and no devices are in a power saving mode. APM Enabled: The computer is powered on, and APM is controlling device power management as needed. APM Standby: Most devices are in their low-power state, the CPU is slowed or stopped, and the system state is saved. The computer can be returned to its former state quickly (in response to activity such as the user pressing a key on the keyboard). APM Suspend: Most devices are powered off, but the system state is saved. The computer can be returned to its former state, but takes a relatively long time. (Hibernation is a special form of the APM Suspend state). Off: The computer is turned off. Device power states APM also defines power states that APM-aware hardware can implement. There is no requirement that an APM-aware device implement all states. The four states are: Device On: The device is in full power mode. Device Power Managed: The device is still powered on, but some functions may not be available, or may have reduced performance. Device Low Power: The device is not working. Power is maintained so that the device may be 'woken up'. Device Off: The device is powered off. CPU The CPU core (defined in APM as the CPU clock, cache, system bus and system timers) is treated specially in APM, as it is the last device to be powered down, and the first device to be powered back up. The CPU core is always controlled through the APM BIOS (there is no option to control it through a driver). Drivers can use APM function calls to notify the BIOS about CPU usage, but it is up to the BIOS to act on this information; a driver cannot directly tell the CPU to go into a power saving state. In ATA drives The ATA specification and SATA specification defines APM provisions for hard drives, which specifies a trade-off between spin-down frequency and always-on performance. Unlike the BIOS-side APM, the ATA APM and SATA APM has never been deprecated. Aggressive spin-down frequencies may reduce drive lifespan by unnecessarily accumulating load cycles; most modern drives are specified to sustain 300,000 cycles and usually last at least 600,000. On the other hand, not spinning down the drive will cause extra power draw and heat generation; high temperatures also reduce the lifespan of hard drives. See also Active State Power Management - hardware power management