[patch/rfc] Revise REGISTER_SIM_REGNO()

[Richard Earnshaw]

> > I need a way the target vector to let REGISTER_NAME() know that the
> > register is "unavailable this session", so that it can return an empty
> > string for the register; so that gdb won't think it exists at all.
> 
> 
> I see.  This is something that depends on both the arch and the target.

Yep, the arch says what registers a processor has, the target says which 
of those it can supply.

Of course, you will only get a useful debugging session if the two overlap 
sufficiently.

I suspect that it will be quite common for the arch to describe registers 
that are only available when the processor is running in a privileged 
mode; clearly you don't need those to debug a user-level program, but they 
can be very helpful when trying to debug an OS.  Certainly this is the way 
I'd like to describe the views of ARM processors.

R.