That makes me curious; I wonder what on a modern PC relies on periodic interrupts, and would lock up the machine if they didn't occur. I know on the original PC and XT, DRAM refresh relied on interrupts but that stopped being the case with the AT:
Isn't preemption still interupt based. Without interrupts, there would be nothing to cause the CPU to stop executing the demo code. With SMP disabled, this means that nothing else will get a chance to run until the demo code yields itself (or re-enables interupts).