Power-Fail

Windows NT supports hot restart after power-fail. Hot restart means that when power is lost, and later comes back on, the system continues operation as though power had not been lost. This feature greatly improves effective availability of the system in environments with unreliable power. It requires hardware support. This is an optional feature.

To support power-fail restart, two hardware features are required. First, main memory must have a backup power source that allows it to reliably retain its contents when main power is lost. This backup source must activate in such a way that transitions between main power and backup power do not cause the memory to lose bits.

Second, a maskable interrupt must be raised when main power is about to be lost. The interrupt must give warning time sufficient to allow the OS to save processor state. (All Windows NT will do is force all processors to dump their register contents into memory, and set marker flags to indicate this has succeeded). The power-fail interrupt should be of higher priority than any other interrupt except for machine error interrupts. It must NOT be NMI.

Power-fail is not shutdown, there is a separate mechanism for that.