add a 'post opcode fetch' to spectrum expansion bus so that interface 1 enable / disables work correctly

mimics behavior found in other emulators, stops nonsense commands from triggering nonsense save operations with interface1 installed which was caused by the rom banking out too early.

src/devices/bus/spectrum/exp.cpp

@@ -107,6 +107,16 @@ void spectrum_expansion_slot_device::opcode_fetch(offs_t offset)
 		 m_card->opcode_fetch(offset);
 }
 
+//-------------------------------------------------
+// fetch_r
+//-------------------------------------------------
+
+void spectrum_expansion_slot_device::opcode_fetch_post(offs_t offset)
+{
+	if (m_card)
+		 m_card->opcode_fetch_post(offset);
+}
+
 //-------------------------------------------------
 //  iorq_r
 //-------------------------------------------------

src/devices/bus/spectrum/exp.h

@@ -74,6 +74,7 @@ public:
 	auto nmi_handler() { return m_nmi_handler.bind(); }
 
 	void opcode_fetch(offs_t offset);
+	void opcode_fetch_post(offs_t offset);
 	uint8_t mreq_r(offs_t offset);
 	void mreq_w(offs_t offset, uint8_t data);
 	uint8_t iorq_r(offs_t offset);
@@ -107,6 +108,7 @@ public:
 
 	// reading and writing
 	virtual void opcode_fetch(offs_t offset) { };
+	virtual void opcode_fetch_post(offs_t offset) { };
 	virtual uint8_t mreq_r(offs_t offset) { return 0xff; }
 	virtual void mreq_w(offs_t offset, uint8_t data) { }
 	virtual uint8_t iorq_r(offs_t offset) { return 0xff; }

src/devices/bus/spectrum/intf1.cpp

@@ -106,6 +106,9 @@ READ_LINE_MEMBER(spectrum_intf1_device::romcs)
 	return m_romcs | m_exp->romcs();
 }
 
+// the Interface 1 looks for specific bus conditions to enable / disable the expansion overlay ROM
+
+// the enable must occur BEFORE the opcode is fetched, as the opcode must be fetched from the expansion ROM
 void spectrum_intf1_device::opcode_fetch(offs_t offset)
 {
 	m_exp->opcode_fetch(offset);
@@ -117,6 +120,19 @@ void spectrum_intf1_device::opcode_fetch(offs_t offset)
 		case 0x0008: case 0x1708:
 			m_romcs = 1;
 			break;
+		}
+	}
+}
+
+// the disable must occur AFTER the opcode fetch, or the incorrect opcode is fetched for 0x0700
+void spectrum_intf1_device::opcode_fetch_post(offs_t offset)
+{
+	m_exp->opcode_fetch_post(offset);
+
+	if (!machine().side_effects_disabled())
+	{
+		switch (offset)
+		{
 		case 0x0700:
 			m_romcs = 0;
 			break;
@@ -124,6 +140,7 @@ void spectrum_intf1_device::opcode_fetch(offs_t offset)
 	}
 }
 
+
 uint8_t spectrum_intf1_device::mreq_r(offs_t offset)
 {
 	uint8_t data = 0xff;

src/devices/bus/spectrum/intf1.h

@@ -37,6 +37,7 @@ protected:
 	virtual const tiny_rom_entry *device_rom_region() const override;
 
 	virtual void opcode_fetch(offs_t offset) override;
+	virtual void opcode_fetch_post(offs_t offset) override;
 	virtual uint8_t mreq_r(offs_t offset) override;
 	virtual void mreq_w(offs_t offset, uint8_t data) override;
 	virtual uint8_t iorq_r(offs_t offset) override;

src/mame/drivers/spec128.cpp

@@ -169,7 +169,16 @@ READ8_MEMBER(spectrum_state::spectrum_128_opcode_fetch_r)
 {
 	/* this allows expansion devices to act upon opcode fetches from MEM addresses */
 	if (BIT(m_port_7ffd_data, 4))
+	{
+		/* this allows expansion devices to act upon opcode fetches from MEM addresses
+		   for example, interface1 detection fetches requires fetches at 0008 / 0708 to
+		   enable paged ROM and then fetches at 0700 to disable it
+		*/
 		m_exp->opcode_fetch(offset);
+		uint8_t retval = m_maincpu->space(AS_PROGRAM).read_byte(offset);
+		m_exp->opcode_fetch_post(offset);
+		return retval;
+	}
 
 	return m_maincpu->space(AS_PROGRAM).read_byte(offset);
 }

src/mame/drivers/spectrum.cpp

@@ -293,10 +293,14 @@ SamRam
 
 READ8_MEMBER(spectrum_state::opcode_fetch_r)
 {
-	/* this allows expansion devices to act upon opcode fetches from MEM addresses */
+	/* this allows expansion devices to act upon opcode fetches from MEM addresses
+	   for example, interface1 detection fetches requires fetches at 0008 / 0708 to
+	   enable paged ROM and then fetches at 0700 to disable it
+	*/
 	m_exp->opcode_fetch(offset);
-
-	return m_maincpu->space(AS_PROGRAM).read_byte(offset);
+	uint8_t retval = m_maincpu->space(AS_PROGRAM).read_byte(offset);
+	m_exp->opcode_fetch_post(offset);
+	return retval;
 }
 
 WRITE8_MEMBER(spectrum_state::spectrum_rom_w)