The example here is based on
1) processigng serial input is more important than running "the main"
2) processigng the serial input characters takes (sometimes) more time than what is between two characters arriving to the UART.

This actually occured. The problem was that the sender needed a response 'immediately' after the last character sent and another process made processing the input data in the main sometimes send the response too late.

The solution was to set all interrupts except T0 (timer 0 was not used in this app) to high priority and move the processing of the input data to the T0 ISR.

The UART ISR
stuff char in the ring buffer
set TF0
get out

the T0 ISR
while (characters in the ring buffer to fetch)
{
....
}
get out

Erik