A.5.103 FSTCW: Store Floating-Point Control Word
FSTCW mem16 ; 9B D9 /7 [8086,FPU]
FNSTCW mem16 ; D9 /7 [8086,FPU]
FSTCW stores the FPU control word (governing things like the rounding
mode, the precision, and the exception masks) into a 2-byte memory area.
See also FLDCW (section A.5.90).
FNSTCW does the same thing as FSTCW, without first waiting for pending
floating-point exceptions to clear.