code: function( address ) {
  		switch( this.mirroring ) {
  		case HORIZONTAL:
  			if ( address >= 0x400 ) {
  				address -= 0x400;
  			}
  			if ( address >= 0x800 ) {
  				address -= 0x400;
  			}
  			break;
  		case VERTICAL:
  			address &= 0x07ff;
  			break;
  		case FOUR_SCREEN:
  			// we still don't implement any mappers that support four screen mirrroring
  			throw new Error("TODO, four screen mirroring");
  		case SINGLE_SCREEN_LOWER:
  		case SINGLE_SCREEN_UPPER:
  			address &= 0x3ff;
  
  			if ( this.mirroring === 4 ) {
  				address += 0x400;
  			}
  			break;
  		}
  
  		return address;
  	}
docs: Map a nametable address to our internal memory, taking mirroring into account.