Subroutine passing value

As a beginner, I have created a subroutine and want to use in another program by calling value.

Please advice the code Am i right here ..

FORM ZX using P .

  IF zemp BETWEEN '00000001' AND '00088888'.

    SELECT  SINGLE * FROM P100 WHERE pernr = zemp

                                   AND subty = 'A'

    desig = P100-desig_desc.

    P = desig.

  ELSE.

    SELECT  SINGLE *  FROM pa9308 WHERE pernr = zemp

                                  AND begda LE sy-datum

                                  AND endda GE sy-datum.

    desig = p200-desig.

   P = desig.

  ENDIF.

ENDFORM.                    " ZX

Please advise the above code for subroutine is fine .or advise the same...

After that i have used the above subroutine in below form in another prog

PERFORM zx(zprogrma) using P.

Please advise how to pass a value here in subroutine.

Regards

Ahmad