Standby – suspends to memory – draws some power to retain memory image.

Hibernation – suspends to disk – no power consumption required

• minimise power consumption
• provides 'instant on'
• orderly shutdown during power outage
• PME events can come via network, modem, scheduled tasks, peripherals

For XP power management features to work, disable all performance management features in the BIOS. ACPI is an OS specification: APM is a BIOS specification. ACPI requires ACPI-compliant applications too. When BIOS and OS contend for control, problems occur. APM BIOS implementations may vary from BIOS to BIOS.

OnNow

Microsoft Initiative

• Boot from power off to usable state in 30 seconds
• resume from hibernate in 20 seconds
• resume from standby in 5 seconds

Screen savers consume energy by keeping the monitor on and consuming CPU cycles. Power management can produce significant cost savings in small/medium/large organisations. Power Options dialogue varies according to the capabilities of the system.

ACPI Power States

Advantages

• Hibernate
• • Preserves data during power outages
  • uses no power
• Standby
• • powers on/off quicker
  • doesn't require disk space

Power Management features require drivers and hardware that support it: built-in VGA driver does not. Critical shutdown (pressing and holding the power button) notifies applications and waits for responses. A hung application will prevent a shutdown. Emergency Shutdown: hold control while you click shutdown. Hibernation available even for non-APM and non-ACPI systems. Contents of RAM saved to disk. This includes part of the MFT: if you reboot to another OS and make changes, when you reboot to XP, old MFT is restored and changes are lost.

On boot NTLDR looks for hiberfil.sys and checks flagged status. Hibernation requires freespace on disk equal to RAM size

Troubleshooting Power Management

System reboots instead of hibernating

faulty device driver could be causing BSOD. In 'Startup and Recovery' select 'Write an Event' and clear Auto-restart. Now you'll be able to see BSOD

For APM system, enable 'NT APM/Legacy Interface Node'

System won't emerge from standby

check for BIOS update

try removing or disabling all peripheral devices. Re-enable one-by-one

If a device is the problem, check for BIOS update

BIOS timeouts set lower than OS: disable BIOS settings

Device doesn't wake when system does

disable option that allows the device to wake computer

Power Saving features of Laptops

• display switches off when lid closes
• display dimmed when running on battery
• processor can be run at lower speeds when on battery
• low battery alarms can cause switch to low power state or run a program, or initiate hibernation
• Power status indicator is a battery when on battery power, and a power cord when on mains

UPS

USB UPSs normally autoinstall. UPS tab disappears: this is for serial UPSs only. USB UPSs adds shutdown times to power schemes and a power meter tab

ACPI also provides

• control of USB and Firewire devices (APM may try to go to standby while these devices are still active)
• support for Wake On LAN and Modem Ring (XP doesn't support these on APM systems)
• User definition of Power and Reset buttons
• Better battery management: separate indicators for each battery
• Dynamic configuration of PC Cards without need for reboot
• Multiprocessor support: APM not available on multiprocessor systems

XP setup chooses ACPI or APM HAL during setup: can be checked from devmgmt

• checks list of incompatible BIOSs
• if later than 01/01/99 then ACPI chosen
• if older than 01/01/99 then checks list of known good BIOSs
• if not then you get APM
• an APM tab in Power Management indicates an APM BIOS
• if you upgrade BIOS, you'll have to reinstall XP to get ACPI HAL, because all hardware must be redetected, support for PnP must be changed, registry must be updated. XP CD can repair an attempt to use Hardware Wizard to upgrade from APM BIOS to ACPI
• APM monitors interrupts, IO data and battery to determine if system is inactive. Once a threshold is passed, BIOS sends message to Windows. Windows replies ready and power transition occurs. If APM is disabled in BIOS, APM tab won't appear in Windows. Enable it and reinstall windows.