From Dave Richards (drichards@mahinetworks.com)

  "While diagnosing an MMX/FPU problem I found a minor problem in the
   code which diagnoses and generates signals for FPU exceptions.  On
   x86 Stack Fault Exception are a subclass of Invalid Operation.  Thus,
   the FPU status register will have both the SF and IF bits set when a
   stack fault occurs.  The code which turns FPU exceptions into signals
   was assuming IF would be clear".