How to call an external subroutine? | SAP-ABAP

  Following are the steps for calling external subroutine:
=>    The external program that contains the subroutine is loaded in the application.
=>    Then there is checking of syntax for the entire external program.
=>    Control goes to form statement in the external program.
=>    The statements which are in the external subroutines are executed.
=>   Then, endform returns control back to the statement following perform statement.