FYL2XP1         Compute Y * log2(x+1)                Exceptions: P
                                                     (operands not checked)
FYL2XP1                                              C3 C2 C1 C0: ? ? * ?

        Logic   ST(1) ← ST(1) * Log2(ST + 1.0)
                pop ST

    FYL2XP1 computes the base-2 logarithm of (ST + 1.0), multiplies the
    logarithm by ST(1), and puts the result in ST(1). Then it pops ST.

    The operand in ST cannot be negative. The absolute value of the
    operand in ST must be greater than zero and less than SQRT(2)/2.


    Note
    If the operand in ST is outside the acceptable range, the result of
    FYL2XP1 is undefined.
    The 80486+ checks for interrupts while executing this instruction.
    It will abort FYL2XP1 to service interrupts.
    FYL2XP1 is more accurate than FYL2X when computing the logarithm of
    numbers very close to 1.


    Opcode      Format
    D9 F9       FYL2XP1


    Timing
    Variations    8087         287        387      486     Pentium
    fyl2xp1     700-1000     700-1000   257-547  171-326   22-103  NP