Add core option Swap Duty Cycles

src/drivers/libretro/libretro.c

@@ -84,6 +84,7 @@ static int soundo = 1;
 unsigned sndsamplerate = 48000;
 unsigned sndquality = 0;
 unsigned sndvolume = 150;
+unsigned swapDuty = 0;
 
 static volatile int nofocus = 0;
 
@@ -600,6 +601,7 @@ void retro_set_environment(retro_environment_t cb)
       { "fceumm_region", "Region Override; Auto|NTSC|PAL|Dendy" },
       { "fceumm_sndquality", "Sound Quality; Low|High|Very High" },
       { "fceumm_sndvolume", "Sound Volume; 150|160|170|180|190|200|210|220|230|240|250|0|10|20|30|40|50|60|70|80|90|100|110|120|130|140" },
+      { "fceumm_swapduty", "Swap Duty Cyles; disabled|enabled" },
       { "fceumm_zapper_mode", "Zapper Mode; absolute|relative" },
       { "fceumm_show_crosshair", "Show Crosshair; enabled|disabled" },
       { NULL, NULL },
@@ -1071,6 +1073,17 @@ static void check_variables(bool startup)
       retro_get_system_av_info(&av_info);
       environ_cb(RETRO_ENVIRONMENT_SET_GEOMETRY, &av_info);
    }
+
+   var.key = "fceumm_swapduty";
+
+   if (environ_cb(RETRO_ENVIRONMENT_GET_VARIABLE, &var) && var.value)
+   {
+      bool newval = (!strcmp(var.value, "enabled"));
+      if (newval != swapDuty)
+      {
+         swapDuty = newval;
+      }
+   }
 }
 
 static int mzx = 0, mzy = 0, mzb = 0;

src/fceu.h

@@ -5,16 +5,23 @@
 
 extern int fceuindbg;
 
+/* Overclocking-related */
 extern unsigned overclock_state;
 extern unsigned overclocked;
 extern unsigned skip_7bit_overclocking;
 extern unsigned DMC_7bit;
-
 extern unsigned normal_scanlines;
 extern unsigned extrascanlines;
 extern unsigned vblankscanlines;
+
+/* Region selection */
 extern unsigned dendy;
 
+/* Audio mods*/
+extern unsigned swapDuty; /* Swap bits 6 & 7 of $4000/$4004 to mimic bug
+                           * found on some famiclones/Dendy models.
+                           */
+
 void ResetGameLoaded(void);
 
 #define DECLFR(x) uint8 FP_FASTAPASS(1) x(uint32 A)

src/sound.c

@@ -195,9 +195,12 @@ static DECLFW(Write_PSG) {
 // FCEU_printf("APU1 %04x:%04x\n",A,V);
 	A &= 0x1F;
 	switch (A) {
-	case 0x0: DoSQ1();
+	case 0x0:
+		DoSQ1();
 		EnvUnits[0].Mode = (V & 0x30) >> 4;
 		EnvUnits[0].Speed = (V & 0xF);
+		if (swapDuty)
+			V = (V & 0x3F) | ((V & 0x80) >> 1) | ((V & 0x40) << 1);
 		break;
 	case 0x1:
 		sweepon[0] = V & 0x80;
@@ -214,30 +217,36 @@ static DECLFW(Write_PSG) {
 		DoSQ2();
 		EnvUnits[1].Mode = (V & 0x30) >> 4;
 		EnvUnits[1].Speed = (V & 0xF);
+		if (swapDuty)
+			V = (V & 0x3F) | ((V & 0x80) >> 1) | ((V & 0x40) << 1);
 		break;
 	case 0x5:
 		sweepon[1] = V & 0x80;
 		break;
-	case 0x6: DoSQ2();
+	case 0x6:
+		DoSQ2();
 		curfreq[1] &= 0xFF00;
 		curfreq[1] |= V;
 		break;
 	case 0x7:
 		SQReload(1, V);
 		break;
-	case 0xa: DoTriangle();
+	case 0xa:
+		DoTriangle();
 		break;
 	case 0xb:
 		DoTriangle();
 		if (EnabledChannels & 0x4)
 			lengthcount[2] = lengthtable[(V >> 3) & 0x1f];
-		TriMode = 1;	// Load mode
+		TriMode = 1;	/* Load mode */
 		break;
-	case 0xC: DoNoise();
+	case 0xC:
+		DoNoise();
 		EnvUnits[2].Mode = (V & 0x30) >> 4;
 		EnvUnits[2].Speed = (V & 0xF);
 		break;
-	case 0xE: DoNoise();
+	case 0xE:
+		DoNoise();
 		break;
 	case 0xF:
 		DoNoise();
@@ -245,7 +254,8 @@ static DECLFW(Write_PSG) {
 			lengthcount[3] = lengthtable[(V >> 3) & 0x1f];
 		EnvUnits[2].reloaddec = 1;
 		break;
-	case 0x10: DoPCM();
+	case 0x10:
+		DoPCM();
 		LoadDMCPeriod(V & 0xF);
 
 		if (SIRQStat & 0x80) {