I know that if I put one of the two interrupts on my uno set to LOW it will fire over and over again as long as that pin stays LOW. When it returns from the ISR, does it execute an instruction from the main code before it goes back into the ISR again? Or will it freeze the main program and run the ISR like a loop?
LOW it will fire over and over again as long as that pin stays LOW
cli ();... sei (); sleep_cpu ();
Yes, but most probably 1/3 of them will be lost.
A LOW interrupt is not queued, and thus nothing is lost.