FCMOVcc (Floating-Point Conditional Move)

Opcodes

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.

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 Descrip-tion column above and in Chapter 8 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 over-head 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 avail-able in earlier IA-32 processors.

Operation

IF condition TRUE
    THEN ST(0) ← ST(i);
FI;

FPU Flags Affected

C1 Set to 0 if stack underflow occurred.
C0, C2, C3 Undefined.

Floating-Point Exceptions

#IS Stack underflow occurred.

Integer Flags Affected

None.

Protected Mode Exceptions

#NM CR0.EM[bit 2] or CR0.TS[bit 3] = 1.
#UD If the LOCK prefix is used.

Real-Address Mode Exceptions

Same exceptions as in protected mode.

Virtual-8086 Mode Exceptions

Same exceptions as in protected mode.

Compatibility Mode Exceptions

Same exceptions as in protected mode.

64-Bit Mode Exceptions

Same exceptions as in protected mode.