if it helps, this is the APU code from my emulator (FreeBASIC) ... the code is kind of sloppy, but it does work and it shouldn't be too (i hope) difficult to decipher.

Code: Select all

```
Dim Shared As LongInt cpuclock = 1789773, samplerate = 44100, sampleticks = 1789773/44100
Dim Shared As Integer buflen, freqmod = 7402, bufpos = 0
Dim Shared As Byte cursample = 0, square1sample = 0, square2sample = 0, trianglesample = 0,_
noisesample = 0, dmcsample = 0, gain = 1, tester = 0, square1=0, square2=0, triangle=0, noise=0,_
tricounter = 0, counterstep = 4, curstep = 1, nosound
Dim Shared As Integer square1_duty = 0, square1_dutypos = 0, square1_loopenv = 0, square1_env = 15,_
square1_disableenv = 0, square1_envperiod = 0, square1_enablesweep = 0, square1_period = 0,_
square1_negative = 0, square1_shift = 0, square1counter = 0, square1_sweep = 0,_
square1_length = 0, square1_enable = 0, square1_write = 0
Dim Shared As LongInt square1_timer = 1, square1_freq = 100000, s1t = 1
Dim Shared As Integer square2_duty = 0, square2_dutypos = 0, square2_loopenv = 0, square2_env = 15,_
square2_disableenv = 0, square2_envperiod = 0, square2_enablesweep = 0, square2_period = 0,_
square2_negative = 0, square2_shift = 0, square2counter = 0, square2_sweep = 0,_
square2_length = 0, square2_enable = 0, square2_write = 0
Dim Shared As LongInt square2_timer = 1, square2_freq = 100000, s2t = 1
Dim Shared As Integer triangle_control = 0, triangle_counter = 0, triangle_length = 0, triangle_period = 0,_
triangle_enable = 0, triangle_write = 0, triangle_halt = 0
Dim Shared As LongInt triangle_timer = 1, triangle_freq = 10000000
Dim Shared As Integer noise_loopenv = 0, noise_disableenv = 0, noise_envperiod = 0, noise_shortmode = 0,_
noise_shift = 1, noise_period = 0, noise_length = 0, noise_write = 0, noise_halt = 0, noise_env = 0,_
noise_enable = 0, noise_loop = 0, noise_origenv = 0
Dim Shared As LongInt noise_timer = 1
Dim Shared As Integer dmc_irqenable = 0, dmc_loop = 1, dmc_freqindex = 0, dmc_dac = 0, dmc_addr = 0,_
dmc_length = 0, dmc_enable = 0
Dim Shared As Integer lengthcounterenable = 1, dmclen = 1, noiselen = 1, trianglelen = 1, square1len = 1, square2len = 1
Dim Shared As Integer frameinterrupt = 0
Dim Shared As Integer squarelookup(32), otherlookup(204)
Dim Shared As UByte lengthtable(&h20) => {_
10,254, 20, 2, 40, 4, 80, 6, 160, 8, 60, 10, 14, 12, 26, 14,_
12, 16, 24, 18, 48, 20, 96, 22, 192, 24, 72, 26, 16, 28, 32, 30 }
Dim Shared As Integer noisefreq(&h10) => {_
4, 8, 16, 32, 64, 96, 128, 160, 202,_
254, 380, 508, 762, 1016, 2034, 4068 }
Dim Shared As UByte tritable(32) => {_
8, 9, 10, 11, 12, 13, 14, 15, 15, 14, 13, 12, 11, 10, 9 ,8,_
7, 6, 5, 4, 3, 2, 1, 0, 0, 1, 2, 3, 4, 5, 6, 7 }
Dim Shared As UByte duty0(16) => { 0, 0, 1, 1, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0 }
Dim Shared As UByte duty1(16) => { 0, 0, 1, 1, 1, 1, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0 }
Dim Shared As UByte duty2(16) => { 0, 0, 1, 1, 1, 1, 1, 1, 1, 1, 0, 0, 0, 0, 0, 0 }
Dim Shared As UByte duty3(16) => { 1, 1, 0, 0, 0, 0, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1 }
Dim Shared buf(11025) As Byte
Function APUreadregs(ByVal addr As UShort) As Integer
Dim tmpstatus As Integer
