Thu Nov 20 23:29:50 EST 2014

gdb stub / cortex m

I've been exploring Staapl ideas through a gdb stub.  Esstentially, if
you stop thinking about gdb as a thing that sets breakpoints and steps
through code, and start thinking of it as a thing that interactively
executes functions and inspects datastructures, you're halfway there.

The stub runs the GDB RSP protocol over USB CDCACM, polled from a main
loop that executes the low priority event loop of the microcontroller
application.  This mainloop keeps running while GDB is attached.  The
"stopped thread" that GDB sees is an empty stack frame that can be
used by GDB to evaluate code.  It is fake in the sense that stepping
through or continuing this current thread makes no sense.

Breakpoints and the continue command are only implemented to the point
that they are necessary to implement the on-target function call

This mechanism would probably be a good candidate for bootstrapping
Staapl on Cortex M.  However, I wonder if it is actually meaningful to
re-implement the interactive protocol.  The only part that is
necessary is a compiler, and maybe a frontend for gdb.