hw/i386.reference/CBW.htm

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   4 <TITLE>80386 Programmer's Reference Manual -- Opcode CBW</TITLE>
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   8 Chapter 17 -- 80386 Instruction Set</A><BR>
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  13 <P>
  14 <H1>CBW/CWDE -- Convert Byte to Word/Convert Word to Doubleword</H1>
  15
  16 <PRE>
  17 Opcode    Instruction     Clocks          Description
  18
  19 98        CBW             3               AX := sign-extend of AL
  20 98        CWDE            3               EAX := sign-extend of AX
  21 </PRE>
  22
  23 <H2>Operation</H2>
  24
  25 <PRE>
  26 IF OperandSize = 16 (* instruction = CBW *)
  27 THEN AX := SignExtend(AL);
  28 ELSE (* OperandSize = 32, instruction = CWDE *)
  29    EAX := SignExtend(AX);
  30 FI;
  31 </PRE>
  32
  33 <H2>Description</H2>
  34
  35 CBW converts the signed byte in AL to a signed word in AX by extending the
  36 most significant bit of AL (the sign bit) into all of the bits of AH. CWDE
  37 converts the signed word in AX to a doubleword in EAX by extending the most
  38 significant bit of AX into the two most significant bytes of EAX. Note that
  39 CWDE is different from CWD. CWD uses DX:AX rather than EAX as a destination.
  40
  41 <H2>Flags Affected</H2>
  42
  43 None
  44
  45 <H2>Protected Mode Exceptions</H2>
  46
  47 None
  48
  49 <H2>Real Address Mode Exceptions</H2>
  50
  51 None
  52
  53 <H2>Virtual 8086 Mode Exceptions</H2>
  54
  55 None
  56
  57
  58 <P>
  59 <HR>
  60 <P>
  61 <B>up:</B> <A HREF="c17.htm">
  62 Chapter 17 -- 80386 Instruction Set</A><BR>
  63 <B>prev:</B><A HREF="CALL.htm"> CALL Call Procedure</A><BR>
  64 <B>next:</B><A HREF="CLC.htm"> CLC Clear Carry Flag</A>
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