.byte $0F, $00, $00, $00
.byte $0F, $00, $00, $00
.byte $0F, $00, $00, $00
.endproc
.macro WRITE_PPU_DATA address, value
bit $2002
lda #.HIBYTE(address)
sta $2006
lda #.LOBYTE(address)
sta $2006
lda #value
sta $2007
.endmacro
.proc init_nametable
bit $2002
WRITE_PPU_DATA $20C8, $02
WRITE_PPU_DATA $20B9, $04
WRITE_PPU_DATA $21CE, $04
WRITE_PPU_DATA $21BA, $04
WRITE_PPU_DATA $22B8, $04
WRITE_PPU_DATA $22E7, $04
WRITE_PPU_DATA $227A, $02
WRITE_PPU_DATA $23CE, %00000001
rts
.endproc
.proc init_sprites
NUM_SPRITES = 2
ldx #$00
@load_sprites_loop:
lda initial_sprite_data, x
sta $0200, x
inx
cpx #(4 * NUM_SPRITES)
bne @load_sprites_loop
rts
initial_sprite_data:
.byte $B0, $00, %00000000, $7A
.byte $B0, $00, %01000000, $82
.endproc
nmi:
bit $20
bpl @next
pha
txa
pha
tya
pha
lda #$00
sta $2003
lda #$02
sta $4014
bit $2002
lda #$00
sta $2005
sta $2005
lda #%01111111
and $20
sta $20
pla
tay
pla
tax
pla
@next:
rti
irq:
rti
|
