hw/i386.reference/s03_11.htm

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   4 <TITLE>80386 Programmer's Reference Manual -- Section 3.11</TITLE>
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   8 Chapter 3 -- Applications Instruction Set</A><BR>
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  11 <P>
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  13 <P>
  14 <H1>3.11  Miscellaneous Instructions</H1>
  15 The following instructions do not fit in any of the previous categories,
  16 but are nonetheless useful.
  17
  18 <H2>3.11.1  Address Calculation Instruction</H2>
  19 <A HREF="LEA.htm">LEA</A> (Load Effective Address)
  20 transfers the offset of the source operand
  21 (rather than its value) to the destination operand.  The source operand must
  22 be a memory operand, and the destination operand must be a general register.
  23 This instruction is especially useful for initializing registers before the
  24 execution of the string primitives (ESI, EDI) or the
  25 <A HREF="XLAT.htm">XLAT</A> instruction (EBX).
  26 The
  27 <A HREF="LEA.htm">LEA</A>
  28 can perform any indexing or scaling that may be needed.
  29 Example:
  30 <PRE>
  31 <A HREF="LEA.htm">LEA</A> EBX, EBCDIC_TABLE
  32 </PRE>
  33 Causes the processor to place the address of the starting location of the
  34 table labeled EBCDIC_TABLE into EBX.
  35
  36 <H2>3.11.2  No-Operation Instruction</H2>
  37 <A HREF="NOP.htm">NOP</A> (No Operation)
  38 occupies a byte of storage but affects nothing but the
  39 instruction pointer, EIP.
  40 <P>
  41 <H2>3.11.3  Translate Instruction</H2>
  42 <A HREF="XLAT.htm">XLAT</A> (Translate) i
  43 replaced a byte in the AL register with a byte from a
  44 user-coded translation table. When
  45 <A HREF="XLAT.htm">XLAT</A> is executed, AL should have the
  46 unsigned index to the table addressed by EBX.
  47 <A HREF="XLAT.htm">XLAT</A> changes the contents of
  48 AL from table index to table entry. EBX is unchanged. The
  49 <A HREF="XLAT.htm">XLAT</A> instruction
  50 is useful for translating from one coding system to another such as from
  51 ASCII to EBCDIC.  The translate table may be up to 256 bytes long.  The
  52 value placed in the AL register serves as an index to the location of the
  53 corresponding translation value.
  54 <P>
  55 <HR>
  56 <P>
  57 <B>up:</B> <A HREF="c03.htm">
  58 Chapter 3 -- Applications Instruction Set</A><BR>
  59 <B>prev:</B> <A HREF="s03_10.htm">3.10  Segment Register Instructions</A><BR>
  60 <B>next:</B> <A HREF="c04.htm">Chapter 4 -- Systems Architecture</A>
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