Sounds like a fun project
Sounds like you want a way to pass parameters to your subroutine.
One option is to use the CPU registers. If the subroutine is used as part of an interrupt handler or may be called recursively it needs to be re-entrant. To make a subroutine re-entrant, save all of the registers used by the subroutine to the stack at the beginning of the subroutine using MOVEM and restore all of the registers at the end of the subroutine.
Another option for parameter passing is to use the stack. See the example tutorial4 that is included with EASy68K for an example. Using the stack for parameter passing and local variables creates subroutines that are also re-entrant.