What exactly happens in a power-fail interrupt?

Question

I came across this term, "power-fail interrupt" in wikipedia here

Since power-failure occurs instantly, what is the use of this interrupt? By the time the computer realizes it has encountered a power-failure, it'll be switched off! What's the point of it? Also this interrupt is given the highest priority

Have I misunderstood the term "power-fail"?

Answer 1

After a power failure, the computer can remain on life support using reserve power (such as a battery).

The article you quote talks specifically about the VAX architecture. The way the power-fail interrupt works there is as follows:

Once a power-failure interrupt has been posted, the processor has approximately 4 milliseconds before power is shut down.

Thus the interrupt handler has four milliseconds to do its thing.