I am a new programmer towards this language of 8051 microcontroller. I have written some subroutines and stuffs like using timers.. etc. But there's one thing which i am not sure of... i knew how to call subroutines from the file itself, but how to call a subroutine which i want to use from another file? What are the steps or methods that is required? Please help me, thanks.

And, what does this instruction meant?
push 7
pop 7

what is it meant by pushing 7 or popping 7?