bus/nes: Added MMC1 variant board SZROM. (#8658)

This fixes corrupt (NVRAM) save games for A Ressha de Ikou, the only known game to use this board.

hash/nes.xml

@@ -44502,7 +44502,7 @@ Also notice that VRAM, WRAM & mirror are probably incorrect for some of these se
 		<info name="release" value="19910821"/>
 		<info name="alt_title" value="A列車で行こう"/>
 		<part name="cart" interface="nes_cart">
-			<feature name="slot" value="sxrom" />
+			<feature name="slot" value="szrom" />
 			<feature name="pcb_model" value="HVC-SZROM-01" />
 			<feature name="mmc1_type" value="MMC1B2" />
 			<dataarea name="prg" size="131072">

src/devices/bus/nes/mmc1.cpp

@@ -41,6 +41,7 @@
 
 DEFINE_DEVICE_TYPE(NES_SXROM, nes_sxrom_device, "nes_sxrom", "NES Cart SxROM (MMC-1) PCB")
 DEFINE_DEVICE_TYPE(NES_SOROM, nes_sorom_device, "nes_sorom", "NES Cart SOROM (MMC-1) PCB")
+DEFINE_DEVICE_TYPE(NES_SZROM, nes_szrom_device, "nes_szrom", "NES Cart SZROM (MMC-1) PCB")
 
 
 
@@ -59,6 +60,11 @@ nes_sorom_device::nes_sorom_device(const machine_config &mconfig, const char *ta
 {
 }
 
+nes_szrom_device::nes_szrom_device(const machine_config &mconfig, const char *tag, device_t *owner, u32 clock)
+	: nes_sxrom_device(mconfig, NES_SZROM, tag, owner, clock)
+{
+}
+
 
 
 void nes_sxrom_device::device_start()
@@ -313,3 +319,32 @@ uint8_t nes_sorom_device::read_m(offs_t offset)
 
 	return get_open_bus();
 }
+
+// SZROM has two RAM banks, the first is not battery backed up, the second is.
+void nes_szrom_device::write_m(offs_t offset, u8 data)
+{
+	LOG_MMC(("szrom write_m, offset: %04x, data: %02x\n", offset, data));
+
+	if (!BIT(m_reg[3], 4) || m_mmc1_type == mmc1_type::MMC1A)  // WRAM enabled
+	{
+		if (BIT(m_reg[BIT(m_reg[0], 4) + 1], 4))
+			m_battery[offset & (m_battery.size() - 1)] = data;
+		else
+			m_prgram[offset & (m_prgram.size() - 1)] = data;
+	}
+}
+
+u8 nes_szrom_device::read_m(offs_t offset)
+{
+	LOG_MMC(("szrom read_m, offset: %04x\n", offset));
+
+	if (!BIT(m_reg[3], 4) || m_mmc1_type == mmc1_type::MMC1A)  // WRAM enabled
+	{
+		if (BIT(m_reg[BIT(m_reg[0], 4) + 1], 4))
+			return m_battery[offset & (m_battery.size() - 1)];
+		else
+			return m_prgram[offset & (m_prgram.size() - 1)];
+	}
+
+	return get_open_bus();
+}

src/devices/bus/nes/mmc1.h

@@ -43,6 +43,9 @@ protected:
 	int m_count;
 };
 
+
+// ======================> nes_sorom_device
+
 class nes_sorom_device : public nes_sxrom_device
 {
 public:
@@ -54,8 +57,22 @@ public:
 };
 
 
+// ======================> nes_szrom_device
+
+class nes_szrom_device : public nes_sxrom_device
+{
+public:
+	// construction/destruction
+	nes_szrom_device(const machine_config &mconfig, const char *tag, device_t *owner, u32 clock);
+
+	virtual u8 read_m(offs_t offset) override;
+	virtual void write_m(offs_t offset, u8 data) override;
+};
+
+
 // device type definition
 DECLARE_DEVICE_TYPE(NES_SXROM, nes_sxrom_device)
 DECLARE_DEVICE_TYPE(NES_SOROM, nes_sorom_device)
+DECLARE_DEVICE_TYPE(NES_SZROM, nes_szrom_device)
 
 #endif // MAME_BUS_NES_MMC1_H

src/devices/bus/nes/nes_carts.cpp

@@ -101,6 +101,7 @@ void nes_cart(device_slot_interface &device)
 // SxROM
 	device.option_add_internal("sxrom",            NES_SXROM);
 	device.option_add_internal("sorom",            NES_SOROM);
+	device.option_add_internal("szrom",            NES_SZROM);
 // TxROM
 	device.option_add_internal("txrom",            NES_TXROM);
 // HKROM

src/devices/bus/nes/nes_ines.hxx

@@ -798,6 +798,8 @@ void nes_cart_slot_device::call_load_ines()
 			break;
 
 		case STD_SXROM:
+			if (mapper == 1 && ines20 && prgram_size == 0x2000 && battery_size == 0x2000 && vrom_size == 0x4000)
+				m_pcb_id = STD_SZROM;
 			if (mapper == 155)
 				m_cart->set_mmc1_type(device_nes_cart_interface::mmc1_type::MMC1A);
 			break;
@@ -1219,6 +1221,12 @@ const char * nes_cart_slot_device::get_default_card_ines(get_default_card_softwa
 				pcb_id = STD_NROM368;
 			break;
 
+		case STD_SXROM:
+			// only A Ressha de Ikou uses SZROM and it can be detected by its profile: 8K WRAM, 8K BWRAM, 16K CHR ROM
+			if (mapper == 1 && ines20 && ROM[10] == 0x77 && ROM[5] == 2)
+				pcb_id = STD_SZROM;
+			break;
+
 		case KONAMI_VRC2:
 			if (mapper == 23 && crc_hack && !submapper)
 				pcb_id = KONAMI_VRC4; // this allows for konami_irq to be installed at reset

src/devices/bus/nes/nes_pcb.hxx

@@ -32,6 +32,7 @@ static const nes_pcb pcb_list[] =
 	{ "un1rom",           STD_UN1ROM },
 	{ "sxrom",            STD_SXROM },
 	{ "sorom",            STD_SOROM },
+	{ "szrom",            STD_SZROM },
 	{ "txrom",            STD_TXROM },
 	{ "hkrom",            STD_HKROM },
 	{ "tqrom",            STD_TQROM },

src/devices/bus/nes/nes_slot.h

@@ -23,7 +23,7 @@ enum
 	STD_CNROM, STD_CPROM,
 	STD_EXROM, STD_FXROM, STD_GXROM,
 	STD_HKROM, STD_PXROM,
-	STD_SXROM, STD_SOROM,
+	STD_SXROM, STD_SOROM, STD_SZROM,
 	STD_TXROM, STD_TXSROM, STD_TKROM, STD_TQROM,
 	STD_UXROM, STD_UN1ROM, UXROM_CC,
 	HVC_FAMBASIC, NES_QJ, PAL_ZZ, STD_EVENT,