Desktop bios "After Power Failure" setting behavior

Solution 1:

Power state information is stored by the BIOS. PSU's are "dumb" units and do not store any persistent power state information unlike a BIOS chip. This is why for instance you must always jump the power-on signal lead on the PSU if you want to start it in a standalone situation (e.g. when not plugged into a motherboard for testing).

The BIOS stores various ACPI power states, including G3 Mechanical Off indicating that zero electric current is flowing through the components, and G2/S5 Soft Off which is similar to G3 but allows for WOL (Wake-On-LAN) and other similar functions. It is also different from S4 which is considered a hibernation power state.

ACPI was introduced in 1996 to supersede APM as the accepted standard and provide a more structured API for operating systems. ACPI provides more fine-grained control, including the S4 power state, for which APM had no equivalent.

When it comes to the BIOS setting, there are typically three options for power-after-failure:

Power Off: Do not take any action in response to a failure
Last State: Resume whatever state the PC was in before the failure (e.g. if it was off, stay off)
Always On: Regardless if the PC was off or not, resume full G0 Working mode after a power failure.