i386/i386.reference/CWD.htm

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   4 <TITLE>80386 Programmer's Reference Manual -- Opcode CWD</TITLE>
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   8 Chapter 17 -- 80386 Instruction Set</A><BR>
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  13 <P>
  14 <H1>CWD/CDQ -- Convert Word to Doubleword/Convert Doubleword to
  15            Quadword</H1>
  16
  17 <PRE>
  18 Opcode    Instruction        Clocks   Description
  19
  20 99        CWD                2        DX:AX := sign-extend of AX
  21 99        CDQ                2        EDX:EAX := sign-extend of EAX
  22 </PRE>
  23
  24 <H2>Operation</H2>
  25
  26 <PRE>
  27 IF OperandSize = 16 (* CWD instruction *)
  28 THEN
  29    IF AX < 0 THEN DX := 0FFFFH; ELSE DX := 0; FI;
  30 ELSE (* OperandSize = 32, CDQ instruction *)
  31    IF EAX < 0 THEN EDX := 0FFFFFFFFH; ELSE EDX := 0; FI;
  32 FI;
  33 </PRE>
  34
  35 <H2>Description</H2>
  36
  37 CWD converts the signed word in AX to a signed doubleword in DX:AX
  38 by extending the most significant bit of AX into all the bits of DX. CDQ
  39 converts the signed doubleword in EAX to a signed 64-bit integer in the
  40 register pair EDX:EAX by extending the most significant bit of EAX
  41 (the sign bit) into all the bits of EDX. Note that CWD is different from
  42 CWDE. CWDE uses EAX as a destination, instead of DX:AX.
  43
  44 <H2>Flags Affected</H2>
  45
  46 None
  47
  48 <H2>Protected Mode Exceptions</H2>
  49
  50 None
  51
  52 <H2>Real Address Mode Exceptions</H2>
  53
  54 None
  55
  56 <H2>Virtual 8086 Mode Exceptions</H2>
  57
  58 None
  59
  60
  61 <P>
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  63 <P>
  64 <B>up:</B> <A HREF="c17.htm">
  65 Chapter 17 -- 80386 Instruction Set</A><BR>
  66 <B>prev:</B><A HREF="CMPS.htm"> CMPS/CMPSB/CMPSW/CMPSD Compare String Operands</A><BR>
  67 <B>next:</B><A HREF="DAA.htm"> DAA Decimal Adjust AL after Addition</A>
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