2 files changed, 236 insertions, 0 deletions
diff --git a/sound/README.md b/sound/README.md
index a099036..90fc84e 100644
--- a/sound/README.md
+++ b/sound/README.md
@@ -1 +1,3 @@
 TBD
+Reference nerdynights: https://nerdy-nights.nes.science/#audio_tutorial-0
+square channels sometimes called pulse channels
diff --git a/sound/select.s b/sound/select.s
new file mode 100644
index 0000000..887a585
--- /dev/null
+++ b/sound/select.s
@@ -0,0 +1,234 @@
+;;;
+;; Enable three sound channels and make them beep depending on the button that
+;; is being pressed.
+
+;;; 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"
+.include "../shared/joypad.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
+
+    ;; Just like beep.s
+    lda #$0F
+    sta APU::m_status
+
+    ;; Square 1 is set up in the same way as in beep.s but it's muted. We will
+    ;; handle playing/muting in the main game loop.
+    lda #%10110000
+    sta APU::m_square_1_envelope
+    lda #$C9
+    sta APU::m_square_1_low
+    lda #$00
+    sta APU::m_square_1_high
+    sta APU::m_square_1_sweep
+
+    ;; The square 2 channel is almost identical to the other square channel. The
+    ;; only difference is with sweeps, for which both square channels are wired
+    ;; up differently. To sum things up, when computing the target period with a
+    ;; negative value, square 1 will use one's complement, while square 2 will
+    ;; use two's complement. You can read more about this here:
+    ;; https://www.nesdev.org/wiki/APU_Sweep.
+    ;;
+    ;; In any case, here we just set the E note with a different duty. We mute
+    ;; it for the same reason as for square 1.
+    lda #%01110000
+    sta APU::m_square_2_envelope
+    lda #$A9
+    sta APU::m_square_2_low
+    lda #$00
+    sta APU::m_square_2_high
+    sta APU::m_square_2_sweep
+
+    ;; The triangle channel is a bit similar to the square ones, but triangle
+    ;; waves have no concept of "duty": there's no percentage of how much time
+    ;; it's in the "up" position because that's not how triangles work.
+    ;;
+    ;; The "control" register from the triangle channel has the most significant
+    ;; bit which, when unset, tells the channel to use the internal
+    ;; counters. The other bits hold a "value" which is only used with the
+    ;; linear counter. Here we set to 1 the most significant bit and hence we
+    ;; will control things manually instead of via the linear counter. If we set
+    ;; the "value" to zero, like in this case, then we mute the channel. To
+    ;; unmute it we can set any value (see below in the main game loop).
+    lda #%10000000
+    sta APU::m_triangle_control
+
+    ;; NOTE: contrary to what you'd expect, address $4009 is not used by the
+    ;; APU.
+
+    ;; And the low/high registers from the triangle channel work in the very
+    ;; same way as their square counterparts. Here we set a G#.
+    ;;
+    ;; That being said, take into account that the pitch of the triangle channel
+    ;; is one octave below the square channels with an equivalent timer
+    ;; value. If we were to have a table with notes for the square channels and
+    ;; we wanted to use them for the triangle channel, we would need to 'lsr'
+    ;; the value to get the same note.
+    lda #$42
+    sta APU::m_triangle_low
+    lda #$00
+    sta APU::m_triangle_high
+
+    cli
+    lda #%10001000
+    sta PPU::m_control
+    lda #%00011110
+    sta PPU::m_mask
+
+@main_game_loop:
+    ;; Read the joypad so we can mute/unmute channels depending on the button
+    ;; being pressed.
+    READ_JOYPAD1
+
+    ;; Mute/Play the square 1 channel
+    bit Joypad::zp_buttons1
+    bpl @reset_square_1
+    lda #%10111000
+    bne @set_square_1
+@reset_square_1:
+    lda #%10110000
+@set_square_1:
+    sta APU::m_square_1_envelope
+
+    ;; Mute/Play the square 2 channel
+    bit Joypad::zp_buttons1
+    bvc @reset_square_2
+    lda #%10111000
+    bne @set_square_2
+@reset_square_2:
+    lda #%10110000
+@set_square_2:
+    sta APU::m_square_2_envelope
+
+    ;; Mute/Play the triangle channel
+    lda Joypad::zp_buttons1
+    and #%00100000
+    beq @reset_triangle
+    lda #%10000001
+    bne @set_triangle
+@reset_triangle:
+    lda #%10000000
+@set_triangle:
+    sta APU::m_triangle_control
+
+    ;; NOTE: done :)
+
+    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