  subroutine exchng1(a, nx, s, e, comm1d, nbrbottom, nbrtop)
  use mpi
  integer nx, s, e, comm1d, nbrbottom, nbrtop
  double precision a(0:nx+1,s-1:e+1)
  integer ierr, req(4)
!
  call MPI_IRECV (&
          a(1,s-1), nx, MPI_DOUBLE_PRECISION, nbrbottom, 0, &
          comm1d, req(1), ierr)
  call MPI_IRECV (&
          a(1,e+1), nx, MPI_DOUBLE_PRECISION, nbrtop, 1, &
          comm1d, req(2), ierr)
  call MPI_ISEND (&
          a(1,e), nx, MPI_DOUBLE_PRECISION, nbrtop, 0, &
          comm1d, req(3), ierr)
  call MPI_ISEND (&
          a(1,s), nx, MPI_DOUBLE_PRECISION, nbrbottom, 1, &
          comm1d, req(4), ierr)
!
  call MPI_WAITALL (4, req, MPI_STATUSES_IGNORE, ierr)
  return
  end
