i386/i386.reference/LEA.htm

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   4 <TITLE>80386 Programmer's Reference Manual -- Opcode LEA</TITLE>
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   7 <B>up:</B> <A HREF="c17.htm">
   8 Chapter 17 -- 80386 Instruction Set</A><BR>
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  13 <P>
  14 <H1>LEA -- Load Effective Address</H1>
  15
  16 <PRE>
  17 Opcode  Instruction  Clocks  Description
  18
  19 8D  /r  LEA r16,m    2       Store effective address for m in register r16
  20 8D  /r  LEA r32,m    2       Store effective address for m in register r32
  21 8D  /r  LEA r16,m    2       Store effective address for m in register r16
  22 8D  /r  LEA r32,m    2       Store effective address for m in register r32
  23 </PRE>
  24
  25 <H2>Operation</H2>
  26
  27 <PRE>
  28 IF OperandSize = 16 AND AddressSize = 16
  29 THEN r16 := Addr(m);
  30 ELSE
  31    IF OperandSize = 16 AND AddressSize = 32
  32    THEN
  33       r16 := Truncate_to_16bits(Addr(m));   (* 32-bit address *)
  34    ELSE
  35       IF OperandSize = 32 AND AddressSize = 16
  36       THEN
  37          r32 := Truncate_to_16bits(Addr(m));
  38       ELSE
  39          IF OperandSize = 32 AND AddressSize = 32
  40          THEN  r32 := Addr(m);
  41          FI;
  42       FI;
  43    FI;
  44 FI;
  45 </PRE>
  46
  47 <H2>Description</H2>
  48
  49 LEA calculates the effective address (offset part) and stores it in the
  50 specified register. The operand-size attribute of the instruction
  51 (represented by OperandSize in the algorithm under "Operation" above) is
  52 determined by the chosen register. The address-size attribute (represented
  53 by AddressSize) is determined by the USE attribute of the segment containing
  54 the second operand. The address-size and operand-size attributes affect the
  55 action performed by LEA, as follows:
  56
  57 <PRE>
  58 Operand Size  Address Size  Action Performed
  59
  60     16            16        16-bit effective address is calculated and
  61                             stored in requested 16-bit register
  62                             destination.
  63
  64     16            32        32-bit effective address is calculated. The
  65                             lower 16 bits of the address are stored in
  66                             the requested 16-bit register destination.
  67
  68     32            16        16-bit effective address is calculated. The
  69                             16-bit address is zero-extended and stored
  70                             in the requested 32-bit register destination.
  71
  72     32            32        32-bit effective address is calculated and
  73                             stored in the requested 32-bit register
  74                             destination.
  75 </PRE>
  76
  77 <H2>Flags Affected</H2>
  78
  79 None
  80
  81 <H2>Protected Mode Exceptions</H2>
  82
  83 #UD if the second operand is a register
  84
  85 <H2>Real Address Mode Exceptions</H2>
  86
  87 Interrupt 6 if the second operand is a register
  88
  89 <H2>Virtual 8086 Mode Exceptions</H2>
  90
  91 Same exceptions as in Real Address Mode
  92
  93
  94 <P>
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  96 <P>
  97 <B>up:</B> <A HREF="c17.htm">
  98 Chapter 17 -- 80386 Instruction Set</A><BR>
  99 <B>prev:</B><A HREF="LAR.htm"> LAR Load Access Rights Byte</A><BR>
 100 <B>next:</B><A HREF="LEAVE.htm"> LEAVE High Level Procedure Exit</A>
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