A SUB subprogram cannot be executed unless linked with a main  program
  unit.   A  SUB  subprogram  does not return a value except by way of a
  modifiable parameter (passed BY REF or BY DESC).  SUB subprograms  are
  called from the main program with the CALL statement.

  The SUB  statement  marks  the  beginning  of  a  SUB  subprogram  and
  specifies its parameters by number and data type.