BACK, EXIT, CANCEL

I had called a screen from an executable program with:

CALL SCREEN 100.

How can I continue processing the remaining code of the program after the execution of flow logic in screen 100? (This will be triggered by an EXECUTE button.)

What are the codes for BACK, EXIT, CANCEL button? If those buttons have following functions:

BACK - go back to screen 100 with data filled previously. Just assumed

screen 100 as customized selection screen.

EXIT - go to SAP easy access screen.

CANCEL - back to the point where program was called.