diff --git a/Makefile b/Makefile
index e866eef..778b508 100644
--- a/Makefile
+++ b/Makefile
@@ -20,7 +20,7 @@ SRC_SRCS_1 += cart.c mapper.c
 SRC_SRCS_1 += sdl_render.c sdl_input.c
 
 MAPDIR = src/map
-MAP_SRCS_1 = nrom.c mmc1.c cnrom.c
+MAP_SRCS_1 = nrom.c mmc1.c cnrom.c mmc3.c
 
 EXT_SRCS_1 = e6502/e6502.c
 
diff --git a/src/map/mmc1.c b/src/map/mmc1.c
index 0486300..0c51be6 100644
--- a/src/map/mmc1.c
+++ b/src/map/mmc1.c
@@ -3,9 +3,6 @@
 #include "map.h"
 
 
-#define MAP_LOG(fmt, ...) //printf("MAP: " fmt "\n" __VA_OPT__(,) __VA_ARGS__)
-
-
 typedef struct {
     // TODO: Does this even support CHR ROM?
     uint8_t* prg_rom;
diff --git a/src/map/mmc3.c b/src/map/mmc3.c
new file mode 100644
index 0000000..887f8d4
--- /dev/null
+++ b/src/map/mmc3.c
@@ -0,0 +1,270 @@
+#include <stdlib.h>
+
+#include "map.h"
+
+typedef enum {
+    mmc3_Flag_Horizontal   = 0b00000001,
+    mmc3_Flag_IRQ_Enabled  = 0b00000010,
+    mmc3_Flag_WRAM_Protect = 0b01000000,
+    mmc3_Flag_WRAM_Enabled = 0b10000000,
+} mmc3_Flag;
+
+typedef enum {
+    mmc3_Bank_Select_Reg    = 0b00000111,
+    mmc3_Bank_Select_PRG    = 0b01000000,
+    mmc3_Bank_Select_CHR    = 0b10000000,
+} mmc3_Bank_Select;
+
+typedef struct {
+    uint8_t* prg_rom;
+    int prg_rom_banks; // 16 KB / 8 KB = 2
+    uint8_t* chr_rom;
+    int chr_rom_banks; // 4 KB / 1 KB = 4
+
+    uint8_t* prg_bank[4]; // 8 KB
+    uint8_t* chr_bank[8]; // 1 KB
+    uint8_t* vram_bank[4];
+
+    mmc3_Flag flags;
+    uint8_t bank_select;
+    uint8_t irq_count;
+    uint8_t irq_latch;
+
+    uint8_t wram[nes_mem_wram_size];
+    uint8_t vram[2][nes_vram_page_size];
+} mmc3_mapper;
+
+static inline uint8_t* mmc3_prg_bank(mmc3_mapper* map, int bank) {
+    return &map->prg_rom[bank << 13];
+}
+
+static inline void mmc3_map_prg(mmc3_mapper* map,
+                                int reg, int bank) {
+    MAP_LOG("PRG ROM: 8k $%04x <- bank %d", reg << 13, bank);
+    map->prg_bank[reg] = mmc3_prg_bank(map, bank);
+}
+
+static inline void mmc3_update_rom_mode(mmc3_mapper* map) {
+    if (!(map->bank_select & mmc3_Bank_Select_PRG)) {
+        mmc3_map_prg(map, 2, map->prg_rom_banks - 2);
+    } else {
+        mmc3_map_prg(map, 0, map->prg_rom_banks - 2);
+    }
+}
+
+static inline void mmc3_update_prg(mmc3_mapper* map, uint8_t bank) {
+    int reg = (map->bank_select & mmc3_Bank_Select_Reg);
+
+    if (!(map->bank_select & mmc3_Bank_Select_PRG)) {
+        if (reg == 7) {
+            mmc3_map_prg(map, 1, bank);
+        } else {
+            mmc3_map_prg(map, 0, bank);
+        }
+
+    } else {
+        if (reg == 7) {
+            mmc3_map_prg(map, 1, bank);
+        } else {
+            mmc3_map_prg(map, 2, bank);
+        }
+    }
+
+}
+
+static inline uint8_t* mmc3_chr_bank(mmc3_mapper* map, int bank) {
+    return &map->chr_rom[bank << 10];
+}
+
+static inline void mmc3_map_2k_chr(mmc3_mapper* map,
+                                   int reg, int bank) {
+    MAP_LOG("CHR ROM: 2k $%04x <- bank %d + %d", reg << 10, bank & ~1, bank | 1);
+    map->chr_bank[reg + 0] = mmc3_chr_bank(map, bank & ~1);
+    map->chr_bank[reg + 1] = mmc3_chr_bank(map, bank | 1);
+}
+
+static inline void mmc3_map_1k_chr(mmc3_mapper* map,
+                                   int reg, int bank) {
