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2006-01-24 Jim Blandy <jimb@redhat.com>

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* gdbint.texinfo (Frames): Document the basics of GDB's register
	unwinding model, and explain the existence of the "sentinel" frame.
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Jim Blandy 2006-01-25 18:48:57 +00:00
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@ -1,3 +1,8 @@
2006-01-24 Jim Blandy <jimb@redhat.com>
* gdbint.texinfo (Frames): Document the basics of GDB's register
unwinding model, and explain the existence of the "sentinel" frame.
2006-01-23 Andrew Stubbs <andrew.stubbs@st.com>
* gdb.texinfo (Choosing files): Mention that -directory is used

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gdb/doc/gdbint.texinfo
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@ -231,6 +231,36 @@ they use.
A frame is a construct that @value{GDBN} uses to keep track of calling
and called functions.
@cindex frame, unwind
@value{GDBN}'s current frame model is the result of an incremental
cleanup of working code, not a fresh design, so it's a little weird.
The natural model would have a frame object, with methods that read
and write that frame's registers. Reading or writing the youngest
frame's registers would simply read or write the processor's current
registers, since the youngest frame is running directly on the
processor. Older frames might have some registers saved on the stack
by younger frames, so accessing the older frames' registers would do a
mix of memory accesses and register accesses, as appropriate.
@findex frame_register_unwind
Instead, @value{GDBN}'s model is that you find a frame's registers by
``unwinding'' them from the next younger frame. That is, to access
the registers of frame #1 (the next-to-youngest frame), you actually
apply @code{frame_register_unwind} to frame #0 (the youngest frame).
But then the obvious question is: how do you access the registers of
the youngest frame itself? How do you ``unwind'' them when they're
not wound up?
@cindex sentinel frame
@findex get_frame_type
@vindex SENTINEL_FRAME
To answer this question, GDB has the @dfn{sentinel} frame, the
``-1st'' frame. Unwinding registers from the sentinel frame gives you
the current values of the youngest real frame's registers. If @var{f}
is a sentinel frame, then @code{get_frame_type (@var{f}) ==
SENTINEL_FRAME}.
@findex create_new_frame
@vindex FRAME_FP
@code{FRAME_FP} in the machine description has no meaning to the