To get the most out of power managing your personal computers, it is helpful to understand the technology underlying power management. This chapter has seven sections. Section 3.2 identifies the parts of a computer system involved in power management, how they function together to reduce energy use, and defines power management modes. Section 3.3 describes power management in PCs, for both x86 processor-based systems and Apple Macintosh® systems. Section 3.4 reviews the implications of networks for PCs. Section 3.5 addresses power management in monitors. Section 3.6 describes power management in workstations. Section 3.7 describes aftermarket devices for PCs and monitors that lack power management features, and Section 3.8 outlines some existing barriers to power management.

Power management in personal computers relies on the fact that for most of the time a typical PC is on, it is not doing anything productive. As long as the computer is idle, energy use can be reduced without interfering with work. Common methods used to reduce energy use are slowing down or stopping the processor clock, spinning down the hard disk, and turning off entire system components such as video or sound cards or disk controllers. Monitors can be power-managed by dimming or blanking the monitor, or by turning off the main beam and possibly also the control electronics.

PC power management was first introduced in laptop computers to allow longer operating times while running on battery power; later, it was brought into the desktop PC market. Many early power management systems had long recovery times, awkward configuration methods, and low energy savings. However, power management has improved rapidly, becoming more powerful, reliable, and easier to use; it also now delivers considerably more energy savings. In 1993, Intel and Microsoft introduced Advanced Power Management (APM), which is becoming an industry standard. The APM protocol supports power management by defining how power management commands are communicated within the PC system.

As Figure 3.1 shows, computers are logically organized as a hierarchy of layers. Those at the top are the software that the user directly interacts with; those closer to the bottom direct the physical control of electrical signals. Power management can involve the application software and the operating system (sometimes these are not involved), and always requires action by the firmware (BIOS), processor and peripheral hardware.

The BIOS (Basic Input/Output System) is a combination of hardware and firmware (software in read-only memory), distinct from the operating system, that intermediates between the processor and other parts of the system. In the first generation of power management (machines built through 1993 or 1994), it was controlled solely by the BIOS. As of 1996, the BIOS is still a key component, but more of the configuration and control is rising into the operating system and occasionally into application software. However, control signals must still pass through each intermediate layer for action to occur.

Accomplishing power management has four components. The first is to monitor activity levels of the processor, input devices (such as the keyboard and mouse), and communication peripherals (network or modem). The second component is to utilize timers to decide when to initiate the shift to a lower power mode. Third, changes in power management status need to be communicated to the correct device and actually occur. Finally, power management needs to recognize when activity resumes and return to a higher power (or full-power) mode.

Figure 3.1 shows the communication paths which allow power management to occur. The BIOS send periodic signals (about once per second) to the operating system to begin power management (see number 2). If this signal is passed through by the operating system, it will trigger the start of the power management timers in the BIOS. The operating system will only pass the signal through if it detects no activity from the application software (number 3). If there is no activity, the operating passes the signal back to the BIOS (number 4), which begins a timer. The BIOS continues to monitor keyboard and mouse activity (number 1). After a specified time with no activity, the BIOS will initiate power management by sending appropriate messages to some or all of the hard disk, peripheral cards, processor, and video card (number 5). After initiating a change in mode, the BIOS begins another timer which indicates when to initiate the next power management mode. If at any time the BIOS receives an interrupt request (keyboard, mouse, or network activity), the BIOS will signal the required peripheral cards, processor, and video card to return to an active mode (usually only a demand for hard disk activity will cause the hard disk to spin up). Examples of peripheral cards include network interfaces and CD-ROM drives.

The ability to enter and leave low-power modes is the key to active PC power management. The following discussion is of power management as defined by APM. Earlier implementations (circa 1993) were similar, but often had fewer modes.

A computer without power management has two power modes, on and off. When a power-managed computer is on, it can be in one of several (usually four) modes. When the computer is active and at full power, this is the full-on mode. After a period of inactivity, the computer can enter doze, the first of three reduced-power modes. In the doze mode, the system does not operate, but is capable of responding to activity with no delay. Doze is not defined by APM, but often added to systems that use APM. With no intervening activity, the computer will enter standby and finally suspend modes after specified delays. Each successive power management mode represents a lower level of CPU function. With lower power management modes, less energy is used, but more time is required to bring the computer back to full activity. Some PCs reach the 30 W Energy Star standard in standby mode, while others need the further savings from the suspend mode (typically about 5 W) to comply.

