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class HardwareSerial : public Stream
{
  protected:
    volatile uint8_t * const _ubrrh;
    volatile uint8_t * const _ubrrl;
    volatile uint8_t * const _ucsra;
    volatile uint8_t * const _ucsrb;
    volatile uint8_t * const _ucsrc;
    volatile uint8_t * const _udr;
    // Has any byte been written to the UART since begin()
    bool _written;

    volatile rx_buffer_index_t _rx_buffer_head;
    volatile rx_buffer_index_t _rx_buffer_tail;
    volatile tx_buffer_index_t _tx_buffer_head;
    volatile tx_buffer_index_t _tx_buffer_tail;

    // Don't put any members after these buffers, since only the first
    // 32 bytes of this struct can be accessed quickly using the ldd
    // instruction.
    unsigned char _rx_buffer[SERIAL_RX_BUFFER_SIZE];
    unsigned char _tx_buffer[SERIAL_TX_BUFFER_SIZE];

  public:
    inline HardwareSerial(
      volatile uint8_t *ubrrh, volatile uint8_t *ubrrl,
      volatile uint8_t *ucsra, volatile uint8_t *ucsrb,
      volatile uint8_t *ucsrc, volatile uint8_t *udr);
    void begin(unsigned long baud) { begin(baud, SERIAL_8N1); }
    void begin(unsigned long, uint8_t);
    void end();
    virtual int available(void);
    virtual int peek(void);
    virtual int read(void);
    int availableForWrite(void);
    virtual void flush(void);
    virtual size_t write(uint8_t);
    inline size_t write(unsigned long n) { return write((uint8_t)n); }
    inline size_t write(long n) { return write((uint8_t)n); }
    inline size_t write(unsigned int n) { return write((uint8_t)n); }
    inline size_t write(int n) { return write((uint8_t)n); }
    using Print::write; // pull in write(str) and write(buf, size) from Print
    operator bool() { return true; }

    // Interrupt handlers - Not intended to be called externally
    inline void _rx_complete_irq(void);
    void _tx_udr_empty_irq(void);
};