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Chapter 17 -- 80386 Instruction Set
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CWD/CDQ -- Convert Word to Doubleword/Convert Doubleword to
Quadword
Opcode Instruction Clocks Description
99 CWD 2 DX:AX := sign-extend of AX
99 CDQ 2 EDX:EAX := sign-extend of EAX
Operation
IF OperandSize = 16 (* CWD instruction *)
THEN
IF AX < 0 THEN DX := 0FFFFH; ELSE DX := 0; FI;
ELSE (* OperandSize = 32, CDQ instruction *)
IF EAX < 0 THEN EDX := 0FFFFFFFFH; ELSE EDX := 0; FI;
FI;
Description
CWD converts the signed word in AX to a signed doubleword in DX:AX
by extending the most significant bit of AX into all the bits of DX. CDQ
converts the signed doubleword in EAX to a signed 64-bit integer in the
register pair EDX:EAX by extending the most significant bit of EAX
(the sign bit) into all the bits of EDX. Note that CWD is different from
CWDE. CWDE uses EAX as a destination, instead of DX:AX.
Flags Affected
None
Protected Mode Exceptions
None
Real Address Mode Exceptions
None
Virtual 8086 Mode Exceptions
None
up:
Chapter 17 -- 80386 Instruction Set
prev: CMPS/CMPSB/CMPSW/CMPSD Compare String Operands
next: DAA Decimal Adjust AL after Addition