diff --git a/core/io_ctrl.c b/core/io_ctrl.c
index 3800f17..c4803ab 100644
--- a/core/io_ctrl.c
+++ b/core/io_ctrl.c
@@ -412,12 +412,10 @@ unsigned int io_68k_read(unsigned int offset)
 
 void io_z80_write(unsigned int offset, unsigned int data, unsigned int cycles)
 {
+  /* I/O Control register */
   if (offset)
   {
-    /* I/O Control register */
-    if (region_code & REGION_USA)
-    {
-      /* 
+    /* 
         Bit  Function
         --------------
         D7 : Port B TH pin output level (1=high, 0=low)
@@ -428,36 +426,33 @@ void io_z80_write(unsigned int offset, unsigned int data, unsigned int cycles)
         D2 : Port B TR pin direction (1=input, 0=output)
         D1 : Port A TH pin direction (1=input, 0=output)
         D0 : Port A TR pin direction (1=input, 0=output)
-      */
+    */
 
-      /* Send TR/TH state to connected peripherals */
-      port[0].data_w((data << 1) & 0x60, (~io_reg[0x0F] << 5) & 0x60);
-      port[1].data_w((data >> 1) & 0x60, (~io_reg[0x0F] << 3) & 0x60);
+    /* Send TR/TH state to connected peripherals */
+    port[0].data_w((data << 1) & 0x60, (~data << 5) & 0x60);
+    port[1].data_w((data >> 1) & 0x60, (~data << 3) & 0x60);
 
-
-      /* Check for TH low-to-high transitions on both ports */
-      if ((!(io_reg[0x0F] & 0x80) && (data & 0x80)) ||
-          (!(io_reg[0x0F] & 0x20) && (data & 0x20)))
-      {
-        /* Latch new HVC */
-        hvc_latch = hctab[cycles % MCYCLES_PER_LINE] | 0x10000;
-     }
-
-      /* Update I/O Control register */
-      io_reg[0x0F] = data;
-    }
-    else
+    /* Japanese model specific */
+    if (region_code == REGION_JAPAN_NTSC)
     {
-      /* TH output is fixed to 0 & TR is always an input on japanese hardware */
-      io_reg[0x0F] = (data | 0x05) & 0x5F;
-
-      /* Port $DD bits D4-D5 return D0-D2 (cf. http://www2.odn.ne.jp/~haf09260/Sms/EnrSms.htm) */
-      io_reg[0x0D] = ((data & 0x01) << 4) | ((data & 0x04) << 3);
+      /* Reading TH & TR pins always return 0 when set as output */
+      data &= 0x0F;
     }
+    
+    /* Check for TH low-to-high transitions on both ports */
+    if ((!(io_reg[0x0F] & 0x80) && (data & 0x80)) ||
+        (!(io_reg[0x0F] & 0x20) && (data & 0x20)))
+    {
+      /* Latch new HVC */
+      hvc_latch = hctab[cycles % MCYCLES_PER_LINE] | 0x10000;
+    }
+
+    /* Update I/O Control register */
+    io_reg[0x0F] = data;
   }
   else
   {
-    /* Update Memory Control register */
+    /* Memory Control register */
     io_reg[0x0E] = data;
 
     /* Switch cartridge & BIOS ROM */
diff --git a/core/memz80.c b/core/memz80.c
index 0165114..c679910 100644
--- a/core/memz80.c
+++ b/core/memz80.c
@@ -251,9 +251,10 @@ void z80_md_port_w(unsigned int port, unsigned char data)
     {
       port &= 0xFF;
 
+      /* write FM chip if enabled */
       if ((port >= 0xF0) && (config.ym2413 & 1))
       {
-        fm_write(Z80.cycles, port&3, data);
+        fm_write(Z80.cycles, port, data);
         return;
       }
 
@@ -299,7 +300,7 @@ unsigned char z80_md_port_r(unsigned int port)
       /* read FM chip if enabled */
       if ((port >= 0xF0) && (config.ym2413 & 1))
       {
-        return YM2413Read(port & 3); 
+        return YM2413Read(); 
       }
 
       return z80_unused_port_r(port);
@@ -329,7 +330,7 @@ void z80_gg_port_w(unsigned int port, unsigned char data)
           return;
         }
 
-        z80_unused_port_w(port & 0xFF, data);
+        z80_unused_port_w(port, data);
         return;
       }
 
@@ -431,13 +432,13 @@ void z80_ms_port_w(unsigned int port, unsigned char data)
     case 0x01:
     {
       /* full address range is decoded by 315-5297 I/O chip (fixes Super Tetris / Power Boggle Boggle) */
-      if ((region_code == REGION_JAPAN_NTSC) && ((port & 0xFE) != 0x3E))
+      if ((region_code != REGION_JAPAN_NTSC) || ((port & 0xFE) == 0x3E))
       {
-        z80_unused_port_w(port & 0xFF, data);
+        io_z80_write(port & 1, data, Z80.cycles + SMS_CYCLE_OFFSET);
         return;
       }
 
-      io_z80_write(port & 1, data, Z80.cycles + SMS_CYCLE_OFFSET);
+      z80_unused_port_w(port & 0xFF, data);
       return;
     }
 
