diff --git a/src/cpu.js b/src/cpu.js
index c5a012d9..db9519cc 100644
--- a/src/cpu.js
+++ b/src/cpu.js
@@ -143,7 +143,7 @@ CPU.prototype = {
       this.irqRequested = false;
     }
 
-    var opinf = this.opdata[this.nes.mmap.load(this.REG_PC + 1)];
+    var opinf = this.opdata[this.loadFromCartridge(this.REG_PC + 1)];
     var cycleCount = opinf >> 24;
     var cycleAdd = 0;
 
@@ -264,8 +264,8 @@ CPU.prototype = {
             (this.mem[(addr & 0xff00) | (((addr & 0xff) + 1) & 0xff)] << 8); // Read from address given in op
         } else {
           addr =
-            this.nes.mmap.load(addr) +
-            (this.nes.mmap.load(
+            this.loadFromCartridge(addr) +
+            (this.loadFromCartridge(
               (addr & 0xff00) | (((addr & 0xff) + 1) & 0xff)
             ) <<
               8);
@@ -1035,11 +1035,21 @@ CPU.prototype = {
     return cycleCount;
   },
 
+  loadFromCartridge: function(addr) {
+    var value = this.nes.mmap.load(addr);
+
+    if (this.nes.gameGenie.enabled) {
+      value = this.nes.gameGenie.applyCodes(addr, value);
+    }
+
+    return value;
+  },
+
   load: function(addr) {
     if (addr < 0x2000) {
       return this.mem[addr & 0x7ff];
     } else {
-      return this.nes.mmap.load(addr);
+      return this.loadFromCartridge(addr);
     }
   },
 
@@ -1047,7 +1057,7 @@ CPU.prototype = {
     if (addr < 0x1fff) {
       return this.mem[addr & 0x7ff] | (this.mem[(addr + 1) & 0x7ff] << 8);
     } else {
-      return this.nes.mmap.load(addr) | (this.nes.mmap.load(addr + 1) << 8);
+      return this.loadFromCartridge(addr) | (this.loadFromCartridge(addr + 1) << 8);
     }
   },
 
@@ -1105,14 +1115,14 @@ CPU.prototype = {
       this.push(status);
 
       this.REG_PC_NEW =
-        this.nes.mmap.load(0xfffa) | (this.nes.mmap.load(0xfffb) << 8);
+        this.loadFromCartridge(0xfffa) | (this.loadFromCartridge(0xfffb) << 8);
       this.REG_PC_NEW--;
     }
   },
 
   doResetInterrupt: function() {
     this.REG_PC_NEW =
-      this.nes.mmap.load(0xfffc) | (this.nes.mmap.load(0xfffd) << 8);
+      this.loadFromCartridge(0xfffc) | (this.loadFromCartridge(0xfffd) << 8);
     this.REG_PC_NEW--;
   },
 
@@ -1125,7 +1135,7 @@ CPU.prototype = {
     this.F_BRK_NEW = 0;
 
     this.REG_PC_NEW =
-      this.nes.mmap.load(0xfffe) | (this.nes.mmap.load(0xffff) << 8);
+      this.loadFromCartridge(0xfffe) | (this.loadFromCartridge(0xffff) << 8);
     this.REG_PC_NEW--;
   },
 
