2.2 Global System State Definitions
Global system states (Gx states) apply to the entire system and are visible to the user.
Global system states are defined by six principal criteria:
- Does application software run?
- What is the latency from external events to application response?
- What is the power consumption?
- Is an OS reboot required to return to a working state?
- Is it safe to disassemble the computer?
- Can the state be entered and exited electronically?
Following is a list of the system states:
- G3 Mechanical Off
A computer state that is entered and left by a mechanical means (for example, turning off the system’s
power through the movement of a large red switch). It is implied by the entry of this off state through a
mechanical means that no electrical current is running through the circuitry and that it can be worked
on without damaging the hardware or endangering service personnel. The OS must be restarted to
return to the Working state. No hardware context is retained. Except for the real-time clock, power
consumption is zero. - G2/S5 Soft Off
A computer state where the computer consumes a minimal amount of power. No user mode or system
mode code is run. This state requires a large latency in order to return to the Working state. The
system’s context will not be preserved by the hardware. The system must be restarted to return to the
Working state. It is not safe to disassemble the machine in this state. - G1 Sleeping
A computer state where the computer consumes a small amount of power, user mode threads are not
being executed, and the system “appears” to be off (from an end user’s perspective, the display is off,
and so on). Latency for returning to the Working state varies on the wake environment selected prior to
entry of this state (for example, whether the system should answer phone calls). Work can be resumed
without rebooting the OS because large elements of system context are saved by the hardware and the
rest by system software. It is not safe to disassemble the machine in this state. - G0 Working
A computer state where the system dispatches user mode (application) threads and they execute. In this
state, peripheral devices (peripherals) are having their power state changed dynamically. The user can
select, through some UI, various performance/power characteristics of the system to have the software
optimize for performance or battery life. The system responds to external events in real time. It is not
safe to disassemble the machine in this state. - S4 Non-Volatile Sleep
A special global system state that allows system context to be saved and restored (relatively slowly)
when power is lost to the motherboard. If the system has been commanded to enter S4, the OS will
write all system context to a file on non-volatile storage media and leave appropriate context markers.
The machine will then enter the S4 state. When the system leaves the Soft Off or Mechanical Off state,
transitioning to Working (G0) and restarting the OS, a restore from a NVS file can occur. This will
only happen if a valid non-volatile sleep data set is found, certain aspects of the configuration of the
machine have not changed, and the user has not manually aborted the restore. If all these conditions are
met, as part of the OS restarting, it will reload the system context and activate it. The net effect for the
user is what looks like a resume from a Sleeping (G1) state (albeit slower). The aspects of the machine
configuration that must not change include, but are not limited to, disk layout and memory size. It
might be possible for the user to swap a PC Card or a Device Bay device, however.
Notice that for the machine to transition directly from the Soft Off or Sleeping states to S4, the system
context must be written to non-volatile storage by the hardware; entering the Working state first so that
the OS or BIOS can save the system context takes too long from the user’s point of view. The
transition from Mechanical Off to S4 is likely to be done when the user is not there to see it.
Because the S4 state relies only on non-volatile storage, a machine can save its system context for an
arbitrary period of time (on the order of many years)
2.3 Device Power State Definitions
Device power states are states of particular devices; as such, they are generally not visible to the user. For
example, some devices may be in the Off state even though the system as a whole is in the Working state.
Device states apply to any device on any bus. They are generally defined in terms of four principal criteria:
Power consumption. How much power the device uses.
Device context. How much of the context of the device is retained by the hardware. The OS is
responsible for restoring any lost device context (this may be done by resetting the device).
Device driver. What the device driver must do to restore the device to full on.
Restore time. How long it takes to restore the device to full on.
The device power states are defined below, although very generically. Many devices do not have all four
power states defined. Devices may be capable of several different low-power modes, but if there is no userperceptible difference between the modes, only the lowest power mode will be used. The Device Class
Power Management Specifications, included in Appendix A of this specification, describe which of these
power states are defined for a given type (class) of device and define the specific details of each power state
for that device class. For a list of the available Device Class Power Management Specifications, see
“Appendix A: Device Class Specifications.”
- D3 (Off)
Power has been fully removed from the device. The device context is lost when this state is entered, so
the OS software will reinitialize the device when powering it back on. Since device context and power
are lost, devices in this state do not decode their address lines. Devices in this state have the longest
restore times. All classes of devices define this state. - D3hot
The meaning of the D3hot State is defined by each device class. Devices in the D3hot State are
required to be software enumerable. In general, D3hot is expected to save more power and optionally
preserve device context. If device context is lost when this state is entered, the OS software will
reinitialize the device when transitioning to D0. Devices in this state can have long restore times. All
classes of devices define this state.
