Interrupt Service Routine (ISR)

I now have the board reading a I/O line state in a loop. This is intended to be hooked up to a reed switch sensor and then calculate speed from wheel rotation. This works on the bench and I can rapidily press a test button and see the resulting km/hr reading. However I suspect that I need an ISR routine as I don't want to risk missing a reed switch trigger when going fast. Here are some cheat links...