Read memory address in target

def read_reg(self, addr):
        """ Read memory address in target """
        # we don't call check_command here because read_reg() function is called
        # when detecting chip type, and the way we check for success (STATUS_BYTES_LENGTH) is different
        # for different chip types (!)
        val, data = self.command(self.ESP_READ_REG, struct.pack('<I', addr))
        if byte(data, 0) != 0:
            raise FatalError.WithResult("Failed to read register address %08x" % addr, data)
        return val