Snippets to implement flow control using c64 user port modems in C

io.c

/** In io_base.c - io_recv_serial is called as part of main loop */

/**
 * io_recv_serial() - Receive and interpret serial data.
 */
void io_recv_serial(void)
{
  recv_buffer_size=io_serial_buffer_size();
  if (recv_buffer_size>XOFF_THRESHOLD && xoff_enabled==false)
    {
      // Ask modem to stop receiving
      io_recv_serial_flow_off();
    }
  else if (recv_buffer_size<XON_THRESHOLD && xoff_enabled==true)
    {
      // Ask modem to start receiving
      io_recv_serial_flow_on();
    }

  if (ser_get(&ch)==SER_ERR_OK)
    {
      // Detect and strip IAC escapes (two consecutive bytes of 0xFF)
      if (ch==0xFF && lastch == 0xFF)
	{
	  lastch=0x00;
	}
      else
	{
	  lastch=ch;
	  ShowPLATO(&ch,1);
	}
    }
}

/** in c64/io.c */

extern uint8_t xoff_enabled;

/**
 * Return the serial buffer size
 */
uint8_t io_serial_buffer_size(void)
{
  return PEEK(0x29B)-PEEK(0x29C)&0xff;
}

/**
 * io_recv_serial_flow_off() - Tell modem to stop receiving.
 */
void io_recv_serial_flow_off(void)
{
  // for now, assume user port.
  POKE(0xD020,0);
  POKE(0xDD01,0);
}

/**
 * io_recv_serial_flow_on() - Tell modem to stop receiving.
 */
void io_recv_serial_flow_on(void)
{
  // For now, assume user port.
  POKE(0xD020,14);
  POKE(0xDD01,1);
}