lonas:
I know that serial is being handle on interrupts in DUE - but data goes to buffer - I would like to handle data right away instad of polling from buffer.
You may want to do it that way, but its not likely to help you.
You have a need to process the serial at a particular time, as indicated by the interrupt happening? Then
just call the relevant handling function from the ISR (making sure its variables are volatile). It can then pull
stuff from the buffer.
If loop() is running often enough (nothing hogs the processor), just poll the buffer there and any other state
you need, no ISR needed, just keep your system from being overloaded and everything progresses nicely.
Maybe you are calling delay() somewhere? That's probably a problem worth solving if so.