3-120 Vol. 2A CMOVcc—Conditional Move
INSTRUCTION SET REFERENCE, A-M
Description
The CMOVcc instructions check the state of one or more of the status flags in the
EFLAGS register (CF, OF, PF, SF, and ZF) and perform a move operation if the flags are
in a specified state (or condition). A condition code (cc) is associated with each
instruction to indicate the condition being tested for. If the condition is not satisfied,
a move is not performed and execution continues with the instruction following the
CMOVcc instruction.
These instructions can move 16-bit, 32-bit or 64-bit values from memory to a
general-purpose register or from one general-purpose register to another. Condi-
tional moves of 8-bit register operands are not supported.
The condition for each CMOVcc mnemonic is given in the description column of the
above table. The terms “less” and “greater” are used for comparisons of signed inte-
gers and the terms “above” and “below” are used for unsigned integers.
Because a particular state of the status flags can sometimes be interpreted in two
ways, two mnemonics are defined for some opcodes. For example, the CMOVA
(conditional move if above) instruction and the CMOVNBE (conditional move if not
below or equal) instruction are alternate mnemonics for the opcode 0F 47H.
The CMOVcc instructions were introduced in P6 family processors; however, these
instructions may not be supported by all IA-32 processors. Software can determine if
the CMOVcc instructions are supported by checking the processor’s feature informa-
tion with the CPUID instruction (see “CPUID—CPU Identification” in this chapter).
In 64-bit mode, the instruction’s default operation size is 32 bits. Use of the REX.R
prefix permits access to additional registers (R8-R15). Use of the REX.W prefix
promotes operation to 64 bits. See the summary chart at the beginning of this
section for encoding data and limits.
Operation
temp ← SRC
IF (64-Bit Mode)
THEN
IF condition TRUE
THEN
IF (OperandSize = 64)
THEN
DEST ← temp;
ELSE
DEST ← temp AND 0x00000000_FFFFFFFF;
FI;
FI;
ELSE
IF condition TRUE
THEN
