C-----------------------------------------------------------------------
C              Get an alphanumeric input string.  
C  In general all alphabetic characters are converted to upper-case, 
C  but if STRING contains "DONT DO IT" on input no conversion is done.
C  This is useful to allow the input of file names in case sensitive 
C  operating systems like UNIX.
C  All null characters are converted to blanks
C  The code should be general for ASCII and EBCDIC.
C  If the first character is a question mark (?) the routine exits to 
C  the system monitor.
C-----------------------------------------------------------------------
      SUBROUTINE ALFNUM (STRING)
      COMMON /IOUASS/ IOUNIT(12)
      CHARACTER STRING*(*),NULL*1
      NULL = CHAR(0)
      ITR = IOUNIT(5)
      ITP = IOUNIT(6)
      IDONT = 0
      IF (LEN(STRING) .GE. 10 .AND. STRING(1:10) .EQ. 'DONT DO IT') 
     $    IDONT = 1
C-----------------------------------------------------------------------
C  Write the prompt - if any - and get the answer
C-----------------------------------------------------------------------
      CALL GWRITE (ITP,'$')
      STRING = ' '
      ILEN = LEN(STRING)
      IF (ILEN .GT. 80) ILEN = 80
      CALL GETLIN (STRING)
      IF (STRING(1:1) .EQ. '?') STOP
      ILEN = LEN(STRING)
      DO 120 I = 1,ILEN
        IF (STRING(I:I) .EQ. NULL) STRING(I:I) = ' '
  120 CONTINUE
      IF (IDONT .EQ. 0) THEN
        LITTLA = ICHAR('a')
        LARGEA = ICHAR('A')
        LITTLZ = ICHAR('z')
        IDIFF = LITTLA - LARGEA
        ILEN = LEN(STRING)
        DO 130 I = 1,ILEN
          ITHIS = ICHAR(STRING(I:I))
          IF (ITHIS .GE. LITTLA .AND. ITHIS .LE. LITTLZ) THEN
            ITHIS = ITHIS - IDIFF
            STRING(I:I) = CHAR(ITHIS)
          ENDIF
  130   CONTINUE
      ENDIF
      RETURN
10000 FORMAT (A)
      END