How does a CPU idle (or run below 100%)?

Question

I first learned about how computers work in terms of a primitive single stored program machine.

Now I\'m learning about multitasking operating systems, scheduling, c

Answer 1

If speaking about x86 architecture when an operating system has nothing to do it can use HLT instruction. HLT instruction stops the CPU till next interrupt. See http://en.m.wikipedia.org/wiki/HLT for details.

Other architectures have similar instruction to give CPU a rest.