gdb macros

Hi all,

I would like to learn more about gdb macros. I have found a
bunch of 
examples of how to set them up so that is pretty cool. What
I would like 
to know more about is what variables does gdb provide
regarding its 
internal state? For instance $SHOW_CONTENT is one. My goal,
besides 
learning more about gdb is to write a macro that traverses
the current 
stack frame from bottom to top and does a list  in each
frame. Can any 
one point me in the right direction?

Thanks
Burlen