The general approach most computer vendors take to battery conservation is to design the CPU,
memory, and I/O devices to have multiple states: on, sleeping, hiberating, and off. To use the
device, it must be on. When the device will not be needed for a short time, it can be put to sleep,
which reduces energy consumption. When it is not expected to be needed for a longer interval, it
can be made to hiberate, which reduces energy consumption even more. The trade-off here is that
getting a device out of hiberating often take more time and energy than getting it out of sleep state.
Finally, when a device is off, it does nothing and consumes no power. Not all devices have all these
states, but when they do, it is up to the operating system to manage the state transition at the
right moments.
Power management brings up a number of questions that the operating system must deal with. Many
of them deal with resource hiberation --- selectively and temporarily turning off devices, or at least
reducing their power consumption when they are idle. Questions that must be answered include these:
Which devices can be controlled? Are they off/on, or do they have intermediate states? How much
power is saved in the low-power state? Is energy expended to restart the device? Must some context
be saced when going to a low-power state? How long does it take to go back to full power? Of course,
the answers to these questions vary from device to device, so the operating system must be able to
deal with a range of possibilities.