! -----------------------------
! Example program
! A simple subroutine 
! -----------------------------
program test
  implicit none
  real(kind(1.d0)):: x,y
  x = 5.d0
  y = 2.d0
  write(*,'("before: x,y=",2f7.2)') x,y
  call swap(x,y)
  write(*,'(" after: x,y=",2f7.2)') x,y
end program test

subroutine swap(a,b)
  implicit none
  real(kind(1.d0)):: a,b,tmp
  tmp = a
  a = b
  b = tmp
end subroutine swap