Virtual Bowling sample playback routine reversing notes

bowling.asm

// Super high-level playroutine overview:
//  Timer reload set to 12, 13, 5 (depending on sample)
//  Timer int fires every ??us (sample rate of ??hz)
//   Store 0x1d in timer control (small interval, zero interrupt enable, clear zero status, timer enable)
//   Output volume to VOICE_1_ENVELOPE_DATA

  // Sample value -> register value lookup table:
  0xffffd258  00 10 20 30 40 50 60 70 80 90 a0 b0 c0 d0 e0 f0

  // Sample playback routine (only thing the timer is used for in this game)
* 0xfff03dc6  64cd0880    ld.w -32760[r4], r11 // r11 = Samples left
  0xfff03dca  4ba5ffff    addi 0xffff, r11, r10
  0xfff03dce  44dd0880    st.w r10, -32760[r4] // Decrement samples left

  0xfff03dd2  0694        bnz 0x6 (0xfff03dd8)
  0xfff03dd4  00a81800    jr 24 (0xfff03dec)

  // Samples left not 0
  0xfff03dd8  20bc0005    movhi 0x500, r0, r1
  0xfff03ddc  41cdc80a    ld.w 2760[r1], r10   // r10 = 0x02000000
  0xfff03de0  60a11d00    movea 0x1d, r0, r11  // r11 = 0x0000001d
  0xfff03de4  6ad12000    st.b r11, 32[r10]    // TIMER_CONTROL_REG = r11
  0xfff03de8  00a82200    jr 34 (0xfff03e0a)

  // Samples left 0 (don't seem to hit this code path)
  0xfff03dec  20bc0005    movhi 0x500, r0, r1
  0xfff03df0  41cdc80a    ld.w 2760[r1], r10   // r10 = 0x02000000
  0xfff03df4  60a11000    movea 0x10, r0, r11  // r11 = 0x00000010
  0xfff03df8  6ad12000    st.b r11, 32[r10]    // TIMER_CONTROL_REG = r11
  0xfff03dfc  4c41        mov 12, r10          // r10 = 12
  0xfff03dfe  44dd1480    st.w r10, -32748[r4] // ??? = r10 (12)
  0xfff03e02  44cd1080    ld.w -32752[r4], r10 // r10 = VOICE_1_ENVELOPE_DATA addr
  0xfff03e06  0ad00000    st.b r0, 0[r10]      // Write out 0 to VOICE_1_ENVELOPE_DATA

  0xfff03e0a  44cd0480    ld.w -32764[r4], r10 // Flip 0x80 bit of -32764[r4] (appears to select which nybble of the data byte we output)
  0xfff03e0e  4ab98000    xori 0x80, r10, r10
  0xfff03e12  44dd0480    st.w r10, -32764[r4]
  0xfff03e16  44cd0480    ld.w -32764[r4], r10 // Branch based on 0x80 bit of -32764[r4]
  0xfff03e1a  4ab58000    andi 0x80, r10, r10
  0xfff03e1e  0694        bnz 0x6 (0xfff03e24)
  0xfff03e20  00a82800    jr 40 (0xfff03e48)

  // Output low nybble
  0xfff03e24  44cd1080    ld.w -32752[r4], r10 // r10 = VOICE_1_ENVELOPE_DATA addr
  0xfff03e28  64cd0c80    ld.w -32756[r4], r11 // r11 = Sample data ptr
  0xfff03e2c  6bc10000    ld.b 0[r11], r11     // r11 = Sample data byte
  0xfff03e30  6bb5ff00    andi 0xff, r11, r11  // r11 &= 0xff
  0xfff03e34  6bb50f00    andi 0xf, r11, r11   // r11 &= 0x0f
  0xfff03e38  2bbc0000    movhi 0x0, r11, r1   // r1 = Volume register LUT index
  0xfff03e3c  61c158d2    ld.b -11688[r1], r11 // r11 = sample
  0xfff03e40  6ad10000    st.b r11, 0[r10]     // Write out byte to VOICE_1_ENVELOPE_DATA
  0xfff03e44  00a82c00    jr 44 (0xfff03e70)

  // Output high nybble and increment data pointer
  0xfff03e48  44cd1080    ld.w -32752[r4], r10 // r10 = VOICE_1_ENVELOPE_DATA addr
  0xfff03e4c  64cd0c80    ld.w -32756[r4], r11 // r11 = Sample data ptr
  0xfff03e50  6bc10000    ld.b 0[r11], r11     // r11 = Sample data byte
  0xfff03e54  6bb5ff00    andi 0xff, r11, r11  // r11 &= 0xff
  0xfff03e58  645d        sar 4, r11           // r11 >>= 4
  0xfff03e5a  2bbc0000    movhi 0x0, r11, r1   // r1 = Volume register LUT index
  0xfff03e5e  61c158d2    ld.b -11688[r1], r11 // r11 = sample
  0xfff03e62  6ad10000    st.b r11, 0[r10]     // Write out byte to VOICE_1_ENVELOPE_DATA

  0xfff03e66  44cd0c80    ld.w -32756[r4], r10 // Increment sample data ptr
  0xfff03e6a  4145        add 1, r10
  0xfff03e6c  44dd0c80    st.w r10, -32756[r4]

  0xfff03e70  1f18        jmp [r31]