Frames
******

   A frame is a construct that GDB uses to keep track of calling and
called functions.

`FRAME_FP'
     in the machine description has no meaning to the
     machine-independent part of GDB, except that it is used when
     setting up a new frame from scratch, as follows:

                create_new_frame (read_register (FP_REGNUM), read_pc ()));

     Other than that, all the meaning imparted to `FP_REGNUM' is
     imparted by the machine-dependent code.  So, `FP_REGNUM' can have
     any value that is convenient for the code that creates new frames.
     (`create_new_frame' calls `INIT_EXTRA_FRAME_INFO' if it is
     defined; that is where you should use the `FP_REGNUM' value, if
     your frames are nonstandard.)

`FRAME_CHAIN'
     Given a GDB frame, determine the address of the calling function's
     frame.  This will be used to create a new GDB frame struct, and
     then `INIT_EXTRA_FRAME_INFO' and `INIT_FRAME_PC' will be called for
     the new frame.