Not all BIOS systems have the same number of power management modes, and even when they do, the same mode may not represent the same level of activity. Table 3.1 shows power management modes for a typical BIOS system. Note that some kinds of activity, such as responding to a network query, may not require the entire system to wake up, so that a system may only shift from standby to doze rather than to full-on.

The timing of the power management modes is determined by settings (usually in BIOS) specifying the delay between each power management mode and the next. Most components are controlled by a single timer. Devices like the hard disk may have an independent timer since they only need to become active for hard disk access. Figure 3.2 shows how the power level changes over time in response to system activity or inactivity.

The fact that some PCs can shift from standby to doze (rather than to full-on) shows the sophistication of APM. Some actions, such as many responses to network activity, do not need all parts of the computer (e.g., not the video card nor the monitor) to be accomplished. Thus, some PCs can move from standby to doze then back to standby to maximize energy savings.

Before the release of Windows 95®, Microsoft operating system software was only minimally involved in desktop PC power management (and application software was only used for monitor power management, not for controlling the PC itself). Thus the BIOS was, and remains, a critical component. Three companies dominate the production of BIOS systems used in x86-based PCs. This means that computers from different manufacturers may use the same power management technology. However, over time, each BIOS manufacturer has produced several versions of BIOS software that operate in different ways. With the introduction of APM, power management is becoming more standard across PCs, which should make power managing them easier and more reliable.

The key to a properly configured power management setup is ensuring that the system properly determines what signals to respond to, and what delay times to use before changing modes. The rest of this section addresses general principles of power management configuration.

While power management systems differ in many details, the configuration mechanisms follow some common patterns. The BIOS on most PCs can only be configured at system start-up (bootup). As power management moves into the operating system, some of the configuration can happen there, usually through a control panel.

For most systems, there is one main "switch" (in the firmware) that, when turned off, disables all power management; on some machines this is called the "green switch" or "green feature". However, turning the main switch on will not necessarily enable all power management features. The subsidiary switches, timers, and other settings must also be set properly. Other switches may enable specific power management modes (e.g., suspend) or specific devices (e.g., the hard disk or monitor). They may also set variables such as the amount of processor speed reduction or the monitor power management method.

The power management timers allow the power management sequence to occur in a sensible manner, compatible with the ways that system is used. Steps which have minimal impact on system operation (those with quick recovery, such as slowing the processor clock speed), can reasonably be set to operate with small delay times, such as 20 seconds or a minute. Modes with a longer recovery time should have longer delay times. The hard disk timer and reactivation controls usually respond only to actual access to the disk, and can operate even when other activity keeps the rest of the system fully operational. Appropriate disk timer settings depend on how long the disk takes to "spin up." For example, if a disk takes 3 to 5 seconds to recover, a 15 to 20 minute delay may make sense; for disks with a shorter recovery time a shorter delay can be considered.

The number and character of the timers varies considerably among machines. On some, each time delay can be adjusted to a specific minute value (e.g., "2 minutes", or "5 minutes"), while on others they are reduced to terms such as "low", "medium", and "high" which imply particular pre-set delay times (e.g., 5 minutes, 10 minutes, and 20 minutes respectively). When specific times can be set, they usually operate in series, with a timer starting each time the computer changes mode. For example, a system with timers set at 5, 10, and 15 minutes would "go off" after 5, 15, and 30 minutes of no activity. In some versions of power management, timers operate in parallel (all timers count from the last activity). In this example, timers in parallel would expire at 5, 10, and 15 minutes after the last activity. Whether the system timers run in series or in parallel should be checked during configuration. The audit discussion in Appendix G has more details.

Some BIOS systems allow the user to specify how much to slow down the CPU in the doze and standby modes (e.g., to one eighth of the normal speed).

While entering low-power modes is controlled by timers, the return to full power (or higher-power modes) is directly triggered by activity from the keyboard, mouse, modem, or network. Such activities generate interrupt requests, or IRQs, which signal the processor that it needs to respond to them. Even when a PC has powered down (but is not off), the BIOS still monitors IRQ activity.

