ADAS Subroutine xxopen

       SUBROUTINE XXOPEN( IUNIT, DSFULL , LEXIST )
C-----------------------------------------------------------------------
C
C  ****************** FORTRAN77 SUBROUTINE: XXOPEN *********************
C
C  PURPOSE: TO INQUIRE & OPEN INPUT DATA FILE & ALLOCATE TO UNIT 'IUNIT'
C           (READ ONLY)  - IF IT DOES NOT EXISTS A MESSAGE IS  SENT  TO
C           THE SCREEN AND LEXIST IS RETURNED AS FALSE.
C
C  CALLING PROGRAM:  GENERAL USE
C
C  SUBROUTINE:
C
C  INPUT :    (I*4)  IUNIT   = UNIT TO WHICH INPUT FILE IS ALLOCATED
C  INPUT :    (C*(*))DSFULL  = FULL INPUT DATA SET NAME (INCL. USERID)
C                              IN FORM FOR DYNAMIC ALLOCATION.
C  OUTPUT:    (L*4)  LEXIST  = .TRUE.  => DATA SETS EXISTS AND IS OPEN
C                            = .FALSE. => DATA SET DOES NOT EXIST
C
C             (I*4)  I4UNIT  = FUNCTION (SEE ROUTINE SECTION BELOW)
C             (C*1)  BSLASH  = '/' - MUST BE FIRST 'DSFULL'CHARACTER
C NOTE:
C
C ROUTINES:
C          ROUTINE    SOURCE    BRIEF DESCRIPTION
C          ------------------------------------------------------------
C          I4UNIT     ADAS      FETCH UNIT NUMBER FOR OUTPUT OF MESSAGES
C          XXTERM     ADAS      TERMINATES ADAS PROGRAM WITH MESSAGE
C
C AUTHOR:  PAUL E. BRIDEN (TESSELLA SUPPORT SERVICES PLC)
C          K1/0/37
C          JET EXT. 2520
C
C DATE:    04/03/91 - ADAS91 VERSION (INCLUDES INQUIRE)
C
C UPDATE:  07/08/91 - PE BRIDEN: ADDED ERROR HANDLING FOR OPEN ERROR.
C
C UPDATE:  23/04/93 - PE BRIDEN - ADAS91: ADDED I4UNIT FUNCTION TO WRITE
C                                         STATEMENTS FOR SCREEN MESSAGES
C
C UPDATE:  24/05/93 - PE BRIDEN - ADAS91: CHANGED I4UNIT(0)-> I4UNIT(-1)
C
C-----------------------------------------------------------------------
      CHARACTER*(*)       DSFULL
      INTEGER             IUNIT
      LOGICAL             LEXIST