I know ISR stands for interrupt-service routine, but what does LSR mean? I hadn't seen that abbreviation anywhere else, so I ran a Google search and located several glossaries and a white paper by ...

The code itself is defined outside of the main code in what is known as an Interrupt Service Routine. It’s not at all obvious when the ISR is going to run; it runs when it’s triggered.

The processor would stop what it was doing to execute the corresponding interrupt service routines and then resume its original task.