hw/i386.reference/XLAT.htm

   1 <!DOCTYPE HTML PUBLIC "-//IETF//DTD HTML 2.0//EN">
   2 <HTML>
   3 <HEAD>
   4 <TITLE>80386 Programmer's Reference Manual -- Opcode XLAT</TITLE>
   5 </HEAD>
   6 <BODY>
   7 <B>up:</B> <A HREF="c17.htm">
   8 Chapter 17 -- 80386 Instruction Set</A><BR>
   9 <B>prev:</B><A HREF="XCHG.htm"> XCHG Exchange Register/Memory with Register</A><BR>
  10 <B>next:</B><A HREF="XOR.htm"> XOR Logical Exclusive OR</A>
  11 <P>
  12 <HR>
  13 <P>
  14 <H1>XLAT/XLATB -- Table Look-up Translation</H1>
  15
  16 <PRE>
  17 D7    XLAT m8    5     Set AL to memory byte DS:[(E)BX + unsigned AL]
  18 D7    XLATB      5     Set AL to memory byte DS:[(E)BX + unsigned AL]
  19 </PRE>
  20
  21 <H2>Operation</H2>
  22
  23 <PRE>
  24 IF AddressSize = 16
  25 THEN
  26    AL := (BX + ZeroExtend(AL))
  27 ELSE (* AddressSize = 32 *)
  28    AL := (EBX + ZeroExtend(AL));
  29 FI;
  30 </PRE>
  31
  32 <H2>Description</H2>
  33
  34 XLAT changes the AL register from the table index to the table entry. AL
  35 should be the unsigned index into a table addressed by DS:BX (for an
  36 address-size attribute of 16 bits) or DS:EBX (for an address-size attribute
  37 of 32 bits).
  38 <P>
  39 The operand to XLAT allows for the possibility of a segment override. XLAT
  40 uses the contents of BX even if they differ from the offset of the operand.
  41 The offset of the operand should have been moved intoBX/EBX with a previous
  42 instruction.
  43 <P>
  44 The no-operand form, XLATB, can be used if the BX/EBX table will always
  45 reside in the DS segment.
  46
  47 <H2>Flags Affected</H2>
  48
  49 None
  50
  51 <H2>Protected Mode Exceptions</H2>
  52
  53 #GP(0) for an illegal memory operand effective address in the CS, DS, ES,
  54 FS, or GS segments; #SS(0) for an illegal address in the SS segment;
  55 #PF(fault-code) for a page fault
  56
  57 <H2>Real Address Mode Exceptions</H2>
  58
  59 Interrupt 13 if any part of the operand would lie outside of the effective
  60 address space from 0 to 0FFFFH
  61
  62 <H2>Virtual 8086 Mode Exceptions</H2>
  63
  64 Same exceptions as in Real Address Mode; #PF(fault-code) for a page fault
  65
  66
  67 <P>
  68 <HR>
  69 <P>
  70 <B>up:</B> <A HREF="c17.htm">
  71 Chapter 17 -- 80386 Instruction Set</A><BR>
  72 <B>prev:</B><A HREF="XCHG.htm"> XCHG Exchange Register/Memory with Register</A><BR>
  73 <B>next:</B><A HREF="XOR.htm"> XOR Logical Exclusive OR</A>
  74 </BODY>