> On Tue, 4 Aug 1998, Mikael Pettersson wrote:
>
> > Right. Floating-point is banned within the Linux kernel, yet some
>
> Uh... Just a stupid question... Why floating-point is banned?
>
The state of the FPU is undefined within the kernel. You would have
to add a 'FINIT' instruction before the actual FP code (the compiler
won't do it for you). Further, if you have a FP fault that traps
to the kernel's FP fault-handler, you probably generated a double-fault
from which you can't recover. Further, if you use FP emulation such
as for the '386, every ESCAPE (preamble for the FP code) generates
a FP trap to the kernel emulator.
The undefined state comes about because, although FPU state is saved and
then restored as part of the context-switch, the FPU is left as it was.
It might have been calculating a FYL2XP1 (y * log2(x+1)) instruction
which is interuptable, then the kernel code pushes two operands onto
the FP unit's stack to perform a multiply. Not good.
Since the only "need" for float would be to perform some sort of
scaling, fixed-point integers will always work.
Cheers,
Dick Johnson
***** FILE SYSTEM MODIFIED *****
Penguin : Linux version 2.1.113 on an i586 machine (66.15 BogoMips).
Warning : It's hard to remain at the trailing edge of technology.
-
To unsubscribe from this list: send the line "unsubscribe linux-kernel" in
the body of a message to majordomo@vger.rutgers.edu
Please read the FAQ at http://www.altern.org/andrebalsa/doc/lkml-faq.html