FCMOVcc
Usage: FCMOVcc dest , src Modifies flags: See Below
Tests the status flags in the EFLAGS register and moves the 'src' operand to the 'dest' operand if the given test condition is true.
Floating-Point Conditional Move
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, 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 () will return a bit which indicates whether conditional moves are supported.
FPU Flags Affected C1 Set to 0 if stack underflow occurred. C0, C2, C3 Undefined.
FCMOVE Floating-point conditional move if equal
FCMOVNE Floating-point conditional move if not equal
FCMOVB Floating-point conditional move if below
FCMOVBE Floating-point conditional move if below or equal
FCMOVNB Floating-point conditional move if not below
FCMOVNBE Floating-point conditional move if not below or equal
FCMOVU Floating-point conditional move if unordered
FCMOVNU Floating-point conditional move if not unordered
EXAMPLE:
fcmovbe st0, st4