PC Power Management

As the standard PC configuration has grown to include capabilities formerly considered options, the power requirements of the system have increased steadily. Larger displays, CD-ROM drives, and audio adapters all need more power to run, and the cost of operating a PC rises steadily.

To address these concerns, several programs and standards are now being developed that are intended to reduce the power needed to run a PC as much as possible. For standard desktop systems, power management is a matter of economy and convenience.

By turning off specific components of the PC when they are not in use, you can reduce the electric bill and avoid having to power the computer up and down manually. For portable systems, power management is far more important.

Adding CD-ROMs, speakers, and other components to a laptop or notebook computer reduces even further what is in many cases a short battery life. By adding new power management technology, a portable system can supply power only to the components it actually needs to run, thus extending the life of the battery charge.

Energy Star Systems

The EPA has started a certification program for energy-efficient PCs and peripherals. To be a member of this program, the PC or display must drop to a power draw at the outlet of 30 watts or less during periods of inactivity. Systems that conform to this specification get to wear the Energy Star logo.

This is a voluntary program; however, many PC manufacturers are finding that it helps them sell their systems if they can advertise these systems as energy efficient. One problem with this type of system is that the motherboard and disk drives can go to sleep, which means they can enter a standby mode in which they draw very little power.

This causes havoc with some of the older power supplies because the low power draw does not provide enough of a load for them to function properly. Most of the newer supplies on the market, which are designed to work with these systems, have a very low minimum-load specification.

I suggest you ensure that the minimum load will be provided by the equipment in your system if you buy a power supply upgrade. Otherwise, when the PC goes to sleep, it might take a power switch cycle to wake it up again. This problem would be most noticeable if you invested in a very high-output supply and used it in a system that draws very little power to begin with.

Advanced Power Management

Advanced Power Management (APM) is a specification jointly developed by Intel and Microsoft that defines a series of interfaces between power management–capable hardware and a computer's operating system. When it is fully activated, APM can automatically switch a computer between five states, depending on the system's current activity.

Each state represents a further reduction in power use, accomplished by placing unused components into a low-power mode. The five system states are as follows:

  • Full On. The system is completely operational, with no power management occurring.

  • APM Enabled. The system is operational, with some devices being power managed. Unused devices can be powered down and the CPU clock slowed or stopped.

  • APM Standby. The system is not operational, with most devices in a low-power state. The CPU clock can be slowed or stopped, but operational parameters are retained in memory. When triggered by a specific user or system activity, the system can return to the APM Enabled state almost instantaneously.

  • APM Suspend. The system is not operational, with most devices unpowered. The CPU clock is stopped, and operational parameters are saved to disk for later restoration. When triggered by a wake-up event, the system returns to the APM Enabled state relatively slowly.

  • Off. The system is not operational. The power supply is off.

APM requires support from both hardware and software to function. In this chapter, you've already seen how ATX-style power supplies can be controlled by software commands using the Power_On signal and the six-pin optional power connector. Manufacturers are also integrating the same type of control features into other system components, such as motherboards, monitors, and disk drives.

Operating systems that support APM, such as Windows, trigger power management events by monitoring the activities performed by the computer user and the applications running on the system. However, the OS does not directly address the power management capabilities of the hardware.

A system can have many hardware devices and many software functions participating in APM functions, which makes communication difficult. To address this problem, both the operating system and the hardware have an abstraction layer that facilitates communication between the various elements of the APM architecture.

The operating system runs an APM driver that communicates with the various applications and software functions that trigger power management activities, while the system's APM-capable hardware devices all communicate with the system BIOS. The APM driver and the BIOS communicate directly, completing the link between the OS and the hardware.

Thus, for APM to function, support for the standard must be built in to the system's individual hardware devices, the system BIOS, and the operating system (which includes the APM driver). Without all these components, APM activities can't occur.

Advanced Configuration and Power Interface

Advanced Configuration and Power Interface (ACPI) is a newer power management and system configuration standard supported by newer system BIOS software running Windows 98 and later operating systems. If your BIOS and operating system support ACPI, full power management control is now done by the operating system, rather than by the BIOS.

ACPI is intended to offer a single place for power management and system configuration control; in the past, with APM you often could make power management settings in the BIOS setup as well as the operating system that overlapped or could have conflicting settings. ACPI is supported in newer systems in lieu of APM.