A.5.113 FXTRACT: Extract Exponent and Significand
FXTRACT ; D9 F4 [8086,FPU]
FXTRACT separates the number in ST0 into its exponent and significand
(mantissa), stores the exponent back into ST0, and then pushes the
significand on the register stack (so that the significand ends up in
ST0, and the exponent in ST1).