BOOT CAMP 271 (15/04/03)


PC Power Management, part 1


To a casual observer the average desktop PC appears to be either on, off, or idle with a screen saver showing but there’s much more to it than that. In fact many PCs built within the last few years can be in one of half a dozen power ‘states’, which can lead to all sorts of fun and games when something goes wrong. This week we’ll take a broad look at computer power management (and apologies for all the acronyms; it’s the nature of the beast…) next week in part two, what you can do to take control.


It all began in the late 1980s when it was realised that rapidly growing numbers of computers in the USA and around the world were wasting vast amounts of energy and contributing to global pollution by being left switched on all day, and sometimes all night, much of the time doing nothing useful. In 1991 the US Government Environmental Protection Agency (EPA) set up the Energy Star programme to encourage manufacturers to reduce the power consumption of computers, monitors and other devices by more than 80 percent when they were not being used.


The computer industry responded with a range of power management initiatives and these evolved into two schemes that are available – but not always fully implemented -- on the majority of today’s desktop and portable PCs. They are: APM (Advanced Power Management), and ACPI (Advanced Configuration and Power Interface). A third system, called PnP-BIOS flourished briefly but has now been largely superseded by ACPI.


APM is a first generation power management system and is it largely controlled by the computer’s BIOS program. It performs adequately well on stand-alone PCs but it can run into problems on PCs connected to peripherals -- it doesn’t recognise USB devices, for example – and it doesn’t get on well with networks and the Internet. APM can also have arguments with Windows (full support wasn’t introduced until Windows ME) and there’s a lack of standardisation, which can lead to compatibility problems.


PCs with all APM features enabled can be in one of five states: Ready, Standby, Suspend, Hibernate or Off. Ready is normal operating mode. Standby is an intermediate state when after a period of inactivity the computer attempts to conserve power by switching off the monitor and disc drives but it comes back on or ‘wakes up’ more or less instantly when you move the mouse or press a key. Suspend is the lowest power consumption level with the CPU still running and able to resume operations quickly. It is mainly used on laptops, to shut the machine down when battery power is running low or the lid is closed. Data is lost in both Standby and Suspend modes if the power supply is interrupted.


Hibernate is a full ‘sleep’ mode and data is saved to the hard disc so that even if the power is disconnected no information is lost. When awoken, by pressing the On button, the computer goes through the normal Power-On Self Test (POST) routine and then returns quickly – usually in less than a minute -- to the state it was in before it went into hibernation. Off is when the computer is powered down and inactive. APM communicates with monitors and video adaptors, telling them to shut down or go into low power mode using Display Power Management Signalling (DPMS).


ACPI, developed by Compaq, Intel, Microsoft, Phoenix and Toshiba was introduced in 1996; it puts power management under joint control of the BIOS and the operating system and makes use of power saving features in compatible ‘plug-and-play’ components and peripherals. ACPI has been incorporated into Windows from Win 98 onwards, enabling a PC to recover from a very low power state to full on in as short a time as possible. This feature has become known as ‘On Now’ and ‘Instantly Available’ and the ultimate goal is to do away with the lengthy delay when your PC’s boots up and loads Windows. With full function ACPI it will appear to be off, and consume less power than a night-light but it will awaken from its deep sleep at the touch of a button and be ready to use in a matter of seconds. 


The key difference with APM is that ACPI knows what peripherals and software the PC is using. This means it can achieve more significant reductions in power consumption, not only when the PC is idle but also when it is working, by reducing processor speed and powering down peripherals and components when they’re not being used. The downside of ACPI is that it’s an ‘open’ standard and manufacturers are free to fiddle around with it, which initially led to a lot of ‘buggy’ implementations but things seem to have settled down now and in general it works quite well. An ACPI enabled PC can be in one of four ‘global’ power states: G0 is On or ‘Working’, G1 is ‘sleep’ mode (there are six levels – S0 to S5 -- with varying speeds of recovery), G2 is ‘Soft Off’ (roughly equivalent to Standby in APM) and G3 is ‘Mechanical Off’, with the power removed completely.


By now you’re probably glazing over, or rightly concluding that the whole business is a nightmare and best left alone. The trouble is most PCs are configured to use one power management system or another – often incorrectly -- and sooner or later something is going to go wrong or your PC starts to behave erratically so next week we’ll show how to take control of your PCs power management features.


Next week – Power Management, part 2





Basic Input Output System, a program stored in a microchip memory on the PC motherboard that tests and configures the hardware, memory and disc drives immediately after switch on and before Windows is loaded



Central Processor Unit - the main microprocessor chip in a PC



Universal Serial Bus, high-speed industry standard connection system for peripherals




Monitors often consume more power than the PC system unit so switching it off, when not in use, either manually or by using the PC’s power management systems can cut your power consumption dramatically but by far the biggest savings can be achieved by changing to an LCD flat screen monitor. These typically consume around 20 to 25 watts, which is between third and a quarter as much power as an equivalent sized CRT monitor. By the way, it’s a myth that screensavers reduce energy consumption; in fact most monitors use almost as much power when they are displaying a screensaver as a normal desktop.


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