Intel 253666-024US User Manual
Page 151
Vol. 2A 3-105
INSTRUCTION SET REFERENCE, A-M
CBW/CWDE/CDQE—Convert Byte to Word/Convert Word to Doubleword/Convert Double-
word to Quadword
CBW/CWDE/CDQE—Convert Byte to Word/Convert Word to
Doubleword/Convert Doubleword to Quadword
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.
Operation
IF OperandSize
=
16 (* Instruction
=
CBW *)
THEN
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 (All Operating Modes)
#UD
If the LOCK prefix is used.
Opcode
Instruction
64-Bit
Mode
Compat/
Leg Mode
Description
98
CBW
Valid
Valid
AX ← sign-extend of AL.
98
CWDE
Valid
Valid
EAX ← sign-extend of AX.
REX.W + 98
CDQE
Valid
N.E.
RAX ← sign-extend of EAX.