tmpstatus = dmc_irqenable*128 + frameinterrupt*64
if (dmc_length) Then tmpstatus = tmpstatus + 16
if (triangle_length) Then tmpstatus = tmpstatus + 4
If (square2_length) Then tmpstatus = tmpstatus + 2
If (square1_length) then tmpstatus = tmpstatus + 1
APUreadregs = tmpstatus
End Function
Sub APUwriteregs(ByVal addr As UShort, ByVal value As UByte)
Select Case As Const (addr)
'START SQUARE 1 HERE
case &h4000:
square1_duty = value Shr 6
if (value And 32) Then square1_disableenv = 1 else square1_disableenv = 0
square1_env = value And &hF
square1_env = square1_env + ((value Shr 1) And &h10)
case &h4001:
if (value And 128) Then square1_enablesweep = 1 else square1_enablesweep = 0
square1_sweep = ((value Shr 4) And 7)+1
if (value And &h10) Then square1_negative = 1 else square1_negative = 0
square1_shift = value And 7
case &h4002:
square1_timer = (square1_timer And &hF00) + value
case &h4003:
square1_length = lengthtable(value Shr 3)
square1_timer = (square1_timer And &hFF) + ((value And &h7) Shl 8)
square1_dutypos = 0
'START SQUARE 2 HERE
case &h4004:
square2_duty = value Shr 6
if (value And 32) Then square2_disableenv = 1 else square2_disableenv = 0
square2_env = value And &hF
square2_env = square2_env + ((value Shr 1) And &h10)
case &h4005:
if (value And 128) Then square2_enablesweep = 1 else square2_enablesweep = 0
square2_sweep = ((value Shr 4) And 7)+1
if (value And &h10) Then square2_negative = 1 else square2_negative = 0
square2_shift = value And 7
case &h4006:
square2_timer = (square2_timer And &hF00) + value
case &h4007:
square2_length = lengthtable(value Shr 3)
square2_timer = (square2_timer And &hFF) + ((value And &h7) Shl 8)
square2_dutypos = 0
'START TRIANGLE HERE
case &h4008:
triangle_halt = value And &h80 Shr 7
if (triangle_control=0) Then triangle_halt = 1
triangle_counter = value And &h7F
case &h400A:
triangle_timer = (triangle_timer And &hF00) + value
case &h400B:
triangle_timer = (triangle_timer And &hFF) + ((value And &h7) Shl 8)
triangle_length = lengthtable(value Shr 3)
triangle_freq = cpuclock / (32*(triangle_timer)+1)
triangle_halt = 1
'START NOISE HERE
case &h400C:
if (value And 32) Then noise_halt = 1 else noise_halt = 0
noise_env = value And &h1F
noise_origenv = noise_env
case &h400E:
if (value And 128) Then noise_loop = 1 else noise_loop = 0
noise_timer = noisefreq(value And &hF)
case &h400F:
noise_length = lengthtable(value Shr 3)
noise_env = noise_origenv
case &h4015:
if (value And 1) Then square1_enable = 1 else square1_enable = 0
if (value And 2) Then square2_enable = 1 else square2_enable = 0
if (value And 4) Then triangle_enable = 1 else triangle_enable = 0
if (value And 8) Then noise_enable = 1 else noise_enable = 0
if (value And 16) Then dmc_enable = 1 else dmc_enable = 0
case &h4017:
if (value And &h80) Then counterstep = 5 else counterstep = 4
curstep = 1
End Select
End Sub
Sub putinbuf()
buf(bufpos) = cursample
bufpos = bufpos + 1
End Sub
Sub square1_clock()
if ((square1_enable=1) And (square1_length>0)) Then
if (square1_disableenv=1) Then square1_env = &hF
Select case (square1_duty)
case 0: if (duty0(square1_dutypos)) Then square1sample = square1_env else square1sample = 0
case 1: if (duty1(square1_dutypos)) Then square1sample = square1_env else square1sample = 0
