CWD/CDQ/CQO — Convert Word to Doubleword/Convert Doubleword to Quadword

Opcode	Instruction	Op/En	64-Bit Mode	Compat/Leg Mode	Description
99	CWD	ZO	Valid	Valid	DX:AX ← sign-extend of AX.
99	CDQ	ZO	Valid	Valid	EDX:EAX ← sign-extend of EAX.
REX.W + 99	CQO	ZO	Valid	N.E.	RDX:RAX← sign-extend of RAX.

Instruction Operand Encoding ¶

Op/En	Operand 1	Operand 2	Operand 3	Operand 4
ZO	NA	NA	NA	NA

Description ¶

Doubles the size of the operand in register AX, EAX, or RAX (depending on the operand size) by means of sign extension and stores the result in registers DX:AX, EDX:EAX, or RDX:RAX, respectively. The CWD instruction copies the sign (bit 15) of the value in the AX register into every bit position in the DX register. The CDQ instruction copies the sign (bit 31) of the value in the EAX register into every bit position in the EDX register. The CQO instruction (available in 64-bit mode only) copies the sign (bit 63) of the value in the RAX register into every bit position in the RDX register.

The CWD instruction can be used to produce a doubleword dividend from a word before word division. The CDQ instruction can be used to produce a quadword dividend from a doubleword before doubleword division. The CQO instruction can be used to produce a double quadword dividend from a quadword before a quadword division.

The CWD and CDQ mnemonics reference the same opcode. The CWD instruction is intended for use when the operand-size attribute is 16 and the CDQ instruction for when the operand-size attribute is 32. Some assemblers may force the operand size to 16 when CWD is used and to 32 when CDQ is used. Others may treat these mnemonics as synonyms (CWD/CDQ) and use the current setting of the operand-size attribute to determine the size of values to be converted, regardless of the mnemonic used.

In 64-bit mode, use of the REX.W prefix promotes operation to 64 bits. The CQO mnemonics reference the same opcode as CWD/CDQ. See the summary chart at the beginning of this section for encoding data and limits.

Operation ¶

IF OperandSize = 16 (* CWD instruction *)
    THEN
        DX ← SignExtend(AX);
    ELSE IF OperandSize = 32 (* CDQ instruction *)
        EDX ← SignExtend(EAX); FI;
    ELSE IF 64-Bit Mode and OperandSize = 64 (* CQO instruction*)
        RDX ← SignExtend(RAX); FI;
FI;

Flags Affected ¶

None

Exceptions (All Operating Modes) ¶

#UD If the LOCK prefix is used.