You provide insufficient information to provide any real answer at all.
But now we know what you want to do, if you can tell us what language, or is assembly language, for what architecture and I'm sure we can provide you with an example of how.
If your answer is "why?" then the answer is to do one or more of the following:
1) To make the program more easily understood (by breaking it into modules)
2) To separate out a routine used many times so that you only need to write it once (also makes the program smaller)
3) To provide an interface (like an API) that can be called externally.