Re: RFA: Recognize 'x' in response to 'p' packet

[Paul Schlie <schlie@comcast.net>]

I would have thought that the BFD etc. description of the target machine
would have correctly identified the register set the machine supported.

(but the desire to support a value of x "un-defined/initialized" may
 make sense if interfaced to a simulator, if the value of x were a
 represent-able gdb value, and could propagate through expression
 evaluations which it presently can't, but would be potentially useful
 if it could to catch subtle bugs; but it's not clear that was the intent?)

> From: Steven Johnson <sjohnson@neurizon.net>
> 
> The PowerPC is such an example, where the Register response usually
> includes floating point registers, but say the MPC860 family doesnt have
> them.   I would imagine the reason to do it is so that, in future, GDB
> can remove from the users view registers that are non existent for a
> target, rather than show them as 0's.
> 
> If so, then this would be a necessary first step (identifying such from
> the target.)
> 
> Steven
> 
> Paul Schlie wrote:
> 
>>> Jim Blandy
>>> * remote.c (fetch_register_using_p): Recognize 'x's for the value
>>>  of the register as indicating that the register's value is not
>>>  available.
>> 
>> 
>> Out of curiosity, under what practical circumstances would the value of a
>> register not be accessible? (and if not, shouldn't an error be returned, as
>> opposed to an 'x' which is converted to a 0 anyway? Which I've noticed "g"
>> packets also assume?)