Legacy Power Management Theory of Operation

Instead of relying on ACPI software, legacy power management uses BIOS and various hardware mechanisms. The scheme relies on the concept of detecting when individual subsystems are idle, detecting when the whole system is idle, and detecting when accesses are attempted to idle subsystems.

However, the operating system is assumed to be at least APM enabled. Without APM calls, there is no quick way to know when the system is idle between keystrokes. The PCH does not support burst modes.

Mobile APM Power Management

In mobile systems, there are additional requirements associated with device power management. To handle this, the PCH has specific SMI traps available. The following algorithm is used:

The periodic SMI timer checks if a device is idle for the require time. If so, it puts the device into a low-power state and sets the associated SMI trap.

When software (not the SMI handler) attempts to access the device, a trap occurs (the cycle does not really go to the device and an SMI is generated).

The SMI handler turns on the device and turns off the trap.

The SMI handler exits with an I/O restart. This allows the original software to continue.

Reset Behavior

System Power Supplies, Planes, and Signals Reset Behavior