+    MAP_LOG("CHR ROM: 1k $%04x <- bank %d", reg << 10, bank);
+    map->chr_bank[reg] = mmc3_chr_bank(map, bank);
+}
+
+static inline void mmc3_update_chr(mmc3_mapper* map, uint8_t bank) {
+    int reg = (map->bank_select & mmc3_Bank_Select_Reg);
+
+    if (!(map->bank_select & mmc3_Bank_Select_CHR)) {
+        if (1 >= reg) {
+            mmc3_map_2k_chr(map, reg * 2, bank);
+        } else {
+            mmc3_map_1k_chr(map, reg + 2, bank);
+        }
+
+    } else {
+        if (1 >= reg) {
+            mmc3_map_2k_chr(map, 4 + (reg * 2), bank);
+        } else {
+            mmc3_map_1k_chr(map, reg - 2, bank);
+        }
+    }
+}
+
+static inline void mmc3_update_vram(mmc3_mapper* map) {
+    if (!(map->flags & mmc3_Flag_Horizontal)) {
+        // Vertical mirroring
+        MAP_LOG("Vertical mirroring");
+        map->vram_bank[0] = map->vram_bank[2] = map->vram[0];
+        map->vram_bank[1] = map->vram_bank[3] = map->vram[1];
+    } else {
+        // Horizontal mirroring
+        MAP_LOG("Horizontal mirroring");
+        map->vram_bank[0] = map->vram_bank[1] = map->vram[0];
+        map->vram_bank[2] = map->vram_bank[3] = map->vram[1];
+    }
+}
+
+static void mmc3_reset(nes_mapper* nes_map) {
+    mmc3_mapper* map = (mmc3_mapper*)nes_map->data;
+    map->flags       = 0;
+    map->bank_select = 0;
+    map->irq_count   = 0;
+    map->irq_latch   = 0;
+    map->prg_bank[3] = mmc3_prg_bank(map, map->prg_rom_banks - 1);
+    mmc3_update_vram(map);
+}
+
+static int mmc3_init(nes_mapper* nes_map, nes_cart* cart) {
+    mmc3_mapper* map = calloc(1, sizeof(mmc3_mapper));
+    nes_map->data = map;
+    if (NULL != map) {
+        map->prg_rom       = cart->prg_rom;
+        map->prg_rom_banks = cart->prg_rom_banks * 2;
+        map->chr_rom       = cart->chr_rom;
+        map->chr_rom_banks = cart->chr_rom_banks * 4;
+        map->flags = (cart->flags & Cart_Flag_Horizontal) ?
+                     mmc3_Flag_Horizontal : 0;
+        mmc3_reset(nes_map);
+    }
+    return (NULL == map ? -1 : 0);
+}
+
+static void mmc3_done(nes_mapper* nes_map) {
+    free(nes_map->data);
+}
+
+static inline uint8_t* mmc3_prg_addr(mmc3_mapper* map,
+                                     uint16_t addr) {
+    return &(map->prg_bank[(addr >> 13) & 3][addr & 0x1FFFU]);
+}
+
+static inline uint8_t* mmc3_wram_addr(mmc3_mapper* map,
+                                      uint16_t addr) {
+    return &(map->wram[addr & 0x1FFFU]);
+}
+
+static uint8_t mmc3_read(nes_mapper* nes_map, uint16_t addr) {
+    uint8_t* ptr = NULL;
+    mmc3_mapper* map = (mmc3_mapper*)nes_map->data;
+    if (addr >= nes_mem_rom_start) {
+        ptr = mmc3_prg_addr(map, addr);
+    } else if ( addr >= nes_mem_wram_start &&
+                (map->flags & mmc3_Flag_WRAM_Enabled)) {
+        ptr = mmc3_wram_addr(map, addr);
+        MAP_LOG("WRAM: $%04x > %02x", addr, *ptr);
+    }
+
+    uint8_t val = (NULL == ptr ? 0 : *ptr);
+
+//    MAP_LOG("$%04x -> %04lx > %02x", addr, ptr - map->prg_rom, val);
+
+    return val;
+}
+
+static void mmc3_write(nes_mapper* nes_map,
+                       uint16_t addr, uint8_t val) {
+    mmc3_mapper* map = (mmc3_mapper*)nes_map->data;
+
+    if (addr >= nes_mem_rom_start) MAP_LOG("$%04x < %02x", addr, val);
+
+    if (addr < nes_mem_wram_start) {
