frame_id question

[Vladimir Prus]

Hello, 
I have some confusion over "frame_id" concept, which is supposed to be
unique identifier of a function frame. 

Frame id consists of a stack address and a program address. The program
address should be start address of a function and many of xxx_this_frame_id
function follow this pattern:

   static void
   xxx_frame_this_id (struct frame_info *next_frame, void **this_cache,
                      struct frame_id *this_id)
   {
      .......
      (*this_id) = frame_id_build (...., 
                                   frame_func_unwind (next_frame));
   }

The question is: why frame id has to include program address at all? It it
ever possible for two frames to have the same stack address? If so, when?

The immediate problem I have is that "frame_func_unwind" requires full debug
info (address boundaries for functions), but in my case assembler modules
have only line information, so frame_func_unwind will always return 0.
Using hardcoded '0' as program address part of frame id does not cause any
problems for me, but I want to be sure.

Thanks in advance,
Volodya