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Vol. 2A 3-255
INSTRUCTION SET REFERENCE, A-M
CWD/CDQ/CQO—Convert Word to Doubleword/Convert Doubleword to Quadword
CWD/CDQ/CQO—Convert Word to Doubleword/Convert Doubleword to
Quadword
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 posi-
tion 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 divi-
dend 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.
Operation
IF OperandSize = 16 (* CWD instruction *)
THEN
DX ← SignExtend(AX);
ELSE IF OperandSize
= 32 (* CDQ instruction *)
EDX ← SignExtend(EAX); FI;
ELSE IF 64-Bit Mode and OperandSize = 64 (* CQO instruction*)
Opcode Instruction 64-Bit
Mode
Compat/
Leg Mode
Description
99 CWD Valid Valid DX:AX ← sign-extend of AX.
99 CDQ Valid Valid EDX:EAX ← sign-extend of EAX.
REX.W + 99 CQO Valid N.E. RDX:RAX← sign-extend of RAX.