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SubjectRe: floating point problem
On Thu, 5 Jul 2001 mdaljeet@in.ibm.com wrote:

> In Linux PPC, the MSR[FP] bit (that is floating point available bit) is off
> (atleast for non-SMP).
>

Yes, so the first FP instruction per process lets "lazy FPU" save/restore
work.


> Due to this, whenever some floating point instruction is executed in 'user
> mode', it leads to a exception 'FPUnavailable'. The exception handler for
> this exception apart from setting the MSR[FP] bit, also sets the MSR[FE0]
> and MSR[FE1] bits. These bits basically enables the floating point
> exceptions so that if there are some floating point exception conditions
> encountered while exeuting a floating point instruction, an appropriate
> exception is raised.
> But whenever some floating point instruction is executed in 'kernel mode',
> 'FPUnavailabe' exception handler code does not set the 'MSR[FE0] and
> MSR[FE1]' bits.
>

The kernel is not supposed to use floating-point.

[SNIPPED...]

I think all you need is this:

/*
* Note FPU control only exists per process. Therefore, you have
* to set up the FPU before you use it in any program.
*/
#include <i386/fpu_control.h>

#define FPU_MASK (_FPU_MASK_IM |\
_FPU_MASK_DM |\
_FPU_MASK_ZM |\
_FPU_MASK_OM |\
_FPU_MASK_UM |\
_FPU_MASK_PM)

void fpu()
{
__setfpucw(_FPU_DEFAULT & ~FPU_MASK);
}


main() {
double zero=0.0;
double one=1.0;
fpu();

one /=zero;
}



Cheers,
Dick Johnson

Penguin : Linux version 2.4.1 on an i686 machine (799.53 BogoMips).

I was going to compile a list of innovations that could be
attributed to Microsoft. Once I realized that Ctrl-Alt-Del
was handled in the BIOS, I found that there aren't any.


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