1 files changed, 171 insertions, 0 deletions
diff --git a/src/sound.s b/src/sound.s
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+.segment "CODE"
+
+;; The sound for this game is extremely simple, coming from a system that only
+;; allowed for 1-bit beeps. Here the sound is a bit different due to a vastly
+;; different audio hardware, and we make use of three channels:
+;;
+;;   1. Square 1: used on level entry and bullets.
+;;   2. Square 2: used for item collection or dropping.
+;;   3. Noise: enemy explosion and rocket launch.
+.scope Sound
+    ;; Bullets can go super fast, and if we delivered the sound effect for each
+    ;; bullet it could potentially be annoying. Moreover, because of the
+    ;; limitations from the ZX Spectrum, the original game didn't spit a sound
+    ;; effect for each bullet either. Hence, wait for some frames before
+    ;; producing a sound. This is why we call Sound::tick() on NMI code, and why
+    ;; the sound effect for bullet is called via Sound::play_bullet_maybe().
+    ;;
+    ;; NOTE: the maximum count value is supposed to be bigger than the timer for
+    ;; bullets creation. This is guaranteed in 'jetpac.s'.
+    zp_frame_count = $DA
+    BULLET_SFX_FRAME_COUNT = HZ / 10
+
+    ;; Period values for square channels.
+    .ifdef PAL
+        BULLET_SFX_LOW  = $29
+        BULLET_SFX_HIGH = $00
+        ENTER_SFX_LOW   = $4D
+        ENTER_SFX_HIGH  = $01
+        PICKUP_SFX_LOW  = $61
+        PICKUP_SFX_HIGH = $01
+        DROP_SFX_LOW    = $3A
+        DROP_SFX_HIGH   = $01
+    .else
+        BULLET_SFX_LOW  = $2C
+        BULLET_SFX_HIGH = $00
+        ENTER_SFX_LOW   = $67
+        ENTER_SFX_HIGH  = $01
+        PICKUP_SFX_LOW  = $7C
+        PICKUP_SFX_HIGH = $01
+        DROP_SFX_LOW    = $52
+        DROP_SFX_HIGH   = $01
+    .endif
+
+    ;; Initialize all the sound channels which are needed and reset some
+    ;; register values.
+    .proc init
+        ;; Enable square 1, 2; and noise.
+        lda #%00001011
+        sta APU::m_status
+
+        ;; Reset sweep registers and frame count.
+        lda #0
+        sta APU::m_square_1_sweep
+        sta APU::m_square_2_sweep
+        sta Sound::zp_frame_count
+
+        ;; Silence channels.
+        lda #0
+        sta APU::m_noise_envelope
+        lda #$30
+        sta APU::m_square_1_envelope
+        sta APU::m_square_2_envelope
+
+        rts
+    .endproc
+
+    ;; Tick the internal frame count for sound effects.
+    ;;
+    ;; NOTE: expected to only be called at the end of NMI code.
+    .proc tick
+        ;; If there is no bullet sound effect to be delivered, don't even sweat
+        ;; it.
+        lda Sound::zp_frame_count
+        beq @end
+
+        ;; Increase the frame counter and check the limit. If we reached that
+        ;; limit, reset it so we don't tick until the next bullet sfx request
+        ;; comes in.
+        clc
+        adc #1
+        cmp #Sound::BULLET_SFX_FRAME_COUNT
+        beq @reset
+        sta Sound::zp_frame_count
+        rts
+
+    @reset:
+        lda #0
+        sta Sound::zp_frame_count
+
+    @end:
+        rts
+    .endproc
+
+    ;; Play the bullet sound effect if we can (i.e. the frame count allows us to
+    ;; do it).
+    .proc play_bullet_maybe
+        ;; If we cannot play the sound yet, skip this altogether.
+        lda Sound::zp_frame_count
+        bne @end
+        inc Sound::zp_frame_count
+
+        lda #$01
+        sta APU::m_square_1_envelope
+        lda #%10000001
+        sta APU::m_square_1_sweep
+        lda #BULLET_SFX_LOW
+        sta APU::m_square_1_low
+        lda #BULLET_SFX_HIGH
+        sta APU::m_square_1_high
+
+    @end:
+        rts
+    .endproc
+.endscope
+
+;; Make an explosion sound via the noise channel.
+.macro SOUND_EXPLOSION
+    lda #$03
+    sta APU::m_noise_envelope
+    lda #$8F
+    sta APU::m_noise_mode
+    lda #$F8
+    sta APU::m_noise_counter
+.endmacro
+
+;; Make a small beep, suitable for level entry.
+.macro SOUND_ENTER_LEVEL
+    lda #%10000100
+    sta APU::m_square_1_envelope
+    lda #Sound::ENTER_SFX_LOW
+    sta APU::m_square_1_low
+    lda #Sound::ENTER_SFX_HIGH
+    sta APU::m_square_1_high
+.endmacro
+
+;; Make a small beep for item pickup.
+.macro SOUND_ITEM_PICKUP
+    lda #%10000100
+    sta APU::m_square_2_envelope
+    lda #Sound::PICKUP_SFX_LOW
+    sta APU::m_square_2_low
+    lda #Sound::PICKUP_SFX_HIGH
+    sta APU::m_square_2_high
+.endmacro
+
+;; Make a small beep for item collection in the droppping zone (i.e. fuel tanks
+;; and shuttle parts making into the shuttle).
+.macro SOUND_ITEM_DROP
+    lda #%10000100
+    sta APU::m_square_2_envelope
+    lda #Sound::DROP_SFX_LOW
+    sta APU::m_square_2_low
+    lda #Sound::DROP_SFX_HIGH
+    sta APU::m_square_2_high
+.endmacro
+
+;; Start the sound effect for the rocket take off animation.
+.macro START_TAKE_OFF_SOUND
+    lda #$38
+    sta APU::m_noise_envelope
+    lda #$0F
+    sta APU::m_noise_mode
+    lda #0
+    sta APU::m_noise_counter
+.endmacro
+
+;; Stop the sound effect for the rocket take off animation.
+.macro STOP_TAKE_OFF_SOUND
+    lda #0
+    sta APU::m_noise_envelope
+.endmacro