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;;;
;; Allow the player to scroll a level which spans more than two screens wide.
;; The heavy lifting is pulled by the engine contained in `include`, which even
;; if it has some big limitations, it's good enough for showing how this can be
;; achieved on the NES/Famicom. Read the comments along this file, but you will
;; have to dig deeper into `include` to better grasp how any of this works.
;;
;; As a final touch, you can press "Select" to switch between different levels,
;; even if I was lazy enough to only provide a second level.
.segment "HEADER"
.byte 'N', 'E', 'S', $1A
.byte $02
.byte $01
.byte $01 ; Vertical mirroring (important since we are using horizontal scrolling!)
.byte $00
.segment "VECTORS"
.addr nmi, reset, irq
.segment "CODE"
.include "include/all.s"
.proc main
;; The PPU mask is zero'ed out so no background nor sprites will be shown at
;; first. This will be the task of the `load_level_x` function which is
;; called during background initialization.
lda #$00
sta PPU::zp_mask
sta PPU::MASK
;; Initialize engine.
jsr Palettes::init
jsr Metatile::init
jsr Driver::init
jsr Background::init
jsr Player::init
;; Accept back interrupts.
cli
;; 7: allow NMI; 5-4: background pattern table at $0000, sprites at $1000.
;;
;; NOTE: we store this same value both into the PPU register and into this
;; special `zp_control` variable. This variable will shadow the contents for
;; the PPU control register. Any operation that requires changing the value
;; for this register, from now on, we will simply touch this variable
;; instead. This is because from now on touching this register is only safe
;; inside of Vblank (a.k.a. NMI code). Our `nmi` handler has special code to
;; check whether there was an update on this register's value and update
;; things accordingly.
lda #%10001000
sta PPU::zp_control
sta PPU::CONTROL
@main_game_loop:
jsr joypad_read
jsr Player::update
jsr Driver::update
;; Set the `render` flag, meaning that the code logic is over and we can
;; safely go into `nmi` code.
lda #%10000000
ora Globals::zp_flags
sta Globals::zp_flags
@wait_for_render:
bit Globals::zp_flags
bmi @wait_for_render
;; Rendering is done, we can perform another iteration of the loop!
jmp @main_game_loop
.endproc
;; The basics are laid out in `basics/sprite.s`. Read the comments below for
;; more info.
.proc nmi
;; Skip this entirely if code is not finished. This is a safeguard, but
;; whenever it happens it means that we are stalling at least for a frame.
bit Globals::zp_flags
bpl @next
;; Save registers
pha
txa
pha
tya
pha
;; Update the sprite of the player. For this simple game, it just means to
;; update its position.
jsr Player::update_sprite
;; Render stuff.
OAM_WRITE_SPRITES
;; Flush any pending background updates. This macro actually hides the most
;; expensive part executed during VBlank. Check the document on `buffer.s`
;; for more information.
FLUSH_PENDING_VRAM_BUFFER
;; Should we update PPU registers? If not, then we can go down to the next
;; section. Otherwise we need to update the PPU registers that have been
;; buffered and set the scroll.
bit Globals::zp_flags
bvc @after_ppu
;; Zero out the "ppu" flag.
lda #%10111111
and Globals::zp_flags
sta Globals::zp_flags
;; Reset the PPU address latch.
bit PPU::STATUS
;; Update the PPU control/mask registers with their buffered values.
lda PPU::zp_control
sta PPU::CONTROL
lda PPU::zp_mask
sta PPU::MASK
;; Update scroll.
lda Background::zp_scroll
sta PPU::SCROLL
lda #$00
sta PPU::SCROLL
@after_ppu:
;; And unset the `render` flag so the `main` code is unblocked.
lda #%01111111
and Globals::zp_flags
sta Globals::zp_flags
;; Restore registers.
pla
tay
pla
tax
pla
@next:
rti
.endproc
;; Unused.
.proc irq
rti
.endproc
.segment "CHARS"
.incbin "../assets/diskun.chr"
|