@@ -462,9 +463,17 @@ void z80_ms_port_w(unsigned int port, unsigned char data)
 
     default:
     {
+      /* write FM chip if enabled */
       if (!(port & 4) && (config.ym2413 & 1))
       {
-        fm_write(Z80.cycles, port & 3, data);
+        fm_write(Z80.cycles, port, data);
+
+        /* 315-5297 I/O chip decodes bit 1 to enable/disable PSG output */
+        if (region_code == REGION_JAPAN_NTSC)
+        {
+          io_reg[6] = (data & 2) ? 0xFF : 0x00;
+          SN76489_Config(Z80.cycles, config.psg_preamp, config.psgBoostNoise, io_reg[6]);
+        }
         return;
       }
 
@@ -506,27 +515,27 @@ unsigned char z80_ms_port_r(unsigned int port)
 
     default:
     {
+      uint8 data = 0xFF;
+
       /* read FM chip if enabled */
       if (!(port & 4) && (config.ym2413 & 1))
       {
-        /* check if I/O ports are disabled */
-        if (io_reg[0x0E] & 0x04)
+        data = YM2413Read();
+
+        /* 315-5297 I/O chip decodes full address range */
+        if (region_code == REGION_JAPAN_NTSC)
         {
-          return YM2413Read(port & 3);
-        }
-        else
-        {
-          return YM2413Read(port & 3) & io_z80_read(port & 1);
+          return data;
         }
       }
 
-      /* check if I/O ports are enabled */
+      /* read I/O ports if enabled */
       if (!(io_reg[0x0E] & 0x04))
       {
-        return io_z80_read(port & 1);
+        data &= io_z80_read(port & 1);
       }
 
-      return z80_unused_port_r(port & 0xFF);
+      return data;
     }
   }
 }
@@ -543,7 +552,7 @@ void z80_m3_port_w(unsigned int port, unsigned char data)
     case 0x00:
     case 0x01:
     {
-      z80_unused_port_w(port, data);
+      z80_unused_port_w(port & 0xFF, data);
       return;
     }
 
@@ -568,9 +577,10 @@ void z80_m3_port_w(unsigned int port, unsigned char data)
 
     default:
     {
+      /* write FM chip if enabled */
       if (!(port & 4) && (config.ym2413 & 1))
       {
-        fm_write(Z80.cycles, port & 3, data);
+        fm_write(Z80.cycles, port, data);
         return;
       }
 
@@ -615,8 +625,8 @@ unsigned char z80_m3_port_r(unsigned int port)
       /* read FM chip if enabled */
       if (!(port & 4) && (config.ym2413 & 1))
       {
-        /* I/O ports are automatically disabled */
-        return YM2413Read(port & 3);
+        /* I/O ports are automatically disabled by hardware */
+        return YM2413Read();
       }
 
       /* read I/O ports   */
@@ -683,7 +693,7 @@ unsigned char z80_sg_port_r(unsigned int port)
 
     default:
     {
-      return z80_unused_port_r(port);
+      return z80_unused_port_r(port & 0xFF);
     }
   }
 }
diff --git a/core/sound/sound.c b/core/sound/sound.c
index 093b638..06bea1a 100644
--- a/core/sound/sound.c
+++ b/core/sound/sound.c
@@ -111,6 +111,7 @@ void sound_reset(void)
   /* reset sound chips */
   YM_Reset();
   SN76489_Reset();
+  SN76489_Config(0, config.psg_preamp, config.psgBoostNoise, 0xff);
 
   /* reset FM buffer ouput */
   fm_last[0] = fm_last[1] = 0;
diff --git a/core/sound/ym2413.c b/core/sound/ym2413.c
index b71fbdf..ad453fa 100644
--- a/core/sound/ym2413.c
+++ b/core/sound/ym2413.c
@@ -1658,14 +1658,14 @@ void YM2413Write(unsigned int a, unsigned int v)
   }
   else
   {
-    /* latched bit (Master System specific) */
+    /* bit 0 enable/disable FM output (Master System / Mark-III FM adapter specific) */
     ym2413.status = v & 0x01;
   }
 }
 
-unsigned int YM2413Read(unsigned int a)
+unsigned int YM2413Read(void)
 {
-  /* D0=latched bit, D1-D2 need to be zero (Master System specific) */
+  /* bit 0 returns latched FM enable status, bits 1-2 return zero (Master System / Mark-III FM adapter specific) */
   return 0xF8 | ym2413.status;
 }
 
diff --git a/core/sound/ym2413.h b/core/sound/ym2413.h
index 73c8c84..ceb48bf 100644
--- a/core/sound/ym2413.h
+++ b/core/sound/ym2413.h
@@ -16,7 +16,7 @@ extern void YM2413Init(void);
 extern void YM2413ResetChip(void);
 extern void YM2413Update(int *buffer, int length);
 extern void YM2413Write(unsigned int a, unsigned int v);
-extern unsigned int YM2413Read(unsigned int a);
+extern unsigned int YM2413Read(void);
 extern unsigned char *YM2413GetContextPtr(void);
 extern unsigned int YM2413GetContextSize(void);