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(nasm.info)Section B.4.108

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B.4.108. `FUCOMxx': Floating-Point Unordered Compare
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     FUCOM fpureg                  ; DD E0+r              [386,FPU]
     FUCOM ST0,fpureg              ; DD E0+r              [386,FPU]

     FUCOMP fpureg                 ; DD E8+r              [386,FPU]
     FUCOMP ST0,fpureg             ; DD E8+r              [386,FPU]

     FUCOMPP                       ; DA E9                [386,FPU]

     FUCOMI fpureg                 ; DB E8+r              [P6,FPU]
     FUCOMI ST0,fpureg             ; DB E8+r              [P6,FPU]

     FUCOMIP fpureg                ; DF E8+r              [P6,FPU]
     FUCOMIP ST0,fpureg            ; DF E8+r              [P6,FPU]

   * `FUCOM' compares `ST0' with the given operand, and sets the FPU
     flags accordingly. `ST0' is treated as the left-hand side of the
     comparison, so that the carry flag is set (for a `less-than'
     result) if `ST0' is less than the given operand.

   * `FUCOMP' does the same as `FUCOM', but pops the register stack
     afterwards. `FUCOMPP' compares `ST0' with `ST1' and then pops the
     register stack twice.

   * `FUCOMI' and `FUCOMIP' work like the corresponding forms of
     `FUCOM' and `FUCOMP', but write their results directly to the CPU
     flags register rather than the FPU status word, so they can be
     immediately followed by conditional jump or conditional move
     instructions.

   The `FUCOM' instructions differ from the `FCOM' instructions (*Note
Section B.4.73::) only in the way they handle quiet NaNs: `FUCOM' will
handle them silently and set the condition code flags to an `unordered'
result, whereas `FCOM' will generate an exception.