Re: [WIP/RFC] MIPS registers overhaul

[cgd@broadcom.com]

At Wed, 14 May 2003 22:05:11 +0000 (UTC), "Kevin Buettner" wrote:
> > ``info registers'' should always display the target's underlying 
> > register set.  In the case of o32 running on a 64 bit ISA, the 64 bit 
> > registers should be displayed.
> 
> Well, in this case the underlying register set really is only 32-bits
> wide.  Volume 1 of the MIPS64 Architecture (revision 1.00, dated
> August 29, 2002) says:
> 
>     For compatibility with MIPS32 processors, the FR bit in the CP0
>     Status register is used by a MIPS64 processor to configure the FPU
>     in a mode in which the FPRs are treated as 32 32-bit registers,
>     each of which is capable of storing only 32-bit data types.  In
>     this mode, the double-precision floating point (type D) data type
>     is stored in even-odd pairs of FPRs, and the long-integer (type L)
>     and paired single (type PS) data types are not supported.
> 
> Figure 2-8 in this manual shows that bits 32-63 (i.e, the high 32 bits)
> are "UNPREDICTABLE" in this mode.

Indeed.


> We still *could* show these extra bits, but displaying the register in
> this way would, most of the time, be next to useless.  If the GDB user
> really wants to see what the higher order bits are, then the raw_fN
> names are useful.

One possible thing to do is display the upper 32 bits *if* you know
that Status:FR is set to 1.  The Status:FR value is really the thing
that determines how the bits are interpreted.

Now, if o32, Status:FR *should* be 0.

However, I don't know if for old binaries being debugged o64 will be
detected properly (or, heck, even for new binaries 8-), and I also
know people who've done Really Special Things with o32 and soft float
and using library calls for all FP and using 64-bit floating point.
8-)

In other words, the real arbiter of the unpredictability of those bits
is Status:FR, so you should probably display them consistently with
what it says (if you have Status... I forget 8-).



I don't know enough about the remote protocol to know what one would
expect from a 64-bit debug agent debugging a 32-bit program (on a
64-bit target).  But i could imagine it could become ugly very
quickly...  8-)


cgd