FSTCW           Store control word                   Exceptions: None
FNSTCW          Store control word, no wait          C3 C2 C1 C0: ? ? ? ?

FSTCW destination
FNSTCW destination

        Logic   destination ← control word

    FSTCW stores the FPU control word register in destination (a 16-bit
    memory location). The no-wait form of this instruction is FNSTCW.
    The FSTCW form checks for unmasked FP error conditions before
    storing the control word; FNSTCW does not make this check.


    Opcode      Format
    9B D9 /7    FSTCW m16
    D9 /7       FNSTCW m16


    Timing
    Variations/
    operand       8087         287        387      486     Pentium
    fstcw  m16   12-18        12-18       15        3       2      NP
    fnstcw m16   12-18        12-18       15        3       2      NP

                The wait version may take additional cycles