CBW/CWDE/CDQE--Convert Byte to Word/Convert Word to Doubleword/Convert Doubleword to Quadword

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.

Description

Double the size of the source operand by means of sign extension (see Figure 7-6 in the IA-32 Intel Architecture Software Developer's 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 high 16 bits of the EAX register.

[...]

Flags Affected

None.

Exceptions (All Operating Modes)

None.

IA-32 Intel Architecture Software Developer's Manual, Volume 2A: Instruction Set Reference, A-M
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