This is a library of functions for Atmel (now Microchip) processors in the AVR family, such as the ATmega8 and ATmega328p as made famous by the ubiquitous Arduino. This is in addition to, and complementing the functions provided by the main AVR library (AVRLIB). The key components are the RS232 ring buffers and the bootstrap mechanism.
The Bootstrap function provides a very simple (but effective) mechanism for being able to update the flash within the chip over the RS232 line. The code occupies about 230 words (460 bytes) at high memory, and once invoked, will disable interrupts, configure the serial port if necessary, and begin communicating with the programming software. For more information on the function, look at the source code and especially the comments in bootstrap.s.
The serial I/O is handled by two 32-character ring buffers, consuming 64 bytes of RAM, as well as four bytes to manage the two head and two tail pointers. In a nutshell, without getting into the workings of ring buffers, the idea is that you try to minimize the handshaking between the main code and the interrupt service routine. On output, the code in sioput.c is used to check if there is any space in the ring buffer. If there is, it just adds the character to the buffer and advances the head pointer. Note that only the sio_enqueue() function can update the head pointer so there is no danger of a collision. It also enables transmit interrupts, just in case they've been turned off. That code is in serinten.S.
When an interrupt occurs on the UDRE (USART Data Register Empty) vector, it should call sio_out() which is in sioint.S. The ISR will pull the next character off the ring buffer, send it out on the serial port, advance the tail pointer (of which it has exclusive ownership), and if the ring buffer is now empty, it will disable TX interrupts.
In this way, it is possible for the character output code to get up to 32 characters into the buffer, before it fills up. If the buffer fills up, the sio_enqueue() function can either wait for the buffer to drain, or return unsuccessfully, immediately. There is also a handy sio_putc() function which calls sio_enqueue(). It will also convert a newline (\n) to a CRLF sequence (\r\n).
On the input side, the ISR for RXC (USART Rx Complete) should be wired to sio_in() which is also in sioint.S. This function will attempt to place the received character into the receive ring. If the ring is full, it will silently drop the character. On the receive side, there is a matching function (sio_dequeue()), defined in sioget.c which will attempt to remove a character from the input ring. Again, the blockf argument specifies whether or not the function should wait until there is a character, or return immediately. The sio_iqueue_empty() can be called to check if the input queue is empty or not. As before, there is a helper function to couple this function with the AVRLIB I/O library. That function is sio_getc() and will block until a character is available on the serial line.
These two helper functions can be added to the AVRLIB I/O subsystem by making the following call during code initialization:
(void )fdevopen(sio_putc, sio_getc);