diff --git a/src/mame/includes/snes.h b/src/mame/includes/snes.h
index c886f4f130b..e52d2cff994 100644
--- a/src/mame/includes/snes.h
+++ b/src/mame/includes/snes.h
@@ -773,6 +773,9 @@ extern DECLARE_READ8_HANDLER(obc1_read);
 extern DECLARE_WRITE8_HANDLER(obc1_write);
 UINT8 CX4_read(UINT32 addr);
 void CX4_write(running_machine &machine, UINT32 addr, UINT8 data);
+UINT8 sdd1_mmio_read(address_space &space, UINT32 addr);
+void sdd1_mmio_write(address_space &space, UINT32 addr, UINT8 data);
+UINT8 sdd1_read(running_machine& machine, UINT32 addr);
 
 
 extern struct snes_cart_info snes_cart;
diff --git a/src/mame/machine/snes.c b/src/mame/machine/snes.c
index 79dfebdd3a8..a0c5e40c4b9 100644
--- a/src/mame/machine/snes.c
+++ b/src/mame/machine/snes.c
@@ -410,17 +410,6 @@ READ8_HANDLER( snes_r_io )
 			return superfx_mmio_read(state->m_superfx, offset);
 		}
 	}
-	else if (state->m_has_addon_chip == HAS_SDD1)
-	{
-		if (offset >= 0x4800 && offset < 0x4808)
-		{
-			return sdd1_mmio_read(space, (UINT32)offset);
-		}
-		if (offset < 0x80)
-		{
-			offset += 0x4300;
-		}
-	}
 	else if (state->m_has_addon_chip == HAS_SPC7110 || state->m_has_addon_chip == HAS_SPC7110_RTC)
 	{
 		UINT16 limit = (state->m_has_addon_chip == HAS_SPC7110_RTC) ? 0x4842 : 0x483f;
@@ -540,19 +529,6 @@ WRITE8_HANDLER( snes_w_io )
 			return;
 		}
 	}
-	else if (state->m_has_addon_chip == HAS_SDD1)
-	{
-		if ((offset >= 0x4300 && offset < 0x4380) ||
-			(offset >= 0x4800 && offset < 0x4808))
-		{
-			sdd1_mmio_write(space, (UINT32)offset, data);
-			return;
-		}
-		if (offset < 0x80)
-		{
-			offset += 0x4300;
-		}
-	}
 	else if (state->m_has_addon_chip == HAS_SPC7110 || state->m_has_addon_chip == HAS_SPC7110_RTC)
 	{
 		UINT16 limit = (state->m_has_addon_chip == HAS_SPC7110_RTC) ? 0x4842 : 0x483f;
@@ -1021,8 +997,6 @@ READ8_HANDLER( snes_r_bank7 )
 	}
 	else if ((state->m_has_addon_chip == HAS_SPC7110 || state->m_has_addon_chip == HAS_SPC7110_RTC) && offset >= 0x100000)
 		value = spc7110_bank7_read(space, offset);
-	else if (state->m_has_addon_chip == HAS_SDD1)
-		value = sdd1_read(space.machine(), offset);
 	else if ((state->m_cart[0].mode & 5) && !(state->m_has_addon_chip == HAS_SUPERFX))      /* Mode 20 & 22 */
 	{
 		if (address < 0x8000)
diff --git a/src/mame/machine/snessdd1.c b/src/mame/machine/snessdd1.c
index 91608a857b8..c326dd296b4 100644
--- a/src/mame/machine/snessdd1.c
+++ b/src/mame/machine/snessdd1.c
@@ -554,15 +554,10 @@ static void sdd1_init(running_machine& machine)
 	snes_sdd1.buffer.ready = 0;
 }
 
-static UINT8 sdd1_mmio_read(address_space &space, UINT32 addr)
+UINT8 sdd1_mmio_read(address_space &space, UINT32 addr)
 {
 	addr &= 0xffff;
 
-	if ((addr & 0x4380) == 0x4300)
-	{
-		return snes_r_io(space, addr & 0x7f);
-	}
-
 	switch(addr)
 	{
 		case 0x4804:
@@ -578,7 +573,7 @@ static UINT8 sdd1_mmio_read(address_space &space, UINT32 addr)
 	return snes_open_bus_r(space, 0);
 }
 
