A.5.114 FYL2X, FYL2XP1: Compute Y times Log2(X) or Log2(X+1)
FYL2X ; D9 F1 [8086,FPU]
FYL2XP1 ; D9 F9 [8086,FPU]
FYL2X multiplies ST1 by the base-2 logarithm of ST0, stores the result
in ST1, and pops the register stack (so that the result ends up in ST0).
ST0 must be non-zero and positive.
FYL2XP1 works the same way, but replacing the base-2 log of ST0 with
that of ST0 plus one. This time, ST0 must have magnitude no greater than
1 minus half the square root of two.