Assuming the hardware is then fast enough to run at the full frame rate, what's it doing once it's done with everything that needed being done? Just burning cycles in a busy loop with no progress?
Typically, yes. It didn't matter back in the day when the CPU had no way to enter an idle state to begin with. Wikipedia says that Intel CPUs didn't actually enter a low-power state on HLT until 1994 [1].
