[jaclaz]

>Nowadays holding it for 5+ seconds still turns the system off, I wonder if this is a BIOS or a hardware configuration. Probably it's the PSU's logic, to power off if the 2 pins are shorted for more than 5 seconds.

It is BIOS+Hardware (the PSU is not involved). As a matter of fact to "switch on" a ATX power supply (not connected to a motheboard) you normally use a paperclip (or a short piece of cable) to connect the green with any of the black see:

https://forum.overclock3d.net/showthread.php?t=394

The whole point is that (unless the PSU has a mains switch and it is turned off) an ATX power supply is always partially ON, powering (parts of) the motherboard at all times (this allows for such things as Wake on Lan or switch on via CTRL+F11 or dedicated key on the keyboard).

[yuhong]

Without ACPI (which NT4 was) the ATX power button was handled entirely by the BIOS (I think in SMI code). With ACPI the power button (when you don't hold it for 5 seconds of course) was handled by the OS.