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|
;;;
;; Make an annoying "beep" sound.
;;; Nothing remarkable here, go down below until you see some comments :)
.segment "HEADER"
.byte 'N', 'E', 'S', $1A
.byte $02, $01
.res $0A, $00
.segment "VECTORS"
.addr nmi, reset, irq
.segment "CHARS"
.incbin "../assets/basic.chr"
.segment "CODE"
.include "../shared/asm.s"
.include "../shared/apu.s"
.include "../shared/oam.s"
.include "../shared/ppu.s"
.include "../shared/clear.s"
.scope Vars
zp_flags = $20
.endscope
.proc reset
sei
cld
ldx #$40
stx APU::m_frame_counter
ldx #$FF
txs
inx
stx PPU::m_control
stx PPU::m_mask
stx APU::m_dmc
bit PPU::m_status
@vblankwait1:
bit PPU::m_status
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
OAM_WRITE_SPRITES
@vblankwait2:
bit PPU::m_status
bpl @vblankwait2
lda #$3F
sta PPU::m_address
lda #$00
sta PPU::m_address
lda #$0F
ldx #$20
@palettes_reset_loop:
sta PPU::m_data
dex
bne @palettes_reset_loop
__fallthrough__ main
.endproc
.proc main
CLEAR_SCREEN
;; In order to get sound, you need to enable/disable the channels you
;; want. One typical setup is to enable all of them except for DMC. You do
;; this by writing into $4015 (APU Status). Note that this register can also
;; be read. Reading from this register will give you the state of
;; interrupts, and the length of the counter for each channel. Yes, each
;; channel has an internal counter, but we can discuss this later.
;;
;; Anyways, if we want to enable all channels except DMC, we are in luck
;; because then we just need to set all bits for the low nibble.
lda #$0F
sta APU::m_status
;; This was the configuration for the APU. Now let's produce sound on the
;; Square 1 channel (registers: $4000-$4003).
;; Let's configure the Square 1 channel before producing any sound.
;;
;; The low nibble controls the volume: 0 for silent, 1 very low, F
;; maximum. Just to be annoying we will be setting the volume at a maximum
;; level.
;;
;; Then we have two bits which seem quite odd at first. Bit 4 sets/unsets
;; whether the volume is to be kept constant. If unset, then an internal
;; counter will tune it down when running out. Similarly, bit 5 sets/unsets
;; whether the length counter is to be accounted or not. See this counter
;; down below. All in all, here we make the sound constant, so the beep
;; never stops until we mute it (which we don't in this example).
;;
;; Finally we have the "duty" bits, which has four possibilities
;; available. These regulate the tone for the note, and it's basically the
;; percentage of time the square wave is in the "up" position.
lda #%10111111
sta APU::m_square_1_envelope
;; The $4001 address contains the sweep register, which is a way that the
;; APU has in order to produce different pitch effects, or workaround known
;; issues on some tones. I'm not touching it here other than resetting to 0.
lda #0
sta APU::m_square_1_sweep
;; The note to play is 11 bits long. This means that we need two bytes for
;; it, which for square 1 are $4002 and $4003. $4002 is the least
;; significant bits, and the three least significant bits from $4003 are the
;; most significant bits from this 11-bit note definition. How to know which
;; note corresponds to what value is easy via:
;; https://www.nesdev.org/wiki/APU_period_table. Thus, in NTSC, setting $0C9
;; to this 11-bit value gives us a C#.
;;
;; The 5 other bits from $4003 correspond to the length counter. That is,
;; you can regulate for how long this note has to be reproduced, and the APU
;; will (magically) track things for you. This is not available on this
;; configuration because we disabled the option when we configured
;; 'APU::m_square_1_envelope' (see above). Hence, we will set these bits to
;; 0.
lda #$C9
sta APU::m_square_1_low
lda #$00
sta APU::m_square_1_high
;; NOTE: and that's it :)
cli
lda #%10001000
sta PPU::m_control
lda #%00011110
sta PPU::m_mask
@main_game_loop:
lda #%10000000
ora Vars::zp_flags
sta Vars::zp_flags
@wait_for_render:
bit Vars::zp_flags
bmi @wait_for_render
jmp @main_game_loop
.endproc
.proc nmi
bit Vars::zp_flags
bpl @next
pha
txa
pha
tya
pha
lda #%01111111
and Vars::zp_flags
sta Vars::zp_flags
pla
tay
pla
tax
pla
@next:
rti
.endproc
.proc irq
rti
.endproc
|
