bit-for-bit IEEE compliance vs performance - strict compliance bit

[Norbert Juffa]

On Wed, 4 Oct 1995, Tom Lynch wrote:

> 
> > person). Where to stick this additional control bit? I would like to
> > propose bit 13 in the FPU control word. It looks like right now the
> 
> Hmm, putting it in the fpu control word could have side effects with
> old code that already mistakenly writes the bit.  I think that some
> supervisor mode protected register would be better.  The bit would not
> be toggled very often, so easy access is not necessary.
> 
> -tom


On a multitasking OS, every task should have the freedom to configure
the FPU to its liking. If you make the bit only writable in supervisor
mode, the OS needs to support a mechanism for user tasks to read/write
this bit. Also, for the purpose of task switching, if the bit is in the
control word, it is automatically switched together with the rest of the 
FPU state. If the bit is in one of the x86 CR registers, it would not
be automatically switched during a task switch, so further changes to
existing OS task switching code are necessary (this may be easy in Linux,
but not other OSs).

-- Norbert