GDB front-end for SimH

Paul Koning paulkoning at
Tue Apr 24 20:10:22 CDT 2018

> On Apr 24, 2018, at 8:14 PM, Maciej W. Rozycki <macro at> wrote:
> On Tue, 24 Apr 2018, Paul Koning via cctalk wrote:
>> One drawback is that there aren't all that many SIMH architectures that 
>> have GDB support.  VAX does, and Alpha; that's about it.  I don't know 
>> how hard it is to do a basic platform in GDB, probably not all that 
>> difficult.  At least for machines like PDP-11.  One's complement might 
>> be harder (PDP-1), never mind things like the IBM 1620...  :-)
> You need target BFD and libopcodes support for a GDB port to be possible.  
> There is such support already present for PDP-11, so wiring in GDB parts 
> should be straightforward.  A remote protocol stub for SimH should also be 
> easy to write as you don't need all the fancy stuff and GDB will be happy 
> to work with just the `?', `G', `M', `c', `g', `m' request packets and the 
> `S' stop reply packet implemented.  The rest is optional.

True.  But if the SIMH code has breakpoint support, GDB can use that as a "hardware breakpoint".  Not all that interesting, perhaps, unless you're putting breakpoints in ROM.  More interesting is watchpoint support, which some SIMH targets have, and GDB supports.

I haven't tried to do a minimal BFD/opcodes implementation for a platform that doesn't yet have one.  PDP-10 perhaps (unless Lars did one years ago?).  One wonders if non-power-of-two wordlengths add complication, as they do in recent GCC.

I don't know if the SIMH execution and control frameworks make it convenient to hook up the GDB remote stub protocol. A stop to the SIMH console would want to be turned into a GDB remote interaction instead, and break signals from GDB would have to be recognized while SIMH is running the emulated machine.


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