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Vol. 2A 3-303
INSTRUCTION SET REFERENCE, A-M
FCMOVcc—Floating-Point Conditional Move
FCMOVcc—Floating-Point Conditional Move
Description
Tests the status flags in the EFLAGS register and moves the source operand (second
operand) to the destination operand (first operand) if the given test condition is true.
The condition for each mnemonic os given in the Description column above and in
Chapter 7 in the Intel® 64 and IA-32 Architectures Software Developer’s Manual,
Volume 1. The source operand is always in the ST(i) register and the destination
operand is always ST(0).
The FCMOVcc instructions are useful for optimizing small IF constructions. They also
help eliminate branching overhead for IF operations and the possibility of branch
mispredictions by the processor.
A processor may not support the FCMOVcc instructions. Software can check if the
FCMOVcc instructions are supported by checking the processor’s feature information
with the CPUID instruction (see “COMISS—Compare Scalar Ordered Single-Precision
Floating-Point Values and Set EFLAGS” in this chapter). If both the CMOV and FPU
feature bits are set, the FCMOVcc instructions are supported.
This instruction’s operation is the same in non-64-bit modes and 64-bit mode.
IA-32 Architecture Compatibility
The FCMOVcc instructions were introduced to the IA-32 Architecture in the P6 family
processors and are not available in earlier IA-32 processors.
Opcode* Instruction 64-Bit
Mode
Compat/
Leg Mode*
Description
DA C0+i FCMOVB ST(0), ST(i) Valid Valid Move if below (CF=1).
DA C8+i FCMOVE ST(0), ST(i) Valid Valid Move if equal (ZF=1).
DA D0+i FCMOVBE ST(0), ST(i) Valid Valid Move if below or equal (CF=1 or
ZF=1).
DA D8+i FCMOVU ST(0), ST(i) Valid Valid Move if unordered (PF=1).
DB C0+i FCMOVNB ST(0), ST(i) Valid Valid Move if not below (CF=0).
DB C8+i FCMOVNE ST(0), ST(i) Valid Valid Move if not equal (ZF=0).
DB D0+i FCMOVNBE ST(0), ST(i) Valid Valid Move if not below or equal (CF=0
and ZF=0).
DB D8+i FCMOVNU ST(0), ST(i) Valid Valid Move if not unordered (PF=0).
NOTES:
* See IA-32 Architecture Compatibility section below.