diff --git a/src/boards/420.c b/src/boards/420.c
new file mode 100644
index 00000000..c012a7c6
--- /dev/null
+++ b/src/boards/420.c
@@ -0,0 +1,63 @@
+/* FCEUmm - NES/Famicom Emulator
+ *
+ * Copyright notice for this file:
+ *  Copyright (C) 2023
+ *
+ * This program is free software; you can redistribute it and/or modify
+ * it under the terms of the GNU General Public License as published by
+ * the Free Software Foundation; either version 2 of the License, or
+ * (at your option) any later version.
+ *
+ * This program is distributed in the hope that it will be useful,
+ * but WITHOUT ANY WARRANTY; without even the implied warranty of
+ * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+ * GNU General Public License for more details.
+ *
+ * You should have received a copy of the GNU General Public License
+ * along with this program; if not, write to the Free Software
+ * Foundation, Inc., 51 Franklin Street, Fifth Floor, Boston, MA  02110-1301  USA
+ */
+
+#include "mapinc.h"
+#include "mmc3.h"
+
+static void M420CW(uint32 A, uint8 V) {
+	uint8 mask = (EXPREGS[1] & 0x80) ? 0x7F : 0xFF;
+	setchr1(A, ((EXPREGS[1] << 1) & 0x100) | ((EXPREGS[1] << 5) & 0x80) | (V & mask));
+}
+
+static void M420PW(uint32 A, uint8 V) {
+	if (EXPREGS[0] & 0x80) {
+		setprg32(0x8000, ((EXPREGS[2] >> 2) & 8) | ((EXPREGS[0] >> 1) & 7));
+	} else {
+		uint8 mask = (EXPREGS[0] & 0x20) ? 0x0F : ((EXPREGS[3] & 0x20) ? 0x1F : 0x3F);
+		setprg8(A, ((EXPREGS[3] << 3) & 0x20) | (V & mask));
+	}
+}
+
+static DECLFW(M420Write) {
+	/* writes possible regardless of MMC3 wram state */
+	EXPREGS[A & 3] = V;
+	FixMMC3PRG(MMC3_cmd);        
+	FixMMC3CHR(MMC3_cmd);
+}
+
+static void M420Reset(void) {
+	EXPREGS[0] = EXPREGS[1] = EXPREGS[2] = EXPREGS[3] = 0;
+	MMC3RegReset();
+}
+
+static void M420Power(void) {
+	EXPREGS[0] = EXPREGS[1] = EXPREGS[2] = EXPREGS[3] = 0;
+	GenMMC3Power();
+	SetWriteHandler(0x6000, 0x7FFF, M420Write);
+}
+
+void Mapper420_Init(CartInfo *info) {
+	GenMMC3_Init(info, 512, 256, 0, 0);
+	cwrap = M420CW;
+	pwrap = M420PW;
+	info->Reset = M420Reset;
+	info->Power = M420Power;
+	AddExState(EXPREGS, 4, 0, "EXPR");
+}
diff --git a/src/ines.c b/src/ines.c
index b7f29d4e..517ec226 100644
--- a/src/ines.c
+++ b/src/ines.c
@@ -807,6 +807,7 @@ INES_BOARD_BEGIN()
 	INES_BOARD( "0353",                     415, Mapper415_Init         )
 	INES_BOARD( "4-in-1/N-32",              416, Mapper416_Init         )
 	INES_BOARD( "",                         417, Mapper417_Init         )
+	INES_BOARD( "A971210",                  420, Mapper420_Init         )
 	INES_BOARD( "SC871115C",                421, Mapper421_Init         )
 	INES_BOARD( "BS-400R/BS-4040",          422, Mapper422_Init         )
 	INES_BOARD( "AB-G1L/WELL-NO-DG450",     428, Mapper428_Init         )
diff --git a/src/ines.h b/src/ines.h
index 2c339ba6..819e5ffb 100644
--- a/src/ines.h
+++ b/src/ines.h
@@ -316,6 +316,7 @@ void Mapper414_Init(CartInfo *);
 void Mapper415_Init(CartInfo *);
 void Mapper416_Init(CartInfo *);
 void Mapper417_Init(CartInfo *);
+void Mapper420_Init(CartInfo *);
 void Mapper421_Init(CartInfo *);
 void Mapper422_Init(CartInfo *);
 void Mapper428_Init(CartInfo *);