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(nasm.info)Section B.4.72

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B.4.72. `FCMOVcc': Floating-Point Conditional Move
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     FCMOVB fpureg                 ; DA C0+r              [P6,FPU]
     FCMOVB ST0,fpureg             ; DA C0+r              [P6,FPU]

     FCMOVE fpureg                 ; DA C8+r              [P6,FPU]
     FCMOVE ST0,fpureg             ; DA C8+r              [P6,FPU]

     FCMOVBE fpureg                ; DA D0+r              [P6,FPU]
     FCMOVBE ST0,fpureg            ; DA D0+r              [P6,FPU]

     FCMOVU fpureg                 ; DA D8+r              [P6,FPU]
     FCMOVU ST0,fpureg             ; DA D8+r              [P6,FPU]

     FCMOVNB fpureg                ; DB C0+r              [P6,FPU]
     FCMOVNB ST0,fpureg            ; DB C0+r              [P6,FPU]

     FCMOVNE fpureg                ; DB C8+r              [P6,FPU]
     FCMOVNE ST0,fpureg            ; DB C8+r              [P6,FPU]

     FCMOVNBE fpureg               ; DB D0+r              [P6,FPU]
     FCMOVNBE ST0,fpureg           ; DB D0+r              [P6,FPU]

     FCMOVNU fpureg                ; DB D8+r              [P6,FPU]
     FCMOVNU ST0,fpureg            ; DB D8+r              [P6,FPU]

   The `FCMOV' instructions perform conditional move operations: each of
them moves the contents of the given register into `ST0' if its
condition is satisfied, and does nothing if not.

   The conditions are not the same as the standard condition codes used
with conditional jump instructions. The conditions `B', `BE', `NB',
`NBE', `E' and `NE' are exactly as normal, but none of the other
standard ones are supported. Instead, the condition `U' and its
counterpart `NU' are provided; the `U' condition is satisfied if the
last two floating-point numbers compared were _unordered_, i.e. they
were not equal but neither one could be said to be greater than the
other, for example if they were NaNs. (The flag state which signals
this is the setting of the parity flag: so the `U' condition is
notionally equivalent to `PE', and `NU' is equivalent to `PO'.)

   The `FCMOV' conditions test the main processor's status flags, not
the FPU status flags, so using `FCMOV' directly after `FCOM' will not
work. Instead, you should either use `FCOMI' which writes directly to
the main CPU flags word, or use `FSTSW' to extract the FPU flags.

   Although the `FCMOV' instructions are flagged `P6' above, they may
not be supported by all Pentium Pro processors; the `CPUID' instruction
(*Note Section B.4.34::) will return a bit which indicates whether
conditional moves are supported.