APM vs ACPI

Advanced Power Management (APM) is an Application Programming Interface (API) that facilitates the interaction between different software programs. It was developed by Microsoft Corporation and Intel in 1992. It enables an IBM compatible operating system to achieve power management.

APM’s five power states for computer systems:

Full On – wherein the computer is on and it is not in a power-saving mode.
APM Enabled – wherein the computer is on and APM is controlling device power management as needed.
APM Standby – wherein the devices are on low power states and the CPU is slowed or stopped; the system state is saved and it takes little time to return to its former state.
APM Suspend – wherein the devices are powered off and the system state is saved. It takes a long time to return it to its former state.
Off – computer is turned off.

APM is controlled by Basic Input/Output System (BIOS) which is built into the PC and is the first code that it runs as it is powered on. Because APM uses BIOS, which usually is not aware of everything the user is doing; it can make a mess of things aside from the fact that it knows nothing about USB devices, add-in cards, and IEEE 1394 devices

Advanced Configuration and Power Interface (ACPI), on the other hand, is intended to be the successor to APM. It replaces APM in functions beyond the capabilities of APM and is compatible with new hardware. It is more advanced and comprehensive compared to APM.

It is centered on the operating system giving it more control over the operating system and the computer’s other components. It is also compatible with products of several different manufacturers.

ACPI’s power states or global states for computer systems:

G0 (S0) – which means it is working.
G1 – which means it is sleeping. Has four states:
S1 – Power to CPU and RAM is maintained but all processor caches are flushed and the CPU has stopped executing instructions.
S2 – CPU is off.
S3 – standby, sleep, or suspend to RAM.
S4 – Hibernation or suspend to disk.

G2 (S5) or Soft Off – wherein some components are on to allow input from keyboard, clock, modem, LAN, and USB devices.
G3 or Mechanical Off – wherein the computer’s power is almost zero and the power cord can be removed.

Since ACPI is new, older devices do not work properly with it, and it is slower than APM and may slow your computer down. It works well with laptops and your computer’s longevity. It also offers a smarter way to power management than APM.

Summary:

1.APM is Advanced Power Management while ACPI is Advanced Configuration and Power Interface.
2.APM is old while ACPI is new.
3.APM is compatible with old devices while ACPI is compatible with new hardware.
4.ACPI is more comprehensive and advanced while APM is not.
5.APM is centered on the BIOS while ACPI is centered on the operating system.

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