@fireplus
IIRC, you can do that by manipulating the ACPI registers (Power Management Registers) in the Power Management controller chip with software. These are standard registers, and I think it's used by windows and other x86 OS to respond to the power-related "messages" such as power-down, suspend to RAM, etc. You can write software for that matter.
Anyway, the ACPI register addresses explanantions are available from the ACPI/Power management controller chip datasheet. I can't recall correctly, wheter the the ACPI controller is embedded inside the southbridge in every motherboard or not. But, I think some of the motherboard has a dedicated power management controller which is integrated with the I/O controller in one chip (in this case, the chip is not part of the southbridge).
There is also a possibility that you can call the BIOS services for power management. But, I don't recommend to venture with this route, since some BIOS are buggy enough to work with in the first place :wink:
goodluck,
PCZ