SUBROUTINE get_lun(lun)

c PURPOSE
c   provide a way to assign fortran logical unit numbers (lun) for 
c   programs to use.  Up to 90 luns can be assigned.  
c
c   Logical unit numbers assigned are  11 -- 100
c
c   Once all the available luns have been assigned, further calls
c   return a value of -1 
c
c   a second entry point,  unget_lun  makes a previously-assigned 
c   lun once again available for use
c
c HISTORY
c   97.03  Original code written by EAC
c   98.06  Documentation expanded by EAC
c
c=======================================================================
      IMPLICIT NONE

      INTEGER*4 lun
      BYTE lunlist(90) /90*0/
      SAVE lunlist

      INTEGER*2 i

      i=1
  100 CONTINUE
        IF(lunlist(i) .EQ. 0) GOTO 200
        IF(i .EQ. 90) GOTO 300
        i= i+1
        GOTO 100

  200 CONTINUE
        lunlist(i)= 1
        lun= i+10
        RETURN

  300 CONTINUE
        WRITE(6,*)'From subroutine get_lun: maximum number of '
        WRITE(6,*)'  assigned LUNs has been reached.  Returning'
        WRITE(6,*)'  a dummy value of -1.'
        lun= -1
        RETURN


      ENTRY unget_lun(lun)
        lunlist(lun-10)= 0
        RETURN

      END