Power good signal
In addition to the voltages and currents that a computer needs to operate, power supplies also provide a signal called the Power-Good signal, sometimes written as Power_OK or Power_Good. Its purpose is to tell the computer all is well with the power supply and that the computer can continue to operate normally. If the Power-Good signal is not present at startup, the CPU is held in reset state. If a Power-Good signal goes down during operation the CPU will shutdown. The Power-Good signal prevents the computer from attempting to operate on improper voltages and damaging itself.
The ATX specification defines the Power-Good signal as a +5 volt (V) signal generated in the power supply when it has passed its internal self-tests and the outputs have stabilized. This normally takes between 0.1 and 0.5 seconds after the power supply is switched on. The signal is then sent to the motherboard, where it is received by the processor timer chip that controls the reset line to the processor.
In the absence of the Power-Good signal, the timer chip continuously resets the processor, which prevents the computer from running under bad or unstable power conditions. When the timer chip receives the Power-Good signal, it stops resetting the processor, and then the processor executes whatever code is at address FFFF:0000 (usually the ROM BIOS).
If the power supply cannot maintain proper outputs, such as when a brownout occurs, the Power-Good signal is withdrawn, and the processor is automatically reset. When proper output is restored, the Power-Good signal is regenerated, and the computer again begins operation. By withdrawing the Power-Good signal, the computer never receives the bad power because it is stopped quickly (reset) rather than allowed to operate on unstable or improper power levels, which can cause parity errors and other problems.
See also
ATX power supply descripiton