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CBW.htm
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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 STYLE="width:80ch">
<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 Description
98 CBW 3 AX := sign-extend of AL
98 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 the
most significant bit of AL (the sign bit) into all of the bits of AH. CWDE
converts the signed word in AX to a doubleword in EAX by extending the most
significant bit of AX into the two most significant bytes of EAX. Note that
CWDE 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>