case 2: if (duty2(square1_dutypos)) Then square1sample = square1_env else square1sample = 0
case 3: if (duty3(square1_dutypos)) Then square1sample = -1*square1_env else square1sample = 0
End Select
square1_dutypos = (square1_dutypos + 1) Mod 16
Else
square1sample = 0
End If
End Sub
Sub square2_clock()
if ((square2_enable=1) and (square2_length>0)) Then
if (square2_disableenv=1) Then square2_env = &hF
Select Case (square2_duty)
case 0: if (duty0(square2_dutypos)) Then square2sample = square2_env else square2sample = 0
case 1: if (duty1(square2_dutypos)) Then square2sample = square2_env else square2sample = 0
case 2: if (duty2(square2_dutypos)) Then square2sample = square2_env else square2sample = 0
case 3: if (duty3(square2_dutypos)) Then square2sample = -1*square2_env else square2sample = 0
End Select
square2_dutypos = (square2_dutypos + 1) Mod 16
Else
square2sample = 0
End If
End Sub
Sub triangle_clock()
if ((triangle_enable=0) or (triangle_length<=0)) Then Exit Sub
trianglesample = tritable(tricounter)
if (trianglesample>15) Then trianglesample = 31 - trianglesample
tricounter = (tricounter + 1) mod 32
End Sub
sub noise_clock()
dim feedback, tmpbit As Integer
if (noise_loop) Then
if (noise_shift And 64) Then tmpbit = 1 else tmpbit = 0
feedback = (noise_shift And 1) Xor tmpbit
Else
if (noise_shift And 2) Then tmpbit = 1 else tmpbit = 0
feedback = (noise_shift And 1) Xor tmpbit
End If
noise_shift = (noise_shift Shr 1) + (feedback Shl 13)
if ((noise_enable) and (noise_length>0) and ((noise_shift And 1)=0) and (noise_timer>=8)) Then
noisesample = noise_env
Else
noisesample = 0
End If
End Sub
Sub APU_frame_seq_tick()
if (counterstep=4) Then
if (triangle_length>0) Then triangle_length = triangle_length - 1
if (square1_length>0) Then square1_length = square1_length - 1
if (square2_length>0) Then square2_length = square2_length - 1
if (noise_length>0) Then noise_length = noise_length - 1
if ((curstep=4) And ((flag_reg And 4)=0)) Then flag_reg = flag_reg or &h14
if (square1_sweep>0) Then square1_sweep = square1_sweep - 1
if (square2_sweep>0) Then square2_sweep = square2_sweep - 1
if ((square1_negative=1) and (square1_enablesweep=1) and (square1_sweep>0) and (curstep<5)) Then
square1_timer = square1_timer - (square1_timer Shr square1_shift)
ElseIf ((curstep<5) And (square1_enablesweep=1) and (square1_sweep>0)) Then
square1_timer = square1_timer + (square1_timer Shr square1_shift)
End If
if ((square2_negative=1) and (square2_enablesweep=1) and (square2_sweep>0) and (curstep<5)) Then
square2_timer = square2_timer - (square2_timer Shr square2_shift)
ElseIf ((curstep<5) and (square2_enablesweep=1) and (square2_sweep>0)) Then
square2_timer = square2_timer + (square2_timer Shr square2_shift)
End If
End If
curstep = (curstep+1) mod counterstep
End Sub
Sub fillaudio Cdecl (ByVal userdata As Any Ptr, ByVal audbuffer As UByte Ptr, ByVal audlen As Integer)
Dim As UByte Ptr audbyte
For n = 0 To buflen-1
audbyte = audbuffer + n
*audbyte = buf(n)
Next n
bufpos = 0
End Sub
```

and then this is at the end of my 6502 opcode execution loop after each instruction:

my sound emulation is NOT perfect, but i've got it improved to the point where it's pretty damn close (imo anyway)... i don't emulate the DPCM channel yet though.