Argthtjtr

Something I've never quite understood is how a computer can operate without using all of its processing power. (Likely this confusion stems from a dreadful OS class in college.)

As far as I know, a thread (such as the OS's scheduler) has to be doing something at any given point in time. If there's nothing to do, it just loops until there is something to do. A loop of this nature, I would think, would always run as fast as it can, which would eat up all of a processor's computing power.

I'm guessing my misunderstanding is a wrong assumption that "if there's nothing to do, it just loops". I don't know what else it could do, however.

Computers have hardware timers that can be used to signal the processor when a particular length of time has passed. One such item is the High Precision Event Timer (HPET), and back in the history of things even the RTC could fulfil a similar purpose.

The CPU also has the ability to halt execution units using the HLT instruction and effectively put itself to sleep. No loops run and for all intents and purposes the CPU is dead except for hardware that waits for interrupt lines to be asserted.

Rather than spin-waiting through a loop the processor simply tells the hardware to alert it when a particular time has passed and then puts the core execution units to sleep. Once the time has passed the timer asserts an interrupt, which in turn wakes the CPU and execution starts again.

Low precision timers such as the RTC would work in a similar way, for every number of ticks you check the count on each tick and sleep until the next one comes along. It's a relatively low cost (processor power wise) way to implement wait states.

Mokubai gives the correct answer for how sleep generally works. But even if the computer were spin-looping, it could still use less power than normal. Different instructions use different amounts of power, depending on what they do. Reading from RAM will use more power than reading from the CPU cache, and while it's in a tiny spin loop it should just use cache. While it's looping the parts of the CPU that perform arithmetic or interact with external devices are inactive, so they don't use any power. If it's a laptop, the screen will be shut down, and that should be a huge power saving.

This all isn't as good as going almost totally dead except for the event timer. But I think some operating systems have "low power mode" that's in between normal running and sleeping, which may incorporate some of this.