Power Supplies - Upgrading and Repairing PCs - page 918

press the power button to turn the system off. The system and all devices are essentially
powered off; however, the system is still plugged in and standby power is coming from the
power supply to the motherboard, allowing the system to wake up (power on) if commanded by
an external device. No hardware context or status is saved. The system must be fully rebooted
to return to the G0 (working) state.
• G3 Mechanical Off —Power is completely removed from the system. In most cases this means
the system must be unplugged or the power turned off via a power strip. This is the only state in
which it is safe to disassemble the system. Except for the CMOS/clock circuitry, power
consumption is completely zero.
In normal use, a system alternates between the G0 (Working) and G1 (Sleeping) states. In the G1
(Working) state, individual devices and processors can be power-managed via the Device Power
(D1-D3) and Processor Power (C1-C3) states. Any device that is selectively turned off can be
quickly powered on in a short amount of time, from virtually instantaneous to only a few seconds
(such as a hard disk spinning up).
When the system is idle (no keyboard or mouse input) for a preset period, the system enters the
Global G1 (Sleeping) state, which means also selecting one of the S1-S4 sleep states. In these states,
the system appears to be off, but all system context and status are saved, enabling the system to return
to exactly where it left off, with varying amounts of latency. For example, returning to the G0
(Working) state from the G1/S4 (Hibernation) state requires more time than when returning from the
G1/S3 (Suspend) state.
When the user presses the power button to turn the system off or selects Shutdown via the OS, the
system enters the G2/S5 (Soft Off) state. In this state, no context is saved, and the system is
completely off except for standby power. Fully disconnecting AC or battery power causes the system
to be in the Global G3 (Mechanical Off) state, which is the only state in which the system should be
disassembled.
During the system setup and boot process, ACPI performs a series of checks and tests to see whether
the system hardware and BIOS support ACPI. If support is not detected or is found to be faulty, the
system typically reverts to standard Advanced Power Management control, which is referred to as
legacy power management under ACPI. Virtually all ACPI problems are the result of partial or
incomplete ACPI implementations or incompatibilities in either the BIOS or the device drivers. If you
encounter any of these errors, contact your motherboard manufacturer for an updated BIOS or the
device manufacturers for updated drivers.
Power Cycling
Should you turn off a system when it is not in use? To answer this frequent question, you should
understand some facts about electrical components and what makes them fail. Combine this
knowledge with information on power consumption, cost, and safety to come to your own conclusion.
Because circumstances can vary, the best answer for your own situation might be different from the
answer for others, depending on your particular needs and applications.
Frequently powering a system on and off does cause deterioration and damage to the components.
This seems logical, but the simple reason is not obvious to most people. Many believe that flipping
system power on and off frequently is harmful because it electrically “shocks” the system. The real
problem, however, is temperature or thermal shock. As the system warms up, the components expand;