FSTSW           Store status word                    Exceptions: None
FNSTSW          Store status word, no wait           C3 C2 C1 C0: ? ? ? ?

FSTSW destination
FNSTSW destination

        Logic   destination ← status word

    FSTSW stores the current value of the status word in destination,
    a 16-bit memory location, or -- in the 287+ -- the AX register of
    the 80x86. FSTSW checks for unmasked FP error conditions before
    storing the status word; FNSTSW does not make this check.


    Note
    FSTSW/FNSTSW are primarily used for conditional branching after a
    comparison instruction (FPREM, FPREM1, or FXAM). They can also be
    used to invoke exception handlers (by polling the exception bits) in
    environments that do not use interrupts.


        Example
        p287
        fstsw ax        ; read FPU status word
        sahf            ; copy condition codes to flags
        jc    @@c0      ; jc ~ C0, jp ~ C2, jz ~ C3


    Opcode      Format
    9B DF /7    FSTSW m16
    9B DF E0    FSTSW AX
    DF /7       FNSTSW m16
    DF E0       FNSTSW AX


    Timing
    Variations/
    operand       8087         287        387      486     Pentium
    fstsw  m16   12-18        12-18       15        3       2      NP
    fstsw  ax      -          10-16       13        3       2      NP
    fnstsw m16   12-18        12-18       15        3       2      NP
    fnstsw ax      -          10-16       13        3       2      NP

                The wait version may take additional cycles