POKEY: implement two-tone mode effect

doc/7-systems/pokey.md

@@ -32,3 +32,6 @@ a sound and input chip developed by Atari for their 8-bit computers (Atari 400,
     - filtered output on channel 2 (I suggest you to set channel 4 volume to 0).
     - use for PWM effects (not automatic!).
   - bit 0: 15KHz mode.
+- 12xx: toggle two-tone mode.
+  - when enabled, channel 2 modulates channel 1. I don't know how, but it does.
+  - only on ASAP core.

src/engine/platform/pokey.cpp

@@ -35,6 +35,7 @@ const char* regCheatSheetPOKEY[]={
   "AUDF4", "6",
   "AUDC4", "7",
   "AUDCTL", "8",
+  "SKCTL", "F",
   NULL
 };
 
@@ -154,6 +155,11 @@ void DivPlatformPOKEY::tick(bool sysTick) {
     }
   }
 
+  if (skctlChanged) {
+    skctlChanged=false;
+    rWrite(15,skctl);
+  }
+
   for (int i=0; i<4; i++) {
     if (chan[i].freqChanged || chan[i].keyOn || chan[i].keyOff) {
       chan[i].freq=parent->calcFreq(chan[i].baseFreq,parent->song.linearPitch?chan[i].pitch:0,chan[i].fixedArp?chan[i].baseNoteOverride:chan[i].arpOff,chan[i].fixedArp,true,0,parent->song.linearPitch?chan[i].pitch2:0,chipClock,CHIP_DIVIDER);
@@ -320,6 +326,10 @@ int DivPlatformPOKEY::dispatch(DivCommand c) {
       audctl=c.value&0xff;
       audctlChanged=true;
       break;
+    case DIV_CMD_STD_NOISE_FREQ:
+      skctl=c.value?0x8b:0x03;
+      skctlChanged=true;
+      break;
     case DIV_CMD_NOTE_PORTA: {
       int destFreq=NOTE_PERIODIC(c.value2);
       bool return2=false;
@@ -385,6 +395,7 @@ void DivPlatformPOKEY::forceIns() {
     chan[i].freqChanged=true;
   }
   audctlChanged=true;
+  skctlChanged=true;
 }
 
 void* DivPlatformPOKEY::getChanState(int ch) {
@@ -408,7 +419,7 @@ unsigned char* DivPlatformPOKEY::getRegisterPool() {
 }
 
 int DivPlatformPOKEY::getRegisterPoolSize() {
-  return 9;
+  return 16;
 }
 
 void DivPlatformPOKEY::reset() {
@@ -430,6 +441,8 @@ void DivPlatformPOKEY::reset() {
 
   audctl=0;
   audctlChanged=true;
+  skctl=3;
+  skctlChanged=true;
 }
 
 bool DivPlatformPOKEY::keyOffAffectsArp(int ch) {

src/engine/platform/pokey.h

@@ -48,8 +48,8 @@ class DivPlatformPOKEY: public DivDispatch {
       QueuedWrite(unsigned char a, unsigned char v): addr(a), val(v) {}
   };
   std::queue<QueuedWrite> writes;
-  unsigned char audctl;
-  bool audctlChanged;
+  unsigned char audctl, skctl;
+  bool audctlChanged, skctlChanged;
   unsigned char oscBufDelay;
   PokeyState pokey;
   AltASAP::Pokey altASAP;

src/engine/sysDef.cpp

@@ -1169,6 +1169,7 @@ void DivEngine::registerSystems() {
     {
       {0x10, {DIV_CMD_WAVE, "10xx: Set waveform (0 to 7)"}},
       {0x11, {DIV_CMD_STD_NOISE_MODE, "11xx: Set AUDCTL"}},
+      {0x12, {DIV_CMD_STD_NOISE_FREQ, "12xx: Toggle two-tone mode"}},
     }
   );