diff --git a/src/boards/235.cpp b/src/boards/235.cpp
index 98ce11bb..aba619ad 100644
--- a/src/boards/235.cpp
+++ b/src/boards/235.cpp
@@ -19,36 +19,79 @@
  */
 
 #include "mapinc.h"
+#include "ines.h"
+
+static uint16 cmdreg = 0;
+static uint8 openbus = 0;
+
+// For carts with extra 128K prg rom (Contra)
+static uint8 unrom = 0;
+static uint8 unromData = 0;
+
+static uint32 PRGROMSize = 0;
 
-static uint16 cmdreg;
 static SFORMAT StateRegs[] =
 {
 	{ &cmdreg, 2, "CREG" },
+	{ &openbus, 1, "OB" },
+	{ &unrom, 1, "UROM" },
+	{ &unromData, 1, "UDTA" },
 	{ 0 }
 };
 
 static void Sync(void) {
-	if (cmdreg & 0x400)
-		setmirror(MI_0);
-	else
-		setmirror(((cmdreg >> 13) & 1) ^ 1);
-	if (cmdreg & 0x800) {
-		setprg16(0x8000, ((cmdreg & 0x300) >> 3) | ((cmdreg & 0x1F) << 1) | ((cmdreg >> 12) & 1));
-		setprg16(0xC000, ((cmdreg & 0x300) >> 3) | ((cmdreg & 0x1F) << 1) | ((cmdreg >> 12) & 1));
-	} else
-		setprg32(0x8000, ((cmdreg & 0x300) >> 4) | (cmdreg & 0x1F));
+	if (unrom) {
+		int PRGPageCount = PRGROMSize / (16 * 1024);
+		setprg16(0x8000, PRGPageCount & 0xC0 | (unromData & 7));
+		setprg16(0xC000, PRGPageCount & 0xC0 | 7);
+		setmirror(MI_V);
+	} else {
+		uint8 bank = ((cmdreg & 0x300) >> 3) | (cmdreg & 0x1F);
+		if (bank >= (PRGROMSize / (32 * 1024))) {
+			openbus = 1;
+		} else {
+			if (cmdreg & 0x400)
+				setmirror(MI_0);
+			else
+				setmirror(((cmdreg >> 13) & 1) ^ 1);
+		 	if (cmdreg & 0x800) {
+				setprg16(0x8000, (bank << 1) | ((cmdreg >> 12) & 1));
+				setprg16(0xC000, (bank << 1) | ((cmdreg >> 12) & 1));
+			} else
+				setprg32(0x8000, bank);
+		}
+	}
+}
+
+static DECLFR(M235Read) {
+	if (openbus) {
+		openbus = 0;
+		return X.DB;
+	}
+	return CartBR(A);
 }
 
 static DECLFW(M235Write) {
 	cmdreg = A;
+	unromData = V;
+	Sync();
+}
+
+static void M235Reset(void) {
+	cmdreg = 0;
+	unromData = 0;
+	if (PRGROMSize & 0x20000)
+		unrom = (unrom + 1) & 1;
 	Sync();
 }
 
 static void M235Power(void) {
 	setchr8(0);
 	SetWriteHandler(0x8000, 0xFFFF, M235Write);
-	SetReadHandler(0x8000, 0xFFFF, CartBR);
+	SetReadHandler(0x8000, 0xFFFF, M235Read);
 	cmdreg = 0;
+	unromData = 0;
+	unrom = 0;
 	Sync();
 }
 
@@ -57,7 +100,11 @@ static void M235Restore(int version) {
 }
 
 void Mapper235_Init(CartInfo *info) {
+	info->Reset = M235Reset;
 	info->Power = M235Power;
 	GameStateRestore = M235Restore;
 	AddExState(&StateRegs, ~0, 0, 0);
+
+	// needs raw, non-pow2 PRGROM size for comparison
+	PRGROMSize = head.ROM_size * 16384;
 }