CBW/CWDE/CDQE

Convert Byte to Word/Convert Word to Doubleword/Convert Doubleword to Quadword

Opcodes

Hex Mnemonic Encoding Long Mode Legacy Mode Description
REX.W + 98 CDQE A Valid N.E. RAX  sign-extend of EAX.
98 CWDE A Valid Valid EAX  sign-extend of AX.
98 CBW A Valid Valid AX  sign-extend of AL.

Instruction Operand Encoding

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

Description

Double the size of the source operand by means of sign extension. The CBW (convert byte to word) instruction copies the sign (bit 7) in the source operand into every bit in the AH register. The CWDE (convert word to doubleword) instruction copies the sign (bit 15) of the word in the AX register into the high 16 bits of the EAX register.

CBW and CWDE reference the same opcode. The CBW instruction is intended for use when the operand-size attribute is 16; CWDE is intended for use when the operand-size attribute is 32. Some assemblers may force the operand size. Others may treat these two mnemonics as synonyms (CBW/CWDE) and use the setting of the operand-size attribute to determine the size of values to be converted.

In 64-bit mode, the default operation size is the size of the destination register. Use of the REX.W prefix promotes this instruction (CDQE when promoted) to operate on 64-bit operands. In which case, CDQE copies the sign (bit 31) of the doubleword in the EAX register into the high 32 bits of RAX.

Pseudo Code

IF OperandSize = 16 (* Instruction = CBW *)
	AX = SignExtend(AL);
ELSE
	IF (OperandSize = 32, Instruction = CWDE) EAX = SignExtend(AX);
	FI;
ELSE
	(* 64-Bit Mode, OperandSize = 64, Instruction = CDQE *)
	RAX = SignExtend(EAX);
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.