3-64
INSTRUCTION SET REFERENCE
CBW/CWDEConvert Byte to Word/Convert Word to Doubleword
Description
These instructions double the size of the source operand by means of sign extension (refer to
Figure 6-5 in Chapter 6, Instruction Set Summary of the Intel Architecture Software Developers
Manual, Volume 1). 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 higher 16 bits of the
EAX register.
The CBW and CWDE mnemonics reference the same opcode. The CBW instruction is intended
for use when the operand-size attribute is 16 and the CWDE instruction for when the operand-
size attribute is 32. Some assemblers may force the operand size to 16 when CBW is used and
to 32 when CWDE is used. Others may treat these mnemonics as synonyms (CBW/CWDE) and
use the current setting of the operand-size attribute to determine the size of values to be
converted, regardless of the mnemonic used.
The CWDE instruction is different from the CWD (convert word to double) instruction. The
CWD instruction uses the DX:AX register pair as a destination operand; whereas, the CWDE
instruction uses the EAX register as a destination.
Operation
IF OperandSize = 16 (* instruction = CBW *)
THEN AX
<
SignExtend(AL);
ELSE (* OperandSize = 32, instruction = CWDE *)
EAX
<
SignExtend(AX);
FI;
Flags Affected
None.
Exceptions (All Operating Modes)
None.
Opcode
Instruction
Description
98
CBW
AX
<
sign-extend of AL
98
CWDE
EAX
<
sign-extend of AX
