Advanced power management (APM) is an application programming interface (API) that allows an operating system running an IBM-compatible personal computer (PC) to interact with the Basic Input Output System (BIOS) to achieve, and regulate, power management. It was created as a joint venture between Intel® and Microsoft® and was made available to the public in 1992. APM has been largely replaced by a more advance management system known as Advanced Configuration and Power Interface (ACPI).
The BIOS is a simple program than is hard-coded into all personal computers (PCs). It controls the hardware on a basic level and can communicate with the operating system as well as any peripheral devices, such as a hard drive. The operating system installs the APM API as part of its installation. The API is essentially a piece of software that facilitates communication between the operating system and the BIOS.
Whenever a peripheral device, such as a monitor or hard drive, is attached, a driver must be installed. The driver is software that allows the device to communicate with the operating system. The driver can be written to APM specifications so that it can have power management options. An example would be a hard drive that has an APM driver; the driver could allow the hard drive to be shut off when not in use.
There are essentially two states in advanced power management: system state and device power states. System state can define five power states for the entire system. Full on and off are the simplest; full on means the system is on and no devices are in power-saving mode and off means the computer is off.
The other three advanced power management system states are APM Enabled, APM Standby and APM Suspend. APM Enabled means the computer is on and APM is available as needed. Standby means most devices are in low-power mode, the central processing unit (CPU) is off or slowed and the system state is saved; in this state, the computer can be started quickly. APM Suspend means the system state is saved but most devices are powered off; in this state, the computer takes a longer time to start processing again.
The device power states, which allow for a device to utilize power saving states, are Device On, Device Power Managed, Device Low Power and Device Off. On and off mean the same as in system state. In Device Power Managed, the device is on but not all functions are available. In the Device Low Power state, the device is not currently active, but the power connection is maintained so it can be started, or “woken up,” quickly.
The only exception in the advanced power management environment is the CPU. Only the BIOS can control the CPU. A driver, either through the system or through a device, can only contact the BIOS; it cannot give an instruction to power down. This is to protect the CPU from being shut down by a poorly written driver.