CWD/CDQ/CQO

Convert Word to Doubleword/Convert Doubleword to Quadword

Opcodes

Hex Mnemonic Encoding Long Mode Legacy Mode Description
REX.W + 99 CQO A Valid N.E. RDX:RAX sign-extend of RAX.
99 CDQ A Valid Valid EDX:EAX  sign-extend of EAX.
99 CWD A Valid Valid DX:AX  sign-extend of AX.

Instruction Operand Encoding

Op/En Operand 0 Operand 1 Operand 2 Operand 3
A 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.

Pseudo Code

IF OperandSize = 16 (* CWD instruction *)
	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

64-Bit Mode Exceptions

Exception Description
#UD If the LOCK prefix is used.

Compatibility Mode Exceptions

Exception Description
#UD If the LOCK prefix is used.

Virtual-8086 Mode Exceptions

Exception Description
#UD If the LOCK prefix is used.

Real-Address Mode Exceptions

Exception Description
#UD If the LOCK prefix is used.

Protected Mode Exceptions

Exception Description
#UD If the LOCK prefix is used.