Desktop bios "After Power Failure" setting behavior

Question

I know what the bios setting does but I have seen 2 different behaviors when this "After Power Failure" setting is set to "power on"

Here are my main questions,

1. does the bios know if the PC was powered down properly then power removed, or if there was a genuine power failure while it was running and did a hard shutdown?

2. Is this power failure detection a function of the PSU and the bios or just the bios?

What I have seen is some desktops will auto power on if shut down properly and then the power cord was removed, then connected back, others do not, they only auto power on if there was a power failure while the PC was running.

3, Can any one confirm these 2 different behaviors in desktops through the years?

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Answer 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.