fixed some MMC3 open bus error

just returned 0 instead of open bus
Roeegg2 · Mar 22, 2024 · 5c3e525 · 5c3e525
1 parent 3d3f332
commit 5c3e525
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Showing 5 changed files with 18 additions and 20 deletions.
diff --git a/src/bus.cpp b/src/bus.cpp
@@ -13,7 +13,7 @@ namespace roee_nes {
         this->controller_1 = controller_1;
         this->controller_2 = controller_2;
         init_palette(palette_path);
-        
+
         cpu->bus = this;
         ppu->bus = this;
 

diff --git a/src/main.cpp b/src/main.cpp
@@ -26,7 +26,7 @@ uint16_t emulator_tick(Bus* bus) {
 }
 
 int main() {
-    const std::string rom_path = "roms/METROID.nes";
+    const std::string rom_path = "roms/SMB2.nes";
     const std::string palette_path = "ntscpalette.pal";
 
     Controller* const controller_1 = new Controller();

diff --git a/src/mapper_n_cart.cpp b/src/mapper_n_cart.cpp
@@ -106,8 +106,8 @@ namespace roee_nes {
         return; // do nothing!
     }
 
-    Save_RAM::Save_RAM(const std::string& save_file_path) 
-    : save_data({ 0 }), path(save_file_path){
+    Save_RAM::Save_RAM(const std::string& save_file_path)
+        : save_data({ 0 }), path(save_file_path) {
         if (std::filesystem::exists(path)) {
             std::cout << "USER INFO: Save game data found. Reading from save data\n";
 
@@ -130,11 +130,11 @@ namespace roee_nes {
     }
 
     uint8_t Save_RAM::mapper_read(const uint16_t addr) {
-        return save_data[addr % 0x2000];
+        return save_data[addr % 0x6000];
     }
 
     void Save_RAM::mapper_write(const uint16_t addr, const uint8_t data) {
-        save_data[addr % 0x2000] = data;
+        save_data[addr % 0x6000] = data;
     }
 
 

diff --git a/src/mappers/mmc3_4.cpp b/src/mappers/mmc3_4.cpp
@@ -21,10 +21,12 @@ namespace roee_nes {
         chr_bank_num = (chr_read_mem->size() / (1 * KILOBYTE)) - 2;
         prg_bank_num = (cart->prg_rom.size() / (8 * KILOBYTE));
 
-        if (cart->header.flag_6.parsed.prg_ram == 1)
+        if (cart->header.flag_6.parsed.prg_ram == 1) {
+            std::cout << "USER INFO: Game has PRG RAM\n";
             save_ram = new Save_RAM(cart->rom_path + ".sav");
-        else
+        } else {
             save_ram = nullptr;
+        }
 
         prg_bank[0] = 0;
         prg_bank[1] = 0;
@@ -38,7 +40,8 @@ namespace roee_nes {
 
     void MMC3_4::cpu_write(uint16_t addr, uint8_t data) {
         if ((0x6000 <= addr) && (addr <= 0x7fff)) {
-            save_ram->mapper_write(addr, data);
+            if (save_ram != nullptr)
+                save_ram->mapper_write(addr, data);
         } else if ((0x8000 <= addr) && (addr <= 0x9ffe)) {
             if ((addr % 2) == 0) { // write to low reg
                 bank_select.comp.even.select = data & 0b0000'0111;
@@ -114,18 +117,13 @@ namespace roee_nes {
         // << " irq reload: " << (int)irq_reload << " \n"
         // << " set irq: " << (int)set_irq << " \n"
         // << "-------------------\n";
-        if ((0x6000 <= addr) && (addr <= 0x7fff)) {
-            return save_ram->mapper_read(addr);
-        } else if ((0x6000 <= addr) && (addr <= 0x7fff)) {
-            if ((prg_ram_protect & 0b1000'0000) == 0)
-                return open_bus_data;
-            else
-                return save_data[addr % 0x6000]; // handle prg_ram
+        if ((0x6000 <= addr) && (addr <= 0x7fff) && (!(prg_ram_protect & 0b1000'0000)) && (save_ram != nullptr)) {
+            return save_data[addr % 0x6000]; // handle prg_ram
         } else if ((0x8000 <= addr) && (addr <= 0xffff)) {
             update_prg(addr);
             return cart->prg_rom[(final_bank * 8 * KILOBYTE) + final_addr];
-        } else {
-            return open_bus_data;
+        } else { // otherwise return open bus
+            return 0; // should return open bus but fuck it, no MMC3 game uses this anyway
         }
     }
 

diff --git a/src/ppu.cpp b/src/ppu.cpp
@@ -23,9 +23,9 @@ namespace roee_nes {
                 vblank_scanline();
 
             if ((bus->mapper->mapper_number == 4) 
-            // && (!checked)
+            && (!checked)
              )
-                ((MMC3_4*)bus->mapper)->clock_irq();
+                check_mmc3_irq_conditions();
 
             increment_cycle(1);
         }