Power Supplies - Upgrading and Repairing PCs - page 1141

deepest sleep state that is consistent with the ability to meet the telephone answering ap-
plication's request.
ACPI enables system designers to implement a range of power-management features that
are compatible with various hardware designs while using the same OS driver. ACPI also
uses the Plug and Play BIOS data structures and takes control over the Plug and Play in-
terface, providing an OS-independent interface for configuration and control.
ACPI defines several system states and substates. There are four Global System states,
labeledfromG0throughG3,withG0beingthefullyoperationalstateandG3beingmech-
anically turned off. Global System states are immediately obvious to the user of the sys-
tem and apply to the entire system as a whole. Within the G0 state, there are four CPU
Power states (C0-C3) and four Device Power states (D0-D3) for each device. Within the
C0 CPU Power state, there are up to 16 CPU Performance states (P0-P15).
Device Power states are states forindividual devices whenthe system isinthe G0(Work-
ing) state. The device states may or may not be visible to the user. For example, it may be
obvious when a hard disk has stopped or when the monitor is off; however, it may not be
obvious that a modem or other device has been shut down. The Device Power states are
somewhat generic; many devices do not have all four Power states defined.
Within the G1 Global Sleep state are four Sleep states (S1-S4). The G2 Global Soft Off
state is also known as the S5 Sleep state , in which case the system is powered off but still
has standby power. Finally, G3 is the Mechanical Off state, where all power is disconnec-
ted from the system.
The following list shows the definitions and nested relationship of the various Global,
CPU/Device Power, and Sleep states:
• G0 Working —This is the normal working state in which the system is running and
fully operational. Within this state, the Processor and Device Power states apply. The
Device Power states are defined as follows:
• G0/D0 Fully-On —The device is fully active.
• G0/D1 —Depends on the device; uses less power than D0.
• G0/D2 —Depends on the device; uses less power than D1.
• G0/D3 Off —The device is powered off (except for wakeup logic).
• The Processor Power states are defined as follows:
• G0/C0 CPU On —Normal processor operation.
• G0/C1 CPU Halted —The processor is halted.
• G0/C2 CPU Stopped —The clock has been stopped.