diff --git a/src/engine/instrument.cpp b/src/engine/instrument.cpp
index b3e386b79..af6721711 100644
--- a/src/engine/instrument.cpp
+++ b/src/engine/instrument.cpp
@@ -97,13 +97,30 @@ void DivInstrument::writeFeatureX1(SafeWriter* w) {
 
 }
 
-#define _C(x) x==a.x
+#define _C(x) x==other.x
 
-bool DivInstrumentFM::operator==(const DivInstrumentFM& a) {
-  return true;
+bool DivInstrumentFM::operator==(const DivInstrumentFM& other) {
+  return (
+    _C(alg) &&
+    _C(fb) &&
+    _C(fms) &&
+    _C(ams) &&
+    _C(fms2) &&
+    _C(ams2) &&
+    _C(ops) &&
+    _C(opllPreset) &&
+    _C(fixedDrums) &&
+    _C(kickFreq) &&
+    _C(snareHatFreq) &&
+    _C(tomTopFreq) &&
+    _C(op[0]) &&
+    _C(op[1]) &&
+    _C(op[2]) &&
+    _C(op[3])
+  );
 }
 
-bool DivInstrumentFM::Operator::operator==(const DivInstrumentFM::Operator& a) {
+bool DivInstrumentFM::Operator::operator==(const DivInstrumentFM::Operator& other) {
   return (
     _C(enable) &&
     _C(am) &&
@@ -130,48 +147,139 @@ bool DivInstrumentFM::Operator::operator==(const DivInstrumentFM::Operator& a) {
   );
 }
 
-bool DivInstrumentGB::operator==(const DivInstrumentGB& a) {
-  return true;
+bool DivInstrumentGB::operator==(const DivInstrumentGB& other) {
+  return (
+    _C(envVol) &&
+    _C(envDir) &&
+    _C(envLen) &&
+    _C(soundLen) &&
+    _C(hwSeqLen) &&
+    _C(softEnv) &&
+    _C(alwaysInit)
+  );
 }
 
-bool DivInstrumentC64::operator==(const DivInstrumentC64& a) {
-  return true;
+bool DivInstrumentC64::operator==(const DivInstrumentC64& other) {
+  return (
+    _C(triOn) &&
+    _C(sawOn) &&
+    _C(pulseOn) &&
+    _C(noiseOn) &&
+    _C(a) &&
+    _C(d) &&
+    _C(s) &&
+    _C(r) &&
+    _C(duty) &&
+    _C(ringMod) &&
+    _C(oscSync) &&
+    _C(toFilter) &&
+    _C(volIsCutoff) &&
+    _C(initFilter) &&
+    _C(dutyIsAbs) &&
+    _C(filterIsAbs) &&
+    _C(noTest) &&
+    _C(res) &&
+    _C(cut) &&
+    _C(hp) &&
+    _C(lp) &&
+    _C(bp) &&
+    _C(ch3off)
+  );
 }
 
-bool DivInstrumentAmiga::operator==(const DivInstrumentAmiga& a) {
-  return true;
+bool DivInstrumentAmiga::operator==(const DivInstrumentAmiga& other) {
+  return (
+    _C(initSample) &&
+    _C(useNoteMap) &&
+    _C(useSample) &&
+    _C(useWave) &&
+    _C(waveLen)
+  );
 }
 
-bool DivInstrumentX1_010::operator==(const DivInstrumentX1_010& a) {
-  return true;
+bool DivInstrumentX1_010::operator==(const DivInstrumentX1_010& other) {
+  return _C(bankSlot);
 }
 
-bool DivInstrumentN163::operator==(const DivInstrumentN163& a) {
-  return true;
+bool DivInstrumentN163::operator==(const DivInstrumentN163& other) {
+  return (
+    _C(wave) &&
+    _C(wavePos) &&
+    _C(waveLen) &&
+    _C(waveMode)
+  );
 }
 
-bool DivInstrumentFDS::operator==(const DivInstrumentFDS& a) {
-  return true;
+bool DivInstrumentFDS::operator==(const DivInstrumentFDS& other) {
+  return (
+    (memcmp(modTable,other.modTable,32)==0) &&
+    _C(modSpeed) &&
+    _C(modDepth) &&
+    _C(initModTableWithFirstWave)
+  );
 }
 
