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FCOM            Compare real                         Exceptions: I D
FCOMP           Compare real and pop                 C3 C2 C1 C0: * * * *
FCOMPP          Compare real and pop twice

FCOM [source]
FCOMP [source]
FCOMPP

        Logic
        ; FCOM                 ; FCOMP                ; FCOMPP
        if source operand      if source operand      CMP ST, ST(1)
           CMP ST, source         CMP ST, source      set condition codes
        else                   else                   pop ST
           CMP ST, ST(1)          CMP ST, ST(1)       pop ST
        endif                  endif
        set condition codes    set condition codes
                               pop ST

                C3 C1 C0
                0  0  0     ST > source
                0  0  1     ST < source
                1  0  0     ST = source
                1  1  1     ST not comparable to source


    FCOM compares a real number to ST and leaves the result encoded in
    the status word as shown above. If no source operand is specified,
    ST(1) is compared to ST. Otherwise, source is compared to ST.

    FCOMP operates like FCOM, except that it also pops the stack.

    FCOMPP compares ST to ST(1), sets the condition codes as shown
    above, and pops the stack twice. It takes no operands.


    Note
    If ST contains a NaN or has an undefined format, or a stack fault
    occurs, the invalid-operand exception is raised and the condition
    bits are set to 'unordered'.
    The sign of zero is ignored, so that -0.0 = +0.0.


    The FCOM instructions perform the same operation as the FUCOM
    instructions. The only difference is how they handle QNaN operands.
    The FCOM instructions raise an invalid-arithmetic-operand exception
    (#IA) when either or both of the operands is a NaN value or is in an
    unsupported format. The FUCOM instructions perform the same
    operation as the FCOM instructions, except that they do not generate
    an invalid-arithmetic-operand exception for QNaNs.


    Opcode      Format
    D8 /2       FCOM m32
    DC /2       FCOM m64
    D8 D0 + i   FCOM ST(i)
    D8 D1       FCOM
    D8 /3       FCOMP m32
    DC /3       FCOMP m64
    D8 D8 + i   FCOMP ST(i)
    D8 D9       FCOMP
    DE D9       FCOMPP


    Timing
    Variations/
    operand      8087         287        387      486     Pentium
    fcom reg     40-50        40-50       24        4      4/1     FX
    fcom m32    (60-70)+EA    60-70       26        4      4/1     FX
    fcom m64    (65-75)+EA    65-75       31        4      4/1     FX
    fcomp        42-52        42-52       26        4      4/1     FX
    fcompp       45-55        45-55       26        5      4/1     FX