Re: Toshiba TX3927 board boot problem.

[Carsten Langgaard <carstenl@mips.com>]

Alice Hennessy wrote:

> Ralf Baechle wrote:
>
> > On Mon, Oct 29, 2001 at 04:17:23PM -0800, Alice Hennessy wrote:
> >
> > > > This doesn't look right, you still need to enable the CU1 bit in the
> > > > status register to let the FP emulator kick-in.  FPU-less CPUs should
> > > > take a coprocessor unusable exception on any floating-point instructions.
> > > > I have been running this on several FPU-less CPUs, and it works fine for
> > > me.
> > >
> > > Maybe the FPU-less CPUs you have been using define the CU1 bit as reserved
> > > or is unused (ignore on write, zero on read)? The TX3927 actually allows
> > > the setting of the CU1 bit.  Have you seen a case where you need to set
> > > the CU1 bit for the emulation to kick-in?   I would think that the CU1
> > > bit should never be set to one for FPU-less CPUs.
> >
> > There are subtle differences in how CUx bits for unimplemented coprocessors
> > are handled in the various processors.  MIPS32 and MIPS64 specifies the
> > behaviour as 0 on read, writes ignored; previous processors such as the
> > R4000 handled this differently and as a consequence a fp instruction on
> > a fpu-less r4000 class cpu may either throw a CU or a reserved instruction
> > exception.  To make things easier for everybody this is documented in the
> > R10000 user's manual ...
> >
> >   Ralf
>
> So,  we should not set CU1 generically for FPU-less CPUs especially since a
> known problem exists
> for the tx3927?  Ie, qualify all setting of CU1 as follows:
>
> if (mips_cpu.options & MIPS_CPU_FPU)
>                    set_cp0_status(ST0_CU1);

And while we are at it, could we handle the CP0 hazard of 4 nops, between setting
the CU1 bit in the status register and executing
the first floating point instruction, on CPU which got a FPU.

>
>
> Alice

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