diff --git a/src/gg.js b/src/gg.js
new file mode 100644
index 00000000..ef48b83e
--- /dev/null
+++ b/src/gg.js
@@ -0,0 +1,123 @@
+
+var GG = function() {
+  this.patches = [];
+  this.enabled = true;
+};
+
+var LETTER_VALUES = 'APZLGITYEOXUKSVN';
+
+function toDigit(letter) {
+  return LETTER_VALUES.indexOf(letter);
+}
+
+function toLetter(digit) {
+  return LETTER_VALUES.substr(digit, 1);
+}
+
+function toHex(n, width) {
+  var s = n.toString(16);
+  return '0000'.substring(0, width - s.length) + s;
+}
+
+
+GG.prototype = {
+  setEnabled: function(enabled) {
+    this.enabled = enabled;
+  },
+
+  addCode: function(code) {
+    this.patches.push(this.decode(code));
+  },
+
+  addPatch: function(addr, value, key) {
+    this.patches.push({addr: addr, value: value, key: key});
+  },
+
+  applyCodes: function(addr, value) {
+    if (!this.enabled) return value;
+
+    for (var i = 0; i < this.patches.length; ++i) { // TODO: optimize data structure?
+      if (this.patches[i].addr === (addr & 0x7fff)) {
+        if (this.patches[i].key === undefined || this.patches[i].key === value) {
+          return this.patches[i].value;
+        }
+      }
+    }
+    return value;
+  },
+
+  decode: function(code) {
+    if (code.indexOf(':') !== -1) return this.decodeHex(code);
+
+    var digits = code.toUpperCase().split('').map(toDigit);
+
+    var value = ((digits[0] & 8) << 4) + ((digits[1] & 7) << 4) + (digits[0] & 7);
+    var addr = ((digits[3] & 7) << 12) + ((digits[4] & 8) << 8) + ((digits[5] & 7) << 8) +
+        ((digits[1] & 8) << 4) + ((digits[2] & 7) << 4) + (digits[3] & 8) + (digits[4] & 7);
+    var key;
+
+    if (digits.length === 8) {
+      value += (digits[7] & 8);
+      key = ((digits[6] & 8) << 4) + ((digits[7] & 7) << 4) + (digits[5] & 8) + (digits[6] & 7);
+    } else {
+      value += (digits[5] & 8);
+    }
+
+    var wantskey = !!(digits[2] >> 3);
+
+    return { value: value, addr: addr, wantskey: wantskey, key: key };
+  },
+
+  encodeHex: function(addr, value, key, wantskey) {
+    var s = toHex(addr, 4) + ':' + toHex(value, 2);
+
+    if (key !== undefined || wantskey) {
+      s += '?';
+    }
+
+    if (key !== undefined) {
+      s += toHex(key, 2);
+    }
+
+    return s;
+  },
+
+  decodeHex: function(s) {
+    var match = s.match(/([0-9a-fA-F]+):([0-9a-fA-F]+)(\?[0-9a-fA-F]*)?/);
+    if (!match) return null;
+
+    var addr = parseInt(match[1], 16);
+    var value = parseInt(match[2], 16);
+    var wantskey = match[3] !== undefined;
+    var key = (match[3] !== undefined && match[3].length > 1) ? parseInt(match[3].substring(1), 16) : undefined;
+
+    return { value: value, addr: addr, wantskey: wantskey, key: key };
+  },
+
+  encode: function(addr, value, key, wantskey) {
+    var digits = Array(6);
+
+    digits[0] = (value & 7) + ((value >> 4) & 8);
+    digits[1] = ((value >> 4) & 7) + ((addr >> 4) & 8);
+    digits[2] = ((addr >> 4) & 7);
+    digits[3] = (addr >> 12) + (addr & 8);
+    digits[4] = (addr & 7) + ((addr >> 8) & 8);
+    digits[5] = ((addr >> 8) & 7);
+
+    if (key === undefined) {
+      digits[5] += value & 8;
+      if (wantskey) digits[2] += 8;
+    } else {
+      digits[2] += 8;
+      digits[5] += key & 8;
+      digits[6] = (key & 7) + ((key >> 4) & 8);
+      digits[7] = ((key >> 4) & 7) + (value & 8);
+    }
+
+    var code = digits.map(toLetter).join('');
+
+    return code;
+  },
+};
+
+module.exports = GG;
diff --git a/src/nes.js b/src/nes.js
index eedc6209..5bdd763f 100644
--- a/src/nes.js
+++ b/src/nes.js
@@ -2,6 +2,7 @@ var CPU = require("./cpu");
 var Controller = require("./controller");
 var PPU = require("./ppu");
 var PAPU = require("./papu");
+var GameGenie = require("./gg");
 var ROM = require("./rom");
 
 var NES = function(opts) {
@@ -34,6 +35,7 @@ var NES = function(opts) {
   this.cpu = new CPU(this);
   this.ppu = new PPU(this);
   this.papu = new PAPU(this);
+  this.gameGenie = new GameGenie();
   this.mmap = null; // set in loadROM()
   this.controllers = {
     1: new Controller(),