Advanced Configuration and Power Interface Specification

ACPI Hardware Specification
SLP_ENx bit. The system will then enter a sleeping state; when one of the enabled wake events
occurs, it will transition the system back to the working state (for more information, see Section 16,
“Waking and Sleeping”).
Another global state transition option while in the G0 “working” state is to enter the G2 “soft off” or
the G3 “mechanical off” state. These transitions represent a controlled transition that allows OSPM
to bring the system down in an orderly fashion (unloading applications, closing files, and so on). The
policy for these types of transitions can be associated with the ACPI power button, which when
pressed generates an event to the power button driver. When OSPM is finished preparing the
operating environment for a power loss, it will either generate a pop-up message to indicate to the
user to remove power, in order to enter the G3 “Mechanical Off” state, or it will initiate a G2 “softoff”
transition by writing the value of the S5 “soft off” system state to the SLP_TYPx register and
setting the SLP_EN bit.
The G1 sleeping state is represented by four possible sleeping states that the hardware can support.
Each sleeping state has different power and wake latency characteristics. The sleeping state differs
from the working state in that the user’s operating environment is frozen in a low-power state until
awakened by an enabled wake event. No work is performed in this state, that is, the processors are
not executing instructions. Each system sleeping state has requirements about who is responsible for
system context and wake sequences (for more information, see Section 16, Waking and Sleeping”).
The G2 “soft off” state is an OS initiated system shutdown. This state is initiated similar to the
sleeping state transition (SLP_TYPx is set to the S5 value and setting the SLP_EN bit initiates the
sequence). Exiting the G2 soft-off state requires rebooting the system. In this case, an ACPI-only
machine will re-enter the G0 state directly (hardware returns the SCI_EN bit set), while an ACPI/
Legacy machine transitions to the Legacy state (SCI_EN bit is clear).
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