-bool DivInstrumentMultiPCM::operator==(const DivInstrumentMultiPCM& a) {
-  return true;
+bool DivInstrumentMultiPCM::operator==(const DivInstrumentMultiPCM& other) {
+  return (
+    _C(ar) &&
+    _C(d1r) &&
+    _C(dl) &&
+    _C(d2r) &&
+    _C(rr) &&
+    _C(rc) &&
+    _C(lfo) &&
+    _C(vib) &&
+    _C(am)
+  );
 }
 
-bool DivInstrumentWaveSynth::operator==(const DivInstrumentWaveSynth& a) {
-  return true;
+bool DivInstrumentWaveSynth::operator==(const DivInstrumentWaveSynth& other) {
+  return (
+    _C(wave1) &&
+    _C(wave2) &&
+    _C(rateDivider) &&
+    _C(effect) &&
+    _C(oneShot) &&
+    _C(enabled) &&
+    _C(global) &&
+    _C(speed) &&
+    _C(param1) &&
+    _C(param2) &&
+    _C(param3) &&
+    _C(param4)
+  );
 }
 
-bool DivInstrumentSoundUnit::operator==(const DivInstrumentSoundUnit& a) {
-  return true;
+bool DivInstrumentSoundUnit::operator==(const DivInstrumentSoundUnit& other) {
+  return _C(switchRoles);
 }
 
-bool DivInstrumentES5506::operator==(const DivInstrumentES5506& a) {
-  return true;
+bool DivInstrumentES5506::operator==(const DivInstrumentES5506& other) {
+  return (
+    _C(filter.mode) &&
+    _C(filter.k1) &&
+    _C(filter.k2) &&
+    _C(envelope.ecount) &&
+    _C(envelope.lVRamp) &&
+    _C(envelope.rVRamp) &&
+    _C(envelope.k1Ramp) &&
+    _C(envelope.k2Ramp) &&
+    _C(envelope.k1Slow) &&
+    _C(envelope.k2Slow)
+  );
 }
 
-bool DivInstrumentSNES::operator==(const DivInstrumentSNES& a) {
-  return true;
+bool DivInstrumentSNES::operator==(const DivInstrumentSNES& other) {
+  return (
+    _C(useEnv) &&
+    _C(sus) &&
+    _C(gainMode) &&
+    _C(gain) &&
+    _C(a) &&
+    _C(d) &&
+    _C(s) &&
+    _C(r)
+  );
 }
 
 void DivInstrument::putInsData2(SafeWriter* w, bool fui, const DivSong* song) {
diff --git a/src/engine/instrument.h b/src/engine/instrument.h
index db9221fcc..feab69c10 100644
--- a/src/engine/instrument.h
+++ b/src/engine/instrument.h
@@ -98,9 +98,9 @@ struct DivInstrumentFM {
   bool fixedDrums;
   unsigned short kickFreq, snareHatFreq, tomTopFreq;
 
-  bool operator==(const DivInstrumentFM& a);
-  bool operator!=(const DivInstrumentFM& a) {
-    return !(*this==a);
+  bool operator==(const DivInstrumentFM& other);
+  bool operator!=(const DivInstrumentFM& other) {
+    return !(*this==other);
   }
   struct Operator {
     bool enable;
@@ -108,9 +108,9 @@ struct DivInstrumentFM {
     unsigned char dam, dvb, egt, ksl, sus, vib, ws, ksr; // YMU759/OPL/OPZ
     unsigned char kvs;
 
-    bool operator==(const Operator& a);
-    bool operator!=(const Operator& a) {
-      return !(*this==a);
+    bool operator==(const Operator& other);
+    bool operator!=(const Operator& other) {
+      return !(*this==other);
     }
     Operator():
       enable(true),
@@ -307,9 +307,9 @@ struct DivInstrumentGB {
     unsigned short data;
   } hwSeq[256];
 
-  bool operator==(const DivInstrumentGB& a);
-  bool operator!=(const DivInstrumentGB& a) {
-    return !(*this==a);
+  bool operator==(const DivInstrumentGB& other);
+  bool operator!=(const DivInstrumentGB& other) {
+    return !(*this==other);
   }
 
   DivInstrumentGB():
@@ -334,9 +334,9 @@ struct DivInstrumentC64 {
   unsigned short cut;
   bool hp, lp, bp, ch3off;
 
-  bool operator==(const DivInstrumentC64& a);
-  bool operator!=(const DivInstrumentC64& a) {
-    return !(*this==a);
+  bool operator==(const DivInstrumentC64& other);
+  bool operator!=(const DivInstrumentC64& other) {
+    return !(*this==other);
   }
 
   DivInstrumentC64():
@@ -380,9 +380,9 @@ struct DivInstrumentAmiga {
   unsigned char waveLen;
   SampleMap noteMap[120];
 
