subprogramas_internos.f90

program mainprog  
implicit none 

   real :: a, b 
   a = 2.0
   b = 3.0

   Print *, "Antes de chamar troca"
   Print *, "a = ", a
   Print *, "b = ", b

   call troca(a, b)

   Print *, "Depois de chamar troca"
   Print *, "a = ", a
   Print *, "b = ", b

contains   
   subroutine troca(x, y)     
      real :: x, y, temp      
      temp = x 
      x = y  
      y = temp   
   end subroutine troca 

end program mainprog