cltd
Convert Long to Double Long
CLTD
Sign-extends EAX into EDX:EAX (also CDQ).
Details
Sign-extends the 32-bit value in EAX into the 64-bit pair EDX:EAX; the sign bit of EAX (bit 31) is copied to all bits of EDX (bits 63–32), and EAX remains unchanged. This is a zero-operand instruction with no flags modified. In 64-bit mode, CDQ is the canonical mnemonic and has the same behavior. Commonly used before IDIV to prepare the dividend.
Pseudocode Operation
EDX ← (EAX[31]) ? 0xFFFFFFFF : 0x00000000;Example
CLTD
Encoding
Binary Layout
99
+0
Operands
Reference (Intel® SDM)
Instruction Forms
| Opcode | Instruction | Op/En | 64/32-bit Mode | CPUID | Description |
|---|---|---|---|---|---|
| 99 | CWD | ZO | Valid Valid | DX:AX := sign-extend of AX. | |
| 99 | CDQ | ZO | Valid Valid | EDX:EAX := sign-extend of EAX. | |
| REX.W + 99 | CQO | ZO | Valid N.E. | RDX:RAX:= sign-extend of RAX. |
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 position 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 dividend 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*) RDX := SignExtend(RAX); FI; FI;
Flags Affected
None. Exceptions (All Operating Modes) #UD If the LOCK prefix is used. CWD/CDQ/CQO—Convert Word to Doubleword/Convert Doubleword to Quadword Vol. 2A 3-257