A.5.104 FSTENV: Store Floating-Point Environment
FSTENV mem ; 9B D9 /6 [8086,FPU]
FNSTENV mem ; D9 /6 [8086,FPU]
FSTENV stores the FPU operating environment (control word, status word,
tag word, instruction pointer, data pointer and last opcode) into
memory. The memory area is 14 or 28 bytes long, depending on the CPU
mode at the time. See also FLDENV (section A.5.91).
FNSTENV does the same thing as FSTENV, without first waiting for pending
floating-point exceptions to clear.