16. Waking and Sleeping

ACPI defines a mechanism to transition the system between the working state (G0) and a sleeping state (G1) or the soft-off (G2) state. During transitions between the working and sleeping states, the context of the user’s operating environment is maintained. ACPI defines the quality of the G1 sleeping state by defining the system attributes of four types of ACPI sleeping states (S1, S2, S3, and S4). Each sleeping state is defined to allow implementations that can tradeoff cost, power, and wake latencies. Additionally, ACPI defines the sleeping states such that an ACPI platform can support multiple sleeping states, allowing the platform to transition into a particular sleeping state for a predefined period of time and then transition to a lower power/higher wake latency sleeping state (transitioning through the G0 state) (See note below).

OSPM uses the RTC wakeup feature or the Time and Alarm Namespace
device to program in the time transition delay. Prior to sleeping, OSPM
will program the alarm to the closest (in time) wakeup event: either a
transition to a lower power sleeping state, or a calendar event (to run
some application).

ACPI defines a programming model that provides a mechanism for OSPM to initiate the entry into a sleeping or soft-off state (S1-S5); this consists of a 3 bit field SLP_TYPx (See note below) that indicates the type of sleep state to enter, and a single control bit SLP_EN to start the sleeping process. On HW-reduced ACPI systems, the register described by the SLEEP_CONTROL_REG field in the FADT is used instead of the fixed SLP_TYPx and SLP_EN register bit fields.

Notice that there can be two fixed PM1x_CNT registers, each pointing
to a different system I/O space region. Normally a register grouping
only allows a bit or bit field to reside in a single register group
instance (a or b); however, each platform can have two instances of the
SLP_TYP (one for each grouping register: a and b). The \_Sx control
method gives a package with two values: the first is the SLP_TYPa value
and the second is the SLP_TYPb value.


Systems containing processors without a hardware mechanism to place the processor in a low-power state may additionally require the execution of appropriate native instructions to place the processor in a low-power state after OSPM sets the SLP_EN bit. The hardware may implement a number of low-power sleeping states and then associate these states with the defined ACPI sleeping states (through the SLP_TYPx fields). The ACPI system firmware creates a sleeping object associated with each supported sleeping state (unsupported sleeping states are identified by the lack of the sleeping object). Each sleeping object contains two constant 3-bit values that OSPM will program into the SLP_TYPa and SLP_TYPb fields (in fixed register space), or, on HW-reduced ACPI platforms, a single 3-bit value that OSPM will write to the register specified by the FADT’s SLEEP_CONTROL_REG field.

On systems that are not HW-reduced ACPI platforms, an alternate mechanism for entering and exiting the S4 state is defined. This mechanism passes control to the platform runtime firmware to save and restore platform context. Context ownership is similar in definition to the S3 state, but hardware saves and restores the context of memory to non-volatile storage (such as a disk drive), and OSPM treats this as an S4 state with implied latency and power constraints. This alternate mechanism of entering the S4 state is referred to as the S4BIOS transition.

Prior to entering a sleeping state (S1-S4), OSPM will execute OEM-specific AML/ASL code contained in the _PTS (Prepare To Sleep) control method. One use of the _PTS control method is that it can indicate to the embedded controller what sleeping state the system will enter. The embedded controller can then respond by executing the proper power-plane sequencing upon sleep state entry.

The _WAK (Wake) control method is then executed. This control method again contains OEM-specific AML/ASL code. One use of the _WAK control method requests OSPM to check the platform for any devices that might have been added or removed from the system while the system was asleep. For example, a PC Card controller might have had a PC Card added or removed, and because the power to this device was off in the sleeping state, the status change event was not generated.

This section discusses the system initialization sequence of an ACPI-enabled platform. This includes the boot sequence, different wake scenarios, and an example to illustrate how to use the system address map reporting interfaces. This sequence is part of the ACPI event programming model.


HW-reduced ACPI platforms do not implement the Legacy Mode nor the S4BIOS state described below.

For detailed information on the power management control methods described above, see Power and Performance Management