Can a program can do exactly some given task?

Is there any way to be sure that a given program will do exactly a specific task?
Let's say I want to make a round-robin scheduler. Is there some way that I can be sure that certain program I've written will do exactly that?

It's pretty easy if you've read your SICP.

1) Figure out how you expect a round-robin to react

2) theorize an "effective" strategy for implementing that reaction

3) implement that reaction

There, you've created a round robin.