IBM ThinkPad T21 Hardware Maintenance Manual page 37

v If you are using the ACPI operating system and have
defined one of the following actions as the event that
causes the system to go into hibernation mode, perform
that action.
– Closing the lid.
– Pressing the power button.
– Pressing Fn+F4 keys.
Also, the computer goes into hibernation mode
automatically in either of the following conditions:
v If a "hibernation time" has been set on the timer, and the
user does not do any operation with the keyboard, the
TrackPoint, the hard-disk drive, the parallel connector, or
the diskette drive within that time.
v If the timer conditions are satisfied in suspend mode.
v If you are using the APM operating system and have set
the mode to Hibernate when battery becomes low,
and the battery charge becomes critically low.
When the power is turned on, the computer returns from
hibernation mode and resumes operation. The hibernation
file in the boot record on the hard-disk drive is read, and
system status is restored from the hard-disk drive.
Related service information
General descriptions
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