Some peripherals are connected to the PC through SCSI (Small Computer Serial Interface) ports. External SCSI devices have their own power sources, unlike the internal hard disk in a typical PC (which usually uses an IDE-Integrated Drive Electronics-interface). Turning the power switch off is always a good power management strategy, particularly for devices such as scanners that are only used occasionally.

A few PC models have a 'hibernate' mode in which the entire system state (including all memory contents) is written to memory and the system halts. All power can then be removed from the system, as coming out of hibernation involves beginning a reboot of the system, but reloading the system state from that saved to disk rather than following the default restart path. This type of power management saves more energy, and is resistant to power failures, but requires a longer recovery time. Sun workstations utilize hibernation, as described in Section 3.6.

The underlying power management technology in Apple Macintosh® systems is similar to that in x86-based systems. Newer models (some PowerMacs) use a power management software utility that offers user-defined timers for the PC and monitor. The power-management utility is available with the system software, but must be specifically installed and enabled to have any effect. Once installed, the utility can be configured via a Control Panel. Most Quadra and PowerMac models turn themselves completely off as a PC power management strategy rather than entering a sleep mode. Since this entails entirely rebooting the computer on wake-up (losing network connections and perhaps even data), power management is rarely enabled on these computers. Some currently available models do not have this problem, but many that do will be in active use in offices for some time. Monitor power management can be successful on any Macintosh (provided the monitor is capable of it), even if processor power management is not an option or does not work for one reason or another. Internal disks on Apple Macintosh systems are generally SCSI (not IDE) drives.

Networks pose special challenges for power management. Depending on the systems (hardware and software), the network can partially or entirely defeat power management, or may require extra configuration changes for it to function.

As more and more computers are connected in networks, people become more dependent on the ability to access individual machines at any time of day. Not only do individual users want to access their own files, but people are relying more on accessing data on other people's systems. As software becomes more sophisticated, we become less aware of how many machines may be required to be on and running to accomplish any particular task.

Local area networks (LANs) are generally united by a single communications protocol, and are usually confined to a single site (or portion of a site). Specialized hardware is used to connect LANs to each other and to the Internet at large. Within a LAN, a network may operate "peer-to-peer" (many systems of equal "rank" within the network), "client-server" (with a small number of high-powered machines providing core computation services for a larger number of individual PCs), or "heterogeneous" (with both peer-to-peer and client-server present) (Bachmann and Brüniger, 1996). A client-server model can be operating at the communications protocol level, or be implemented solely in software. The type of network may affect whether power management is possible for a given system, and how it must be configured to successfully do it.

Maintaining network connections during power management was a problem for many earlier systems. Some new systems continue to have problems with this, but many have been tested to work properly under common networking systems, with the results listed in the ENERGY STAR tables. Currently, most power management problems with networked PCs are a result of the way the network operating system works rather than with the PC hardware. You may need to do your own tests with your particular networking environment.

In many networks, a central server will send out periodic "Are you there?" messages to see which computers are still on and connected to the network. For many computers, these messages cause enough activity to keep the PC and monitor awake, defeating power management. If the PC does successfully go to sleep, it may fail to respond to the "Are you there?" message, so that the server (or other computers on the network) assumes the machine is off and terminates network services to it. When the user brings the PC back to full-on operation, the network connection has been lost. This latter problem usually results in power management being disabled, though it may not be necessary to disable all power management on such computers to maintain the network connection. For example, a processor that is operating more slowly in a low-power mode may not react quickly enough to maintain the network connection, so that increasing its doze speed may solve the problem.

Some newer BIOS systems are able to treat network activity differently from other activity, such as from the keyboard. Network activity on these PCs will only power up those parts of the system needed to respond to the network request. For example, the processor may switch from 'stopped' to half of the normal speed, process the task, then return to a stopped mode. The PC is not returned to a full-on mode, and the monitor is not activated.

A growing number of systems are able to successfully power manage and maintain full network services. Some of these have smart network interface cards that respond to the routine messages without bothering the CPU. Others can awaken only partly and briefly to handle the network request before resuming the low-power mode. Others have a chip installed that bypasses operating system operation that would otherwise defeat power management.

