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Re: [Lightning] Detecting use of floating point

From: Sandro Magi
Subject: Re: [Lightning] Detecting use of floating point
Date: Sat, 23 Feb 2008 00:57:45 -0500

On Sat, Feb 23, 2008 at 12:35 AM, tim <address@hidden> wrote:
> For X86, the TS bit of CR0 (X86), if on ,forces a fault if FP is used.
>  It is set by a hardware context switch and reset by a privileged
>  instruction.
>  The EM bit may also be useful.
>  The OS formally does this for you. If you are writing the OS, you woudl
>  have the authority to MOV into CR0.
>  Some CPUs have this facility and others do not.

Thanks for the pointers. I knew it could be done with ring-0
privilege, as I've studied OSes that use this optimization. I was just
wondering whether it could be done at user level. I suspect that most
operating systems don't expose an API to access this state. The C99
fenv.h header [1] exposes an interface to some floating point state,
but it's mainly limited to exceptional conditions and rounding modes.



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