-  bool operator==(const DivInstrumentAmiga& a);
-  bool operator!=(const DivInstrumentAmiga& a) {
-    return !(*this==a);
+  bool operator==(const DivInstrumentAmiga& other);
+  bool operator!=(const DivInstrumentAmiga& other) {
+    return !(*this==other);
   }
 
   /**
@@ -426,9 +426,9 @@ struct DivInstrumentAmiga {
 struct DivInstrumentX1_010 {
   int bankSlot;
 
-  bool operator==(const DivInstrumentX1_010& a);
-  bool operator!=(const DivInstrumentX1_010& a) {
-    return !(*this==a);
+  bool operator==(const DivInstrumentX1_010& other);
+  bool operator!=(const DivInstrumentX1_010& other) {
+    return !(*this==other);
   }
 
   DivInstrumentX1_010():
@@ -439,9 +439,9 @@ struct DivInstrumentN163 {
   int wave, wavePos, waveLen;
   unsigned char waveMode;
 
-  bool operator==(const DivInstrumentN163& a);
-  bool operator!=(const DivInstrumentN163& a) {
-    return !(*this==a);
+  bool operator==(const DivInstrumentN163& other);
+  bool operator!=(const DivInstrumentN163& other) {
+    return !(*this==other);
   }
 
   DivInstrumentN163():
@@ -457,9 +457,9 @@ struct DivInstrumentFDS {
   // this is here for compatibility.
   bool initModTableWithFirstWave;
 
-  bool operator==(const DivInstrumentFDS& a);
-  bool operator!=(const DivInstrumentFDS& a) {
-    return !(*this==a);
+  bool operator==(const DivInstrumentFDS& other);
+  bool operator!=(const DivInstrumentFDS& other) {
+    return !(*this==other);
   }
 
   DivInstrumentFDS():
@@ -474,9 +474,9 @@ struct DivInstrumentMultiPCM {
   unsigned char ar, d1r, dl, d2r, rr, rc;
   unsigned char lfo, vib, am;
 
-  bool operator==(const DivInstrumentMultiPCM& a);
-  bool operator!=(const DivInstrumentMultiPCM& a) {
-    return !(*this==a);
+  bool operator==(const DivInstrumentMultiPCM& other);
+  bool operator!=(const DivInstrumentMultiPCM& other) {
+    return !(*this==other);
   }
 
   DivInstrumentMultiPCM():
@@ -518,9 +518,9 @@ struct DivInstrumentWaveSynth {
   bool oneShot, enabled, global;
   unsigned char speed, param1, param2, param3, param4;
 
-  bool operator==(const DivInstrumentWaveSynth& a);
-  bool operator!=(const DivInstrumentWaveSynth& a) {
-    return !(*this==a);
+  bool operator==(const DivInstrumentWaveSynth& other);
+  bool operator!=(const DivInstrumentWaveSynth& other) {
+    return !(*this==other);
   }
 
   DivInstrumentWaveSynth():
@@ -541,9 +541,9 @@ struct DivInstrumentWaveSynth {
 struct DivInstrumentSoundUnit {
   bool switchRoles;
 
-  bool operator==(const DivInstrumentSoundUnit& a);
-  bool operator!=(const DivInstrumentSoundUnit& a) {
-    return !(*this==a);
+  bool operator==(const DivInstrumentSoundUnit& other);
+  bool operator!=(const DivInstrumentSoundUnit& other) {
+    return !(*this==other);
   }
 
   DivInstrumentSoundUnit():
@@ -582,9 +582,9 @@ struct DivInstrumentES5506 {
   Filter filter;
   Envelope envelope;
 
-  bool operator==(const DivInstrumentES5506& a);
-  bool operator!=(const DivInstrumentES5506& a) {
-    return !(*this==a);
+  bool operator==(const DivInstrumentES5506& other);
+  bool operator!=(const DivInstrumentES5506& other) {
+    return !(*this==other);
   }
 
   DivInstrumentES5506():
@@ -605,9 +605,9 @@ struct DivInstrumentSNES {
   unsigned char gain;
   unsigned char a, d, s, r;
 
-  bool operator==(const DivInstrumentSNES& a);
-  bool operator!=(const DivInstrumentSNES& a) {
-    return !(*this==a);
+  bool operator==(const DivInstrumentSNES& other);
+  bool operator!=(const DivInstrumentSNES& other) {
+    return !(*this==other);
   }
 
   DivInstrumentSNES():