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 rank, coord, ierr
!
  call MPI_COMM_RANK(comm1d, rank, ierr)
  call MPI_CART_COORDS(comm1d, rank, 1, coord, ierr)
  if (mod(coord, 2) .eq. 0) then
     call MPI_SEND(a(1,e), nx, MPI_DOUBLE_PRECISION, &
                   nbrtop, 0, comm1d, ierr)
     call MPI_RECV(a(1,s-1), nx, MPI_DOUBLE_PRECISION, &
                   nbrbottom, 0, comm1d, MPI_STATUS_IGNORE, ierr)
     call MPI_SEND(a(1,s), nx, MPI_DOUBLE_PRECISION, &
                   nbrbottom, 1, comm1d, ierr)
     call MPI_RECV(a(1,e+1), nx, MPI_DOUBLE_PRECISION, &
                   nbrtop, 1, comm1d, MPI_STATUS_IGNORE, ierr)
  else
     call MPI_RECV(a(1,s-1), nx, MPI_DOUBLE_PRECISION, &
                   nbrbottom, 0, comm1d, MPI_STATUS_IGNORE, ierr)
     call MPI_SEND(a(1,e), nx, MPI_DOUBLE_PRECISION, &
                   nbrtop, 0, comm1d, ierr)
     call MPI_RECV(a(1,e+1), nx, MPI_DOUBLE_PRECISION, &
                   nbrtop, 1, comm1d, MPI_STATUS_IGNORE, ierr)
     call MPI_SEND(a(1,s), nx, MPI_DOUBLE_PRECISION, &
                   nbrbottom, 1, comm1d, ierr)
  endif
  return
  end