A.5.105 FSTSW: Store Floating-Point Status Word
FSTSW mem16 ; 9B DD /7 [8086,FPU]
FSTSW AX ; 9B DF E0 [286,FPU]
FNSTSW mem16 ; DD /7 [8086,FPU]
FNSTSW AX ; DF E0 [286,FPU]
FSTSW stores the FPU status word into AX or into a 2-byte memory area.
FNSTSW does the same thing as FSTSW, without first waiting for pending
floating-point exceptions to clear.