added mRAM cartridge type

Author: JetSetIlly

hardware/memory/external/fingerprint.go

@@ -166,6 +166,25 @@ func FingerprintBlob(filename string, d []uint8, mapper string) (CartridgeInsert
 				}, nil
 			}
 
+			// mRAM chip in flat ROM cartridge (no bankswitching)
+			//
+			// it's not clear if the mRAM chip (with a masked address line) can be used in
+			// conjunction with a bankswitching method. as it stands, I think only 'Rescue On
+			// Fractalus' uses this type of RAM chip and that has a flat ROM map
+			if cartType == 0x0080 {
+				return CartridgeInsertor{
+					data: d,
+					creator: func(ctx Context, d []uint8) (Bus, error) {
+						return NewMRAM(ctx, d[dataStart:])
+					},
+					Controller: controller,
+					spec:       spec,
+					chips:      chips,
+					UseHSC:     useHSC,
+					UseSavekey: useSavekey,
+				}, nil
+			}
+
 			// activision
 			if cartType == 0x0100 {
 				// if cartridge name contians the '(OM)' string then the cartridge has been dumped

hardware/memory/external/mram.go

@@ -0,0 +1,42 @@
+package external
+
+// MRAM is created when bit 0x0080 of the a78 cartridge type field is on. Example ROM is the
+// prototype of Rescue on Fractalus. The name mRAM comes from the A7800 rom.cpp file which describes
+// this type of cartridge as "no bankswitch + mRAM chip"
+type MRAM struct {
+	data   []byte
+	origin uint16
+	ram    []byte
+}
+
+func NewMRAM(_ Context, d []byte) (*MRAM, error) {
+	return &MRAM{
+		data:   d,
+		origin: uint16(0x10000 - len(d)),
+		ram:    make([]byte, 0x4000),
+	}, nil
+}
+
+func (ext *MRAM) Label() string {
+	return "mRAM"
+}
+
+func (ext *MRAM) Access(write bool, address uint16, data uint8) (uint8, error) {
+	if address < 0x4000 {
+		return 0, nil
+	}
+
+	if address < 0x8000 {
+		address &= 0xfeff
+		if write {
+			ext.ram[address-0x4000] = data
+			return 0, nil
+		}
+		return ext.ram[address-0x4000], nil
+	}
+
+	if address < ext.origin {
+		return 0, nil
+	}
+	return ext.data[address-ext.origin], nil
+}