-static void sdd1_mmio_write(address_space &space, UINT32 addr, UINT8 data)
+void sdd1_mmio_write(address_space &space, UINT32 addr, UINT8 data)
 {
 	addr &= 0xffff;
 
@@ -604,8 +599,6 @@ static void sdd1_mmio_write(address_space &space, UINT32 addr, UINT8 data)
 				snes_sdd1.dma[channel].size = (snes_sdd1.dma[channel].size &   0x00ff) + (data <<  8);
 				break;
 		}
-		snes_ram[addr] = data;
-		snes_io_dma_w(space, addr, data);
 		return;
 	}
 
@@ -633,7 +626,7 @@ static void sdd1_mmio_write(address_space &space, UINT32 addr, UINT8 data)
 	}
 }
 
-static UINT8 sdd1_read(running_machine& machine, UINT32 addr)
+UINT8 sdd1_read(running_machine& machine, UINT32 addr)
 {
 	unsigned char *ROM = machine.root_device().memregion("cart")->base();
 
diff --git a/src/mess/drivers/snes.c b/src/mess/drivers/snes.c
index f4462be32f6..33bdbb68426 100644
--- a/src/mess/drivers/snes.c
+++ b/src/mess/drivers/snes.c
@@ -141,6 +141,9 @@ static READ8_HANDLER( snes_lo_r )
 	if (state->m_has_addon_chip == HAS_DSP4 
 		&& (offset >= 0x300000 && offset < 0x400000 && (offset & 0x8000) == 0x8000))
 		return ((offset & 0xffff) < 0xc000) ? dsp_get_dr() : dsp_get_sr();
+	if (state->m_has_addon_chip == HAS_SDD1
+		&& (offset < 0x400000 && (offset & 0xffff) >= 0x4800 && (offset & 0xffff) < 0x4808))
+		return sdd1_mmio_read(space, (UINT32)(offset & 0xffff));
 	
 	// base cart access
 	if (offset < 0x300000)
@@ -191,6 +194,11 @@ static READ8_HANDLER( snes_hi_r )
 	if (state->m_has_addon_chip == HAS_DSP4 
 		&& (offset >= 0x300000 && offset < 0x400000 && (offset & 0x8000) == 0x8000))
 		return ((offset & 0xffff) < 0xc000) ? dsp_get_dr() : dsp_get_sr();
+	if (state->m_has_addon_chip == HAS_SDD1
+		&& (offset < 0x400000 && (offset & 0xffff) >= 0x4800 && (offset & 0xffff) < 0x4808))
+		return sdd1_mmio_read(space, (UINT32)(offset & 0xffff));
+	if (state->m_has_addon_chip == HAS_SDD1 && offset >= 0x400000)
+		return sdd1_read(space.machine(), offset - 0x400000);;
 	
 	// base cart access
 	if (offset < 0x400000)
@@ -259,6 +267,15 @@ static WRITE8_HANDLER( snes_lo_w )
 		else
 		{	dsp_set_sr(data);	return;	}
 	}
+	if (state->m_has_addon_chip == HAS_SDD1 && offset < 0x400000)
+	{
+		if (((offset & 0xffff) >= 0x4300 && (offset & 0xffff) < 0x4380) ||
+			((offset & 0xffff) >= 0x4800 && (offset & 0xffff) < 0x4808))
+		{
+				sdd1_mmio_write(space, (UINT32)(offset & 0xffff), data);
+				// here we don't return, but we let the w_io happen...
+		}
+	}
 	
 	// base cart access
 	if (offset < 0x300000)
@@ -333,6 +350,15 @@ static WRITE8_HANDLER( snes_hi_w )
 		else
 		{	dsp_set_sr(data);	return;	}
 	}
+	if (state->m_has_addon_chip == HAS_SDD1 && offset < 0x400000)
+	{
+		if (((offset & 0xffff) >= 0x4300 && (offset & 0xffff) < 0x4380) ||
+			((offset & 0xffff) >= 0x4800 && (offset & 0xffff) < 0x4808))
+		{
+			sdd1_mmio_write(space, (UINT32)(offset & 0xffff), data);
+			// here we don't return, but we let the w_io happen...
+		}
+	}
 	
 	// base cart access
 	if (offset < 0x400000)