> +enum {
> +  /* Normal sane architecture.  The corresponding register cache
> +     entry is zero filled and the register is marked as unavailable.  */
> +  SIM_REGNO_UNAVAILABLE = -1,
> 
> Does:
>     SIM_REGNO_DNE = -1,
> sound better?

(I spelt it out - SIM_REGNO_DOES_NOT_EXIST)

> For the moment remote-sim.c would mark the register as unavailable (fixme comment).  At least, though, the name is right.
> 
> +  /* For possible backward compatibility.  The register cache doesn't
> +     have a corresponding name.  Skip the register entirely.  */
> +  LEGACY_SIM_REGNO_IGNORE = -2
> +};

as per the attached.  Committed.

Andrew