Re: RFA/ARM: Switch mode when setting PC

Daniel Jacobowitz
2004-01-16T14:41:40Z

On Fri, Jan 16, 2004 at 02:34:23PM +0000, Richard Earnshaw wrote:
>
> >
> > Unless the "Thumb bit" is being stripped out by GDB, then I suspect that
> > this is a bug in the gdb/simulator binding layer. Any attempt to force
> > the PC value by the debugger should be taken as a potential state change.
> > If that is not happening, then all sorts of things may not work.
> >
> > I've suspected that there is a problem in the way that gdb drives the
> > simulator for a while now.
> >
>
> sim_store_register in sim/arm/wrapper.c is currently usring ARMul_SetReg
> to set the PC. I think this is wrong.
>
> RDI_CPUwrite in sim/arm/armrdi.c uses ARMul_SetPC in a similar context.
>
> ARMul_SetReg should only be used on the PC in specific circumstances,
> specifically from within the main simulation loop. Even then it should
> probably be using ARMul_SetR15.
>
> I suspect that this is why several rather gross hacks were introduced over
> the years to make single stepping work, and what you are seeing now may be
> another artifact of this general problem.

OK, but I don't think that's relevant. Take a look at what those
functions do:

void
ARMul_SetR15 (ARMul_State * state, ARMword value)
{
if (ARMul_MODE32BIT)
state->Reg[15] = value & PCBITS;
else
{
state->Reg[15] = value;
ARMul_R15Altered (state);
}
FLUSHPIPE;
}

void
ARMul_SetPC (ARMul_State * state, ARMword value)
{
if (ARMul_MODE32BIT)
state->Reg[15] = value & PCBITS;
else
state->Reg[15] = R15CCINTMODE | (value & R15PCBITS);
FLUSHPIPE;
}

As I said in my other message, none of these have the potential to
switch the simulator from Thumb to ARM mode. I spent some time looking
yesterday and I believe that none of the functions used to set the PC
do, with the exception of WriteR15Branch; and it's pretty obvious to me
that the sim interface should not be using WriteR15Branch, so the
simulator's client needs to handle setting the CPSR.

--
Daniel Jacobowitz
MontaVista Software Debian GNU/Linux Developer