ACPI Power Management States

These are the power management states as defined in the ACPI specification. I’ll go through them briefly but let's get a big picture of it first. The details of each and every state will be described in the following sections.

Global system states

• The entire system state which is visible to user.
• Divided into 4 states – G0, G1, G2, G3

Sleeping states

• They are the sleeping states resides in global system state G1 (except S5)
• Divided into 5 states – S1, S2, S3, S4, S5

Device Power states

• This power state is usually invisible to user.
• When a device is at ‘on’ state, another one might be in ‘off’ state.
• Divided into 4 states – D0, D1, D2, and D3.

CPU Power states

• Also known as CPU sleep states.
• It is within the global system state G0
• So far divided into 5 states – C0, C1, C2, C3, and C4.
• In future, there will be up to C6 states (Penryn).

CPU / Device Performance states

• CPU / Device power management when it is still active.
• Usually clock speed and voltage varies depend on workload.
• The amount of P-state is CPU / Device specific
• Eg. Higher clock ratio CPU will have more P-states than a lower clock ratio CPU

CPU Thermal Monitor

• It throttles the CPU to a lower performance state when temperature exceeds the threshold.
• In TM1, throttling is done via changing its duty cycle.
• In TM2, throttling is done via changing its clock speed and core voltage (P-state).