FCLEX           Clear exceptions, with error check   Exceptions: None
FNCLEX          Clear exceptions, no wait            C3 C2 C1 C0: ? ? ? ?

FCLEX
FNCLEX

        Logic   status word ← status word AND 7F00h

    These instructions clear the FP exception flags, the exception
    status flag (bit 7 -- the interrupt request flag in the 8087), and
    the busy flag of the FPU status word.
    Before clearing the exception flags, FCLEX checks for unmasked FP
    error conditions (using an FWAIT); FNCLEX does not make this check.

    FCLEX/FNCLEX can be used by an exception handler after it has
    processed an exception. If the exception flags are not cleared, a
    second interrupt request would be issued immediately.


    Opcode      Format
    9B DB E2    FCLEX
    DB E2       FNCLEX


    Timing
    Variations    8087         287        387      486     Pentium
    fclex         2-8          2-8        11        7       9      NP
    fnclex        2-8          2-8        11        7       9      NP

                The wait version may take additional cycles