Typically, there would be a jump table at a fixed offset containing a table of functions that the application could/would call. This has to be strictly adhered to.

This is how in the days of MS/other DOS people would be able to give printer/graphic card drivers as loadable modules. There were a fixed list of entry points in a table which could be called by the host application to render its output on the medium. Example of such a program is 'Easytrax' later 'Autotrax'

I hope this will guide you on your way.

Another idea to explore is the concept of a DLL(dynamically linked library) and how it works.