fcmovnb
Floating-Point Conditional Move If Not Below
FCMOVNB ST(0), ST(i)
Moves ST(i) to ST(0) if CF=0.
Details
Conditionally moves the value in ST(i) to ST(0) if the carry flag (CF) is clear, indicating the first operand is not below the second in an x87 FPU comparison context. This is part of the x87 conditional move instruction set available on P6 and later processors. No flags are modified by this instruction.
Pseudocode Operation
if (CF == 0) { ST(0) ← ST(i); }Example
FCMOVNB st(0), st(1)
Encoding
Binary Layout
DB
+0
Operands
-
src
Register
Reference (Intel® SDM)
Instruction Forms
| Opcode | Instruction | Op/En | 64/32-bit Mode | CPUID | 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). |
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 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 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.
Operation
IF condition TRUE THEN ST(0) := ST(i); FI;
Exceptions
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.
FCMOVcc—Floating-Point Conditional Move Vol. 2A 3-323