Power management has been more successful in monitors than in PCs, even though the PC must be the initiator. Compared to power managing PCs, monitors are usually simpler, have much more energy savings potential, power manage more reliably, and are less likely to interfere with operation or network connections. Because of this, it is even more important to enable monitors for power management than it is to enable PCs.

Monitor power management is in most cases independent of PC power management in that the monitor can power down even if the PC doesn't, and vice-versa. However, the monitor is still dependent on the PC for power management initiation; this is necessary since the monitor does not directly receive the activity information needed to know when to begin and end power management. Once the first low-power mode is entered, however, the monitor has an internal timer and will shift to succeeding low-power modes even if the PC doesn't send additional signals. While delay times may differ, for the most part, the monitor and PC are driven by the same activity for beginning and ending low-power modes (though network activity is an exception to this).

Some PCs will "dim" the monitor so that the image is present, but not bright enough for use. This results in some energy savings, but the savings are small compared to sleep modes. However, dimming does provide a transition mode to full blanking of the screen as well as having instant reactivation.

Most computer monitors are controlled through Display Power Management Signaling (DPMS) which defines a method for the PC to send power management signals to the monitor. For DPMS to work, both the monitor and (usually) the PC must be designed to use DPMS. Also, the PC must be properly enabled and send the correct signals. DPMS has modes similar to the PC power management modes defined by APM (standby, sleep, etc.), with two reduced power levels. In most cases, both power levels appear the same-a blank screen is displayed-but the lower power level has a longer reactivation time.

The structure of monitor power management control is shown in Figure 3.1. The PC always initiates the process, with the initial timers within the BIOS, or special software that comes with the video card. When the timer indicates that the monitor should be put into sleep mode, the BIOS (or software) signals the video card which in turn sends the appropriate DPMS signal to the monitor. Successive power management modes can be activated by either the PC's timer or the monitor's internal timer.

Some video cards can send out DPMS signals even in PCs that cannot power manage on their own, provided the appropriate software is installed and enabled. In addition, software is available that will allow a non-DPMS video card to send DPMS signals. As the monitor is connected to the video card, not directly to other parts of the processor, it is dependent on the video card to pass through DPMS signals. When reviving the monitor is called for (usually by keyboard or mouse activity) the PC sends the proper DPMS signal, and the video card redisplays the current image.

While most monitors are power-managed with DPMS, there are two other methods available on some systems: blanked screens, and switched monitor outlets. These were particularly useful in early power management applications, but over time may disappear.

A "universal" monitor will begin power management from either a DPMS signal or a blanked screen. Rather than originating with the BIOS, however, a blanked screen (no color at all-entirely black) is usually accomplished by a screensaver or video card control software. "Universal" monitors are so named because they do not require the PC to have DPMS to power manage (although they can also respond to DPMS signals). When a "universal" monitor receives the blanked out video signal, it begins power management. The monitor's internal timer can then activate subsequent power management modes. The ENERGY STAR list suggests that about 10% of power managing monitor models are universal.

Some PCs have a "convenience" electrical outlet on the back for plugging in the monitor. PCs that can switch off the power to this outlet allow energy savings from monitors that can't power manage on their own. Some of these PCs cannot send DPMS signals, so rely on the switched outlet for monitor power management. In addition, if the monitor is plugged into the PC (whether the outlet is power-managed or not), then the monitor will be switched off when the PC is switched off (otherwise they often remain on, albeit with a blank screen). These outlets are often designed for flat 3-prong plugs and do not accommodate ordinary 2- or 3-prong plugs.

Every Sun® desktop workstation offered for sale as of January, 1994 complies with the EPA ENERGY STAR requirements, though larger systems (servers) do not. Power management is similar to that for PCs in that the monitor and other devices (such as hard disks) can be powered down independently, depending on activity. Sun power management differs from that in PCs in that there is only one low-power state for the processor, a hibernation state, and that powering the system down and back up can be accomplished (on some models) by a time clock as well as by system activity (or lack of it). On Sun systems, all power management controls are within a single system utility.