+        // Nothing prior to WRAM
+
+    } else if (addr < nes_mem_rom_start) {
+        if (    (map->flags & mmc3_Flag_WRAM_Enabled) &&
+                !(map->flags & mmc3_Flag_WRAM_Protect)) {
+//            MAP_LOG("WRAM: $%04x < %02x", addr, val);
+            *(mmc3_wram_addr(map, addr)) = val;
+        }
+
+    } else if (addr < 0xA000U) {
+        if (addr & 1) {
+            // Bank data
+            if ((map->bank_select & mmc3_Bank_Select_Reg) >= 6) {
+                mmc3_update_prg(map, val);
+            } else {
+                mmc3_update_chr(map, val);
+            }
+        } else {
+            // Bank select
+            map->bank_select = val;
+            mmc3_update_rom_mode(map);
+        }
+
+    } else if (addr < 0xC000U) {
+        if (addr & 1) {
+            MAP_LOG("WRAM %s, %s", (val & mmc3_Flag_WRAM_Enabled) ? "enabled" : "disabled", (val & mmc3_Flag_WRAM_Protect) ? "protected" : "writable");
+            // WRAM protection
+            map->flags &= ~(mmc3_Flag_WRAM_Enabled |
+                            mmc3_Flag_WRAM_Protect);
+            map->flags |= (val & (mmc3_Flag_WRAM_Enabled |
+                                  mmc3_Flag_WRAM_Protect));
+        } else {
+            // Mirroring
+            map->flags &= ~mmc3_Flag_Horizontal;
+            map->flags |= (val & mmc3_Flag_Horizontal);
+            mmc3_update_vram(map);
+        }
+
+    } else if (addr < 0xE000U) {
+        if (addr & 1) {
+            // TODO: IRQ reload
+        } else {
+            // TODO: IRQ latch
+        }
+
+    } else {
+        if (addr & 1) {
+            // TODO: IRQ enable
+        } else {
+            // TODO: IRQ disable
+        }
+    }
+}
+
+static uint8_t* mmc3_chr_addr(nes_mapper* nes_map,
+                              uint16_t addr) {
+    mmc3_mapper* map = (mmc3_mapper*)nes_map->data;
+    return &map->chr_bank[(addr >> 10) & 7][addr & 0x3FFU];
+}
+
+static uint8_t* mmc3_vram_addr(nes_mapper* nes_map,
+                               uint16_t addr) {
+    mmc3_mapper* map = (mmc3_mapper*)nes_map->data;
+    return &map->vram_bank[(addr >> 10) & 3][addr & 0x3FFU];
+}
+
+static void mmc3_chr_write(nes_mapper* map,
+                           uint16_t addr, uint8_t val) {
+    // ROM only.
+    printf("CHR ROM Write: $%04x < %02x\n", addr, val);
+}
+
+nes_mapper mapper_mmc3 = {
+    .init = mmc3_init,
+    .reset = mmc3_reset,
+    .done = mmc3_done,
+    .read = mmc3_read,
+    .write = mmc3_write,
+    .chr_addr = mmc3_chr_addr,
+    .vram_addr = mmc3_vram_addr,
+    .chr_write = mmc3_chr_write,
+};
diff --git a/src/mapper.c b/src/mapper.c
index 2eadbde..7828047 100644
--- a/src/mapper.c
+++ b/src/mapper.c
@@ -4,10 +4,12 @@
 extern nes_mapper mapper_nrom;
 extern nes_mapper mapper_mmc1;
 extern nes_mapper mapper_cnrom;
+extern nes_mapper mapper_mmc3;
 
 
 nes_mapper* nes_mappers[256] = {
     [  0] = &mapper_nrom,
     [  1] = &mapper_mmc1,
     [  3] = &mapper_cnrom,
+    [  4] = &mapper_mmc3,
 };
diff --git a/src/mapper.h b/src/mapper.h
index 0f6fd26..1aacb06 100644
--- a/src/mapper.h
+++ b/src/mapper.h
@@ -4,6 +4,9 @@
 #include <stdint.h>
 
 
+#define MAP_LOG(fmt, ...) printf("MAP: " fmt "\n" __VA_OPT__(,) __VA_ARGS__)
+
+
 struct nes_cart_t;
 
 typedef struct nes_mapper_t {