What does volatile do, and when must you use it?

volatile tells the compiler that a variable's value can change outside the normal flow of the program, so it must perform every read and write to the actual memory location exactly as written, no caching the value in a register, no eliding "redundant" reads, no reordering relative to other volatile accesses. Two situations make it mandatory. First, memory-mapped hardware registers: the hardware updates a status register independently of your code, so a polling loop without volatile reads it once, caches it, and spins forever on the stale value. Second, variables shared between an ISR and the main flow: the optimizer can't see that the interrupt modifies the flag, so it may hoist the read out of a loop; volatile forces a re-read each iteration. Without it you get bugs that appear only at higher optimization levels, works in debug, breaks in release.