Writing Interrupt Handlers in C
An interrupt handler is the only function in your program that nothing calls. You write it, you never reference it, and yet it runs — sometimes millions of times a second, at a moment you did not choose, on top of whatever the main program happened to be doing. That inversion is the whole difficulty. Every rule below follows from it: you cannot pass arguments to something nobody calls, you cannot return a value to nobody, and you cannot assume anything about the state of the code you interrupted.