NOTE: The D3hot state differs from the D3 state in two distinct parameters; the main power rail is
present and software can access a device in D3hot. For devices that support both D3hot and D3
exposed to OSPM via _PR3, device software/drivers must always assume OSPM will target D3and
must assume device context will be lost. - D2
The meaning of the D2 Device State is defined by each device class. Many device classes may not
define D2. In general, D2 is expected to save more power and preserve less device context than D1 or
D0. Buses in D2 may cause the device to lose some context (for example, by reducing power on the
bus, thus forcing the device to turn off some of its functions). - D1
The meaning of the D1 Device State is defined by each device class. Many device classes may not
define D1. In general, D1 is expected to save less power and preserve more device context than D2. - D0 (Fully-On)
This state is assumed to be the highest level of power consumption. The device is completely active
and responsive, and is expected to remember all relevant context continuously.
2.4 Sleeping State Definitions
Sleeping states (Sx states) are types of sleeping states within the global sleeping state, G1. The Sx states are
briefly defined below. For a detailed definition of the system behavior within each Sx state, see section
7.3.4, “System _Sx States.” For a detailed definition of the transitions between each of the Sx states, see
section 15.1, “Sleeping States.”
- S1 Sleeping State
The S1 sleeping state is a low wake latency sleeping state. In this state, no system context is lost (CPU
or chip set) and hardware maintains all system context. - S2 Sleeping State
The S2 sleeping state is a low wake latency sleeping state. This state is similar to the S1 sleeping state
except that the CPU and system cache context is lost (the OS is responsible for maintaining the caches
and CPU context). Control starts from the processor’s reset vector after the wake event. - S3 Sleeping State
The S3 sleeping state is a low wake latency sleeping state where all system context is lost except
system memory. CPU, cache, and chip set context are lost in this state. Hardware maintains memory
context and restores some CPU and L2 configuration context. Control starts from the processor’s reset
vector after the wake event. - S4 Sleeping State
The S4 sleeping state is the lowest power, longest wake latency sleeping state supported by ACPI. In
order to reduce power to a minimum, it is assumed that the hardware platform has powered off all
devices. Platform context is maintained. - S5 Soft Off State
The S5 state is similar to the S4 state except that the OS does not save any context. The system is in
the “soft” off state and requires a complete boot when it wakes. Software uses a different state value to
distinguish between the S5 state and the S4 state to allow for initial boot operations within the BIOS to
distinguish whether or not the boot is going to wake from a saved memory image.
2.5 Processor Power State Definitions
Processor power states (Cx states) are processor power consumption and thermal management states within
the global working state, G0. The Cx states possess specific entry and exit semantics and are briefly defined
below. For a more detailed definition of each Cx state, see section 8.1, “Processor Power States.”Definition of Terms 43
- C0 Processor Power State
While the processor is in this state, it executes instructions. - C1 Processor Power State
This processor power state has the lowest latency. The hardware latency in this state must be low
enough that the operating software does not consider the latency aspect of the state when deciding
whether to use it. Aside from putting the processor in a non-executing power state, this state has no
other software-visible effects. - C2 Processor Power State
The C2 state offers improved power savings over the C1 state. The worst-case hardware latency for
this state is provided via the ACPI system firmware and the operating software can use this
information to determine when the C1 state should be used instead of the C2 state. Aside from putting
the processor in a non-executing power state, this state has no other software-visible effects. - C3 Processor Power State
The C3 state offers improved power savings over the C1 and C2 states. The worst-case hardware
latency for this state is provided via the ACPI system firmware and the operating software can use this
information to determine when the C2 state should be used instead of the C3 state. While in the C3
state, the processor’s caches maintain state but ignore any snoops. The operating software is
responsible for ensuring that the caches maintain coherency.
2.6 Device and Processor Performance State Definitions
Device and Processor performance states (Px states) are power consumption and capability states within the
active/executing states, C0 for processors and D0 for devices. The Px states are briefly defined below. For a
more detailed definition of each Px state from a processor perspective, see section 8.4.4, “Processor
Performance Control.” For a more detailed definition of each Px state from a device perspective see section
3.6, “Device and Processor Performance States,” and the device class specifications in Appendix A.
- P0 Performance State
While a device or processor is in this state, it uses its maximum performance capability and may
consume maximum power. - P1 Performance State
In this performance power state, the performance capability of a device or processor is limited below
its maximum and consumes less than maximum power. - Pn Performance State
In this performance state, the performance capability of a device or processor is at its minimum level
and consumes minimal power while remaining in an active state. State n is a maximum number and is
processor or device dependent. Processors and devices may define support for an arbitrary number of
performance states not to exceed 16.
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