MBC2 working

src/addressSpace.hpp

@@ -97,7 +97,10 @@ public:
   Byte *MBCRead(Word address);
   // prevents seg faults when programs with no MBC try to write to ROM
   Byte dummyVal = 0;
+
+  // called after every op
   void MBCUpdate();
+
   void loadRomBank();
   void createRamBank();
   void loadRamBank();
@@ -110,6 +113,7 @@ public:
   bool dmaTransferRequested = false;
   void dmaTransfer();
 
+  // MBC registers
   // Selected ROM Bank = (Secondary Bank << 5) + ROM Bank
   Byte selectedRomBank = 0;
   Byte romBankRegister = 0x00;
@@ -117,6 +121,10 @@ public:
   Byte twoBitBankRegister = 0x0;
   Byte selectedExternalRamBank = 0;
   Byte romRamSelect = 0x00;
+  // 0xA if enabled in MBC1/3, else off
+  // in MBC2 also the rom bank number
+  // in MBC3 also timer enable
+  // reading ram while disabled is undefined behaviour
   Byte ramEnable = 0x00;
   // MBC3
   Byte latchClockData = 0x00;
@@ -139,6 +147,9 @@ public:
     if (address < 0xC000) {
       if (externalRamSize == 0)
         return 0xFF;
+      // MBC2 echos 15 times and only stores lower 4 bits
+      if (MBC == MBC2 || MBC == MBC2Battery)
+        return memoryLayout.externalRam[(address - 0xA000) & 0x01FF] | 0xF0;
       return memoryLayout.externalRam[address - 0xA000];
     }
     if (address < 0xD000)

src/mbc.cpp

@@ -64,6 +64,14 @@ Byte *AddressSpace::MBCRead(const Word address) {
     if (address <= 0x7FFF)
       return &romRamSelect;
   }
+  if (MBC == MBC2 || MBC == MBC2Battery) {
+    if (address <= 0x3FFF) {
+      if (address & 0x0100)
+        return &romBankRegister;
+      else
+        return &ramEnable;
+    }
+  }
   return &dummyVal;
 }
 
@@ -93,8 +101,7 @@ void AddressSpace::MBCUpdate() {
 
     loadRomBank();
     loadRamBank();
-  }
-  if (MBC == MBC1Ram || MBC == MBC1RamBattery) {
+  } else if (MBC == MBC1Ram || MBC == MBC1RamBattery) {
     // Selected ROM Bank = (Secondary Bank << 5) + ROM Bank
     romBankRegister &= 0b11111;
     twoBitBankRegister &= 0b11;
@@ -119,7 +126,9 @@ void AddressSpace::MBCUpdate() {
     loadRomBank();
     loadRamBank();
   } else if (MBC == MBC2 || MBC == MBC2Battery) {
-    // TODO
+    selectedRomBank = romBankRegister & 0x0F;
+    loadRomBank();
+    loadRamBank();
   } else if (MBC == MBC3 || MBC == MBC3TimerBattery) {
     romBankRegister &= 0b11111;
     twoBitBankRegister &= 0b11;
@@ -168,7 +177,8 @@ void AddressSpace::createRamBank() {
 }
 
 void AddressSpace::loadRamBank() {
-  if (cartridgeRam != nullptr)
+  if (cartridgeRam != nullptr) {
     memoryLayout.externalRam =
         cartridgeRam + (RAM_BANK_SIZE * selectedExternalRamBank);
+  }
 }