FXTRACT         Extract exponent and significand     Exceptions: I
                                                     C3 C2 C1 C0: ? ? * ?
FXTRACT

        Logic   temp ← ST
                ST ← exponent(ST)
                push significand(temp) into ST

    FXTRACT separates out the exponent and significand of the value in
    ST. It puts the exponent into ST and then pushes the significand
    onto the stack. (Both are represented as real numbers.)
    It leaves the exponent in ST(1) and the significand in ST.


    Note
    FXTRACT performs a superset of the IEEE-recommended logb(x)
    function.
    If the original operand is zero, FXTRACT leaves negative infinity as
    the exponent in ST(1) and sets ST to zero with the same sign as the
    original operand. The zero-divide exception is raised in this case.
    ST(7) must be empty to avoid an invalid-operation exception.

    Note also that the exponent generated by FXTRACT is the true
    exponent, not the biased exponent used internally by the FPU.


    Opcode      Format
    D9 F4       FXTRACT


    Timing
    Operand       8087         287        387      486     Pentium
    -            27-55        27-55      70-76    16-20    13      NP