Add support for iNES Mapper 007 (AxROM), Mirroring

Add support for iNES Mapper 007 (AxROM) which should enable Battletoads
and Marble Madness. Add support for one-screen mirroring.

(bit broken at the moment)

src/cartridge.cpp

@@ -6,6 +6,7 @@
 #include "mappers/mapper2.hpp"
 #include "mappers/mapper3.hpp"
 #include "mappers/mapper4.hpp"
+#include "mappers/mapper7.hpp"
 #include "ppu.hpp"
 #include "cartridge.hpp"
 
@@ -57,6 +58,7 @@ void load(const char* fileName)
         case 2:  mapper = new Mapper2(rom); break;
         case 3:  mapper = new Mapper3(rom); break;
         case 4:  mapper = new Mapper4(rom); break;
+        case 7:  mapper = new Mapper7(rom); break;
     }
 
     CPU::power();

src/mappers/mapper7.cpp

@@ -0,0 +1,38 @@
+#include "ppu.hpp"
+#include "mappers/mapper7.hpp"
+
+/* Based off of https://wiki.nesdev.com/w/index.php/AxROM */
+
+/* Apply the registers state */
+void Mapper7::apply()
+{
+   /*
+    * 32 kb PRG ROM Banks
+    * 0x8000 - 0xFFFF swappable
+    */
+    map_prg<32>(0, regs[0] & 0x7);
+
+    /* 8k of CHR (ram) */
+    map_chr<8>(0, 0);
+
+    /* Mirroring based on bit 5 */
+    set_mirroring((regs[0] & 0x10) ? PPU::ONE_SCREEN_HI : PPU::ONE_SCREEN_LO);
+}
+
+u8 Mapper7::write(u16 addr, u8 v)
+{
+    /* check for bus contingency? (addr & 0x8000 == v?) nah */
+
+    /* bank switching */
+    if (addr & 0x8000)
+    {
+        regs[0] = v;
+        apply();
+    }
+    return v;
+}
+
+u8 Mapper7::chr_write(u16 addr, u8 v)
+{
+    return chr[addr] = v;
+}

src/mappers/mapper7.hpp

@@ -0,0 +1,19 @@
+#pragma once
+#include "mapper.hpp"
+#include "ppu.hpp"
+
+class Mapper7 : public Mapper
+{
+    u8 regs[1];
+    void apply();
+
+  public:
+    Mapper7(u8* rom) : Mapper(rom)
+    {
+        regs[0] = 0;
+        apply();
+    }
+
+    u8 write(u16 addr, u8 v);
+    u8 chr_write(u16 addr, u8 v);
+};

src/ppu.cpp

@@ -42,6 +42,9 @@ u16 nt_mirror(u16 addr)
     {
         case VERTICAL:    return addr % 0x800;
         case HORIZONTAL:  return ((addr / 2) & 0x400) + (addr % 0x400);
+        case ONE_SCREEN_LO:
+        case ONE_SCREEN_HI:
+                          addr = (addr & 0x3ff) + ((mirroring == ONE_SCREEN_HI) ? 0x400 : 0x0);
         default:          return addr - 0x2000;
     }
 }

src/ppu.hpp

@@ -5,7 +5,7 @@ namespace PPU {
 
 
 enum Scanline  { VISIBLE, POST, NMI, PRE };
-enum Mirroring { VERTICAL, HORIZONTAL };
+enum Mirroring { VERTICAL, HORIZONTAL, ONE_SCREEN_HI, ONE_SCREEN_LO, FOUR_SCREEN };
 
 /* Sprite buffer */
 struct Sprite