subroutine exchng1( a, nx, s, e, comm1d, bottom_nbr, top_nbr ) 
      include 'mpif.h' 
      integer nx, s, e 
      double precision a(0:nx+1,s-1:e+1) 
      integer comm1d, bottom_nbr, top_nbr 
      integer status_array(MPI_STATUS_SIZE,4), ierr, req(4) 
C 
      call MPI_IRECV ( a(1,s-1), nx, MPI_DOUBLE_PRECISION, bottom_nbr,  
     *     0, comm1d, req(1), ierr ) 
      call MPI_ISEND ( a(1,e), nx, MPI_DOUBLE_PRECISION, top_nbr, 0,  
     *     comm1d, req(3), ierr ) 
      call MPI_IRECV ( a(1,e+1), nx, MPI_DOUBLE_PRECISION, top_nbr, 1,  
     *     comm1d, req(2), ierr ) 
      call MPI_ISEND ( a(1,s), nx, MPI_DOUBLE_PRECISION, bottom_nbr, 1,  
     *     comm1d, req(4), ierr ) 
C 
      call MPI_WAITALL ( 4, req, status_array, ierr ) 
      return 
      end