Re: WIP: Register doco

Daniel Jacobowitz
2002-07-21T17:04:49Z

On Sun, Jul 21, 2002 at 12:40:59PM -0400, Andrew Cagney wrote:
>
> >No, I think we need to draw the GDB developer's eye *to* those glossy
> >user-level ISA specs. :) Figure 8-1 --- the first diagram in the
> >chapter titled "Programming with the X87 FPU" --- has R0 -- R7 right
> >there. The second diagram, figure 8-2, shows how the TOP field
> >affects the relationship between ST(i) and Ri. The fact that there is
> >a fixed set of registers accessed as a rotating stack is very much
> >part of the ISA documentation.
>
> I was talking generally.
>
> However looking at specific manual set(1)
> (http://developer.intel.com/design/pentium4/manuals), vol1 is the user
> stuff; vol3 is the system stuff. The GDB developer needs to look beyond
> vol1 and and into vol3. Vol1 8.1.10 Saving the x87 FPU's State with the
> FXSAVE Instruction, for instance, just points the reader at volume 3.
> In addition, the GDB developer ends up studying kernel interfaces and
> too often (ulgh!) kernel sources.
>
> Taking a step back. Cooked registers are at the level of the user
> and/or ABI. Raw registers are at the level of the underlying
> system/hardware. The GDB developer needs to be familar with both. More
> importantly, and as I mentioned last time, we need to be very careful to
> ensure that the GDB developer looks beyond that user model and on down
> to the lower level details of the architecture.
>
> >As a sanity check, assuming that SPARC register windows are analogous:
> >the SPARC ISA spec talks about register windows immediately, as well.
> >Figure 2 in the chapter on Registers shows "Three Overlapping Windows
> >and the Eight Global Registers". (For some reason, that makes me
> >think of Goldilocks and the Three Bears.)
>
> Just FYI, an example involving the SPARC is on my things todo list for
> frames. It turns out that the OS for a register-window architecture
> typically flushes all but the inner most window to memory before
> transfering control to GDB. Consequently the only raw registers that
> GDB sees are those that are innermost. It is the frame, and not the
> register cache code, that needs to handle this one.

"Typically" doesn't seem terribly useful... especially when debugging
through a monitor rather than ptrace. I just worked on a GDB port to
an architecture which does not do this (and let me tell you, it's
awkward to deal with in GDB. It will be much easier when some more of
your regcache work is finished.). The frame code had to figure out
where in the register file to find the registers for the current (or
any other) frame.

--
Daniel Jacobowitz Carnegie Mellon University
MontaVista Software Debian GNU/Linux Developer