B.4.113. `FXTRACT': Extract Exponent and Significand
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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').