 <p>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 <a href="http://en.m.wikipedia.org/wiki/HLT" rel="nofollow">http://en.m.wikipedia.org/wiki/HLT</a> for details.</p> <p>Other architectures have similar instruction to give CPU a rest.</p>