mmSwitchRegister() finished.

src/mmc.js

   
   mmSwitchRegister(){
     return (function(byte){
-      // TODO: Check the bits to determin which bank and bank index is being switched on.
+      let mmcidx = (byte & 0xF0) >> 4;
+      if (mmcidx < this.__banks.length){
+        this.__banks[mmcidx].idx = (byte & 0x0F);
+      }
     }).bind(this);
   }
 }