Processor power management is based on an inactivity timer, but also can include times during the day in which processor power management is locked out (such as the user's typical work hours). When a workstation determines that the system has been inactive and ready for power management, all processes are stopped, all devices instructed to power off (if they are capable of it), and the entire system state (including all memory contents) written to a disk before the system is powered off. In the suspend state the system consumes minimal power, and unplugging the system entirely does not affect its ability to later recover to full operation. Note that 'suspend' on Suns is different from the suspend mode defined in APM for PCs.

Monitor power management on Suns is accomplished by DPMS signals as with PCs, though not all Sun monitors are power management capable. Some external devices (e.g. scanner, disks) can power off on signals from the processor; others must be manually turned off after the system reaches suspend and turned back on before the system resumes in order to save energy.

The system can be brought back to active operation by either pressing the 'power' key on the keyboard (which can also be used to power off the system), or (on some models) by a time clock at a pre-set time. The time to full recovery is typically less than one minute.

Aside from saving energy, this form of power management allows the system to be unplugged and moved, protects it from power loss within the building, and is quicker than rebooting the system from scratch after turning it off. There are potential problems with using power management; these are discussed in the section on enabling Sun power management in Appendix D (D.6).

Some IBM RS/6000 systems running OS/2 can power-manage, as can some clones of Sun workstations.

For PCs or monitors that lack built-in power management features, "Power Controlling" (or "aftermarket") devices are available that sense activity (usually keyboard or mouse) and cut power to a device when appropriate. Most commonly these are used with monitors, sensing either keyboard/mouse activity, or the presence of a person in front of the screen.

Power controlling devices can substantially reduce the cost of operating computers, monitors, printers, copiers and fax machines by turning off the equipment when it is not being used. These devices are most beneficial in offices where equipment is normally left on continuously or if the equipment has no power reduction functions. The add-on device is connected externally to the PC or monitor and may be controlled by software (which is often installed incorrectly or not at all). These factors, combined with your electricity cost (dollars per kWh), will determine the payback period for power management devices. See the EPA ENERGY STAR listing for control devices for specific model information.

For PCs, some control devices work with software that will save any open documents and include a "book marking" feature to return to the file previously being worked on when the machine is powered back up. Some printer control devices intercept commands sent to the printer and store them until the printer is back up and fully functioning.

Even though a computer or monitor may have power management features, power management may not always operate effectively. There are many reasons why power management can be defeated in systems that have the feature.

Computer networks pose special challenges for power management. Once a PC is connected to a network, the user may want to access the machine remotely, others may rely on being able to connect to it at any time, and services such as disk backups may operate during nights or weekends. This can mean that the simplest power management strategy, simply turning the machine off, can no longer be used for the PC (though the monitor can and should be turned off). Remote access by modem has the same effect as does a computer set up to receive faxes.

Power management also affects a PC's network response (see Section 3.4). If a machine cannot successfully wake up when warranted by network activity, or if it loses its network connection while power managing, the user will want power management disabled. In some cases these types of network problems cannot be fixed without some change in the network operating system or the PC hardware. These type of network responses should be tested before ordering large numbers of PCs (see Chapter 5 for purchasing guidelines).

Power management capabilities may change when PCs are upgraded by replacing the processor, the motherboard, or add-on cards (e.g., the network interface). Software upgrades of operating systems or utility software can cause power management to be disabled. Before upgrading many similar machines, determine if the proposed change interferes with power management. If it does, consider looking for alternatives that do not.

Some application software can interfere with power management, depending on how it is configured and the particular machine it is used on. One example is the 'auto-save' feature on many word processors and spreadsheets. If the feature saves the document even if no changes have been made, this will unnecessarily cause the processor and hard disk to stay awake, defeating power management partially or entirely. Some screensavers will periodically load complex images from the hard disk, keeping the disk from powering down. A screensaver can keep the monitor and processor from power managing, unless set to a specific power management mode; many screensavers lack a power management mode and so need to be turned off for power management to occur. If you need to leave your PC on overnight, try to exit any applications with such auto-save features before leaving for the day so that power manage can operate. Operating system software is also key for power management, and can make power management easy, difficult, or, in some cases, impossible.

Continue to Chapter 4: What can I do with my existing stock of PCs?

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