dev108 - add SN period range compat flag

there's room for possible improvement

papers/format.md

@@ -32,6 +32,7 @@ these fields are 0 in format versions prior to 100 (0.6pre1).
 
 the format versions are:
 
+- 108: Furnace dev108
 - 107: Furnace dev107
 - 106: Furnace dev106
 - 105: Furnace dev105
@@ -331,7 +332,8 @@ size | description
   1  | broken outVol (>=99) or reserved
   1  | E1xy and E2xy stop on same note (>=100) or reserved
   1  | broken initial position of porta after arp (>=101) or reserved
-  7  | reserved
+  1  | SN periods under 8 are treated as 1 (>=108) or reserved
+  6  | reserved
  --- | **virtual tempo data**
   2  | virtual tempo numerator of first song (>=96) or reserved
   2  | virtual tempo denominator of first song (>=96) or reserved

src/engine/engine.h

@@ -46,8 +46,8 @@
 #define BUSY_BEGIN_SOFT softLocked=true; isBusy.lock();
 #define BUSY_END isBusy.unlock(); softLocked=false;
 
-#define DIV_VERSION "dev107"
-#define DIV_ENGINE_VERSION 107
+#define DIV_VERSION "dev108"
+#define DIV_ENGINE_VERSION 108
 
 // for imports
 #define DIV_VERSION_MOD 0xff01

src/engine/fileOps.cpp

@@ -175,6 +175,7 @@ bool DivEngine::loadDMF(unsigned char* file, size_t len) {
     ds.brokenOutVol=true; // ???
     ds.e1e2StopOnSameNote=true;
     ds.brokenPortaArp=false;
+    ds.snNoLowPeriods=true;
 
     // 1.1 compat flags
     if (ds.version>24) {
@@ -1063,6 +1064,9 @@ bool DivEngine::loadFur(unsigned char* file, size_t len) {
     if (ds.version<101) {
       ds.brokenPortaArp=true;
     }
+    if (ds.version<108) {
+      ds.snNoLowPeriods=true;
+    }
     ds.isDMF=false;
 
     reader.readS(); // reserved
@@ -1475,7 +1479,12 @@ bool DivEngine::loadFur(unsigned char* file, size_t len) {
       } else {
         reader.readC();
       }
-      for (int i=0; i<7; i++) {
+      if (ds.version>=108) {
+        ds.snNoLowPeriods=reader.readC();
+      } else {
+        reader.readC();
+      }
+      for (int i=0; i<6; i++) {
         reader.readC();
       }
     }
@@ -3719,7 +3728,8 @@ SafeWriter* DivEngine::saveFur(bool notPrimary) {
   w->writeC(song.brokenOutVol);
   w->writeC(song.e1e2StopOnSameNote);
   w->writeC(song.brokenPortaArp);
-  for (int i=0; i<7; i++) {
+  w->writeC(song.snNoLowPeriods);
+  for (int i=0; i<6; i++) {
     w->writeC(0);
   }
 

src/engine/platform/sms.cpp

@@ -188,7 +188,11 @@ void DivPlatformSMS::tick(bool sysTick) {
     if (chan[i].freqChanged) {
       chan[i].freq=parent->calcFreq(chan[i].baseFreq,chan[i].pitch,true,0,chan[i].pitch2,chipClock,toneDivider);
       if (chan[i].freq>1023) chan[i].freq=1023;
-      if (chan[i].freq<8) chan[i].freq=1;
+      if (parent->song.snNoLowPeriods) {
+        if (chan[i].freq<8) chan[i].freq=1;
+      } else {
+        if (chan[i].freq<0) chan[i].freq=0;
+      }
       //if (chan[i].actualNote>0x5d) chan[i].freq=0x01;
       rWrite(0,0x80|i<<5|(chan[i].freq&15));
       rWrite(0,chan[i].freq>>4);
@@ -203,7 +207,9 @@ void DivPlatformSMS::tick(bool sysTick) {
   if (chan[3].freqChanged || updateSNMode) {
     chan[3].freq=parent->calcFreq(chan[3].baseFreq,chan[3].pitch,true,0,chan[3].pitch2,chipClock,noiseDivider);
     if (chan[3].freq>1023) chan[3].freq=1023;
-    if (chan[3].actualNote>0x5d) chan[3].freq=0x01;
+    if (parent->song.snNoLowPeriods) {
+      if (chan[3].actualNote>0x5d) chan[3].freq=0x01;
+    }
     if (snNoiseMode&2) { // take period from channel 3
       if (updateSNMode || resetPhase) {
         if (snNoiseMode&1) {

src/engine/song.h

@@ -500,6 +500,7 @@ struct DivSong {
   bool brokenOutVol;
   bool e1e2StopOnSameNote;
   bool brokenPortaArp;
+  bool snNoLowPeriods;
 
   std::vector<DivInstrument*> ins;
   std::vector<DivWavetable*> wave;
@@ -600,7 +601,8 @@ struct DivSong {
     volMacroLinger(true),
     brokenOutVol(false),
     e1e2StopOnSameNote(false),
-    brokenPortaArp(false) {
+    brokenPortaArp(false),
+    snNoLowPeriods(false) {
     for (int i=0; i<32; i++) {
       system[i]=DIV_SYSTEM_NULL;
       systemVol[i]=64;

src/gui/compatFlags.cpp

@@ -139,6 +139,10 @@ void FurnaceGUI::drawCompatFlags() {
     if (ImGui::IsItemHovered()) {
       ImGui::SetTooltip("if enabled, no checks for the presence of a volume macro will be made.\nthis will cause the last macro value to linger unless a value in the volume column is present.");
     }
+    ImGui::Checkbox("Treat SN76489 periods under 8 as 1",&e->song.snNoLowPeriods);
+    if (ImGui::IsItemHovered()) {
+      ImGui::SetTooltip("when enabled, any SN period under 8 will be written as 1 instead.\nthis replicates DefleMask behavior, but reduces available period range.");
+    }
 
     ImGui::Text("Pitch linearity:");
     if (ImGui::RadioButton("None",e->song.linearPitch==0)) {