Hibernate (OS feature)

Hibernation (or suspend to disk) in computing is powering down a computer while retaining its state. Upon hibernation, the computer saves the contents of its random access memory (RAM) to a hard disk or other non-volatile storage. Upon resumption, the computer is exactly as it was before entering hibernation. Like a powered-down system, the power source from a system in hibernation can usually be removed without severe state loss risk.

Hibernation saves electrical power. After hibernating, the hardware is powered down like a regular shutdown. Hibernation is a means of avoiding the burden of saving unsaved data before shutting down and restoring all running programs after powering back on. Hibernation is used in laptops, which have limited battery power available. It can be set to happen automatically on a low battery alarm. Most desktops also support hibernation, mainly as a general energy saving measure.

Many systems also support a low-power sleep mode in which the processing functions of the machine are powered down, using a little power to preserve the contents of RAM and support waking up. Instantaneous resumption is one of the advantages of sleep mode over hibernation. A hibernated system must start up and read data back to RAM, which typically takes time. A system in sleep mode only needs to power up the CPU and display, which is almost instantaneous. On the other hand, a system in sleep mode still consumes power to keep the data in the RAM. Detaching power from a system in sleep mode results in data loss, while cutting the power of a system in hibernation has no risk; the hibernated system can resume when and if the power is restored. Both shut down and hibernated systems may consume standby power unless they are unplugged.

Sleep mode and hibernation can be combined: The contents of RAM are copied to the non-volatile storage and the computer enters sleep mode. This approach combines the benefits of sleep mode and hibernation: The machine can resume instantaneously, and its state, including open and unsaved files, survives a power outage. Hybrid sleep consumes as much power as sleep mode while hibernation powers down the computer.

Early implementations of hibernation used the BIOS, but modern operating systems usually handle hibernation. Hibernation is defined as sleeping mode S4 in the ACPI specification.

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