scc68070: limit external memory addresses to 24 bits

src/devices/cpu/m68000/scc68070.cpp

@@ -21,4 +21,34 @@ void scc68070_base_device::device_start()
 {
 	m68000_musashi_device::device_start();
 	init_cpu_scc68070();
+
+	m_readimm16 = [this](offs_t address) -> u16  { return m_oprogram16.read_word(translate_addr(address)); };
+	m_read8   = [this](offs_t address) -> u8     { return m_program16.read_byte(translate_addr(address)); };
+	m_read16  = [this](offs_t address) -> u16    { return m_program16.read_word(translate_addr(address)); };
+	m_read32  = [this](offs_t address) -> u32    { return m_program16.read_dword(translate_addr(address)); };
+	m_write8  = [this](offs_t address, u8 data)
+		{
+			address = translate_addr(address);
+			m_program16.write_word(address & ~1, data | (data << 8), address & 1 ? 0x00ff : 0xff00);
+		};
+	m_write16 = [this](offs_t address, u16 data) { m_program16.write_word(translate_addr(address), data); };
+	m_write32 = [this](offs_t address, u32 data) { m_program16.write_dword(translate_addr(address), data); };
+}
+
+offs_t scc68070_base_device::translate_addr(offs_t address) const
+{
+	// internal addresses (0x80000000-bfffffff) are only accessible in supervisor mode;
+	// all other accesses use the external 24-bit address bus.
+	if (!(supervisor_mode() && (address >> 30) == 0x2))
+		address &= 0xffffff;
+
+	return address;
+}
+
+bool scc68070_base_device::memory_translate(int spacenum, int intention, offs_t &address, address_space *&target_space)
+{
+	target_space = &space(spacenum);
+	if (spacenum == AS_PROGRAM)
+		address = translate_addr(address);
+	return true;
 }

src/devices/cpu/m68000/scc68070.h

@@ -18,6 +18,10 @@ class scc68070_base_device : public m68000_musashi_device
 	// device-level overrides
 	virtual void device_start() override ATTR_COLD;
 
+	offs_t translate_addr(offs_t address) const;
+
+	virtual bool memory_translate(int spacenum, int intention, offs_t &address, address_space *&target_space) override;
+
 	scc68070_base_device(const machine_config &mconfig, const char *tag, device_t *owner, u32 clock,
 						const device_type type, address_map_constructor internal_map);
 };