FXCH            Exchange register contents           Exceptions: I
                                                     C3 C2 C1 C0: ? ? 0 ?
FXCH [destination]

                ; FXCH          ; FXCH dest
        Logic   temp ← ST(1)    temp ← ST(i)
                ST(1) ← ST      ST(i) ← ST
                ST ← temp       ST ← temp

    FXCH swaps the contents of the destination and stack-top registers.
    If no destination is specified, ST is exchanged with ST(1).


    Note
    Many FPU instructions operate only on ST. FXCH provides a handy way
    to operate on lower stack elements.


        Example
        ; compute SQRT of ST(4)
        fxch ST(4)
        fsqrt
        fxch ST(4)


    Opcode      Format
    D9 C8 + i   FXCH ST(i)
    D9 C9       FXCH


    Timing
    Operand       8087         287        387      486     Pentium
    -            10-15        10-15       18        4      0-1     *

        * FCXH is pairable in the V pipe with all FX pairable instructions