Does fnop pop the register stack?

it's a floating point unit instruction used to create a small delay between two floating point instructions.

I think it's there because of the pipeline (enables cpu to do multiple instructions at the same time..kind of) which under some circumstances messes up and processes parts of instructions in the wrong order, adding a small wait of a few cycles inbetween solves this.

FNOP does nothing except take up code space. It's in the manual.

As for where it might be useful, I have no idea.