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PC Power Management Guide Rev. 2.0
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Computer performance have increased at an amazing rate in recent years, and unfortunately so does power consumption. An ultimate gaming system equipped with a quad-core processor, two NVIDIA GeForce 8800 Ultra, 4 sticks of DDR2 memory and a few hard drives can easily consume 200W without doing anything! To reduce power wastage, a few industry standards have been developed to make our computers work more efficiently.

In January 1992, Intel and Microsoft developed APM (Advanced Power Management) to manage power when a computer system is idling. Later in December 1996, the successor of APM – the Advanced Configuration and Power Interface (ACPI) specification was developed by Compaq, Microsoft, Intel, Phoenix and Toshiba as the industry open-standard power management interface. What's the difference? Let's take a look :

Advanced Power Management (APM)
Advanced Configuration and Power Interface (ACPI)

 • Cheap implementation, but not effective.
 • Application and driver send control to APM driver directly.
 • Device power is managed by its own driver.
 • Other hardware like CPU is managed by APM BIOS.
 • Power management state machine is done by APM BIOS since it is simple.

 • Implementation is more costly, but effective.
 • Application doesn't need to manage power.
 • Device driver uses ACPI to interface with hardware power management.
 • ACPI is abstract, thus OS and hardware can evolve separately.
 • Power management state machine is complex, hence handle by the operating system.

In this article, I will not go into APM as most PC use ACPI these days.



ACPI Power Management States

The Big Picture

Global System States (G-States)

G0 Working States
G1 Sleeping State
G2 Soft Off
G3 Mechanical Off

Sleeping States (S-States)

S1 State
S2 State
S3 State
S4 State

Device States (D-States)

D1 State
D2 State
D3 State

CPU Power States (C-States)

C0 State (Active)
C1 State (Halt)
C2 State (Stop Grant)
C3 State (Deep Sleep)
C4 State (Deeper Sleep)
C5 State
C6 State

C-States In Multi-Core Processors

CPU / Device Performance States (P-States)

P-State Lookup Tables

C-States In Multi-Core Processors
Single Core
Dual Core
Quad Core (Intel)
Quad Core (AMD)

Other P-State Features
Super Low Frequency Mode
Combining CPU C-state & P-state
CPU Thermal Monitor



Next Page : ACPI Power Management States >>>


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