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<!DOCTYPE HTML PUBLIC "-//IETF//DTD HTML 2.0//EN"><HTML><HEAD><TITLE>80386 Programmer's Reference Manual -- Opcode CBW</TITLE></HEAD><BODY><B>up:</B> <A HREF="c17.htm">Chapter 17 -- 80386 Instruction Set</A><BR><B>prev:</B><A HREF="CALL.htm"> CALL Call Procedure</A><BR><B>next:</B><A HREF="CLC.htm"> CLC Clear Carry Flag</A><P><HR><P><H1>CBW/CWDE -- Convert Byte to Word/Convert Word to Doubleword</H1><PRE>Opcode Instruction Clocks Description98 CBW 3 AX := sign-extend of AL98 CWDE 3 EAX := sign-extend of AX</PRE><H2>Operation</H2><PRE>IF OperandSize = 16 (* instruction = CBW *)THEN AX := SignExtend(AL);ELSE (* OperandSize = 32, instruction = CWDE *) EAX := SignExtend(AX);FI;</PRE><H2>Description</H2>CBW converts the signed byte in AL to a signed word in AX by extending themost significant bit of AL (the sign bit) into all of the bits of AH. CWDEconverts the signed word in AX to a doubleword in EAX by extending the mostsignificant bit of AX into the two most significant bytes of EAX. Note thatCWDE is different from CWD. CWD uses DX:AX rather than EAX as a destination.<H2>Flags Affected</H2>None<H2>Protected Mode Exceptions</H2>None<H2>Real Address Mode Exceptions</H2>None<H2>Virtual 8086 Mode Exceptions</H2>None<P><HR><P><B>up:</B> <A HREF="c17.htm">Chapter 17 -- 80386 Instruction Set</A><BR><B>prev:</B><A HREF="CALL.htm"> CALL Call Procedure</A><BR><B>next:</B><A HREF="CLC.htm"> CLC Clear Carry Flag</A></BODY>?? 快捷鍵說明
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