hw/i386.reference/LODS.htm

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   4 <TITLE>80386 Programmer's Reference Manual -- Opcode LODS</TITLE>
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   8 Chapter 17 -- 80386 Instruction Set</A><BR>
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  11 <P>
  12 <HR>
  13 <P>
  14 <H1>LODS/LODSB/LODSW/LODSD -- Load String Operand</H1>
  15
  16 <PRE>
  17 Opcode  Instruction   Clocks   Description
  18
  19 AC      LODS m8       5        Load byte [(E)SI] into AL
  20 AD      LODS m16      5        Load word [(E)SI] into AX
  21 AD      LODS m32      5        Load dword [(E)SI] into EAX
  22 AC      LODSB         5        Load byte DS:[(E)SI] into AL
  23 AD      LODSW         5        Load word DS:[(E)SI] into AX
  24 AD      LODSD         5        Load dword DS:[(E)SI] into EAX
  25 </PRE>
  26
  27 <H2>Operation</H2>
  28
  29 <PRE>
  30 IF AddressSize = 16
  31 THEN use SI for source-index
  32 ELSE (* AddressSize = 32 *)
  33    use ESI for source-index;
  34 FI;
  35 IF byte type of instruction
  36 THEN
  37    AL := [source-index]; (* byte load *)
  38    IF DF = 0 THEN IncDec := 1 ELSE IncDec := -1; FI;
  39 ELSE
  40    IF OperandSize = 16
  41    THEN
  42       AX := [source-index]; (* word load *)
  43       IF DF = 0 THEN IncDec := 2 ELSE IncDec := -2; FI;
  44    ELSE (* OperandSize = 32 *)
  45       EAX := [source-index]; (* dword load *)
  46       IF DF = 0 THEN IncDec := 4 ELSE IncDec := -4; FI;
  47    FI;
  48 FI;
  49 source-index := source-index + IncDec
  50 </PRE>
  51
  52 <H2>Description</H2>
  53
  54 LODS loads the AL, AX, or EAX register with the memory byte, word,
  55 or doubleword at the location pointed to by the source-index register.
  56 After the transfer is made, the source-index register is automatically
  57 advanced. If the direction flag is 0 (<A HREF="CLD.htm">CLD</A> was executed), the source index
  58 increments; if the direction flag is 1 (<A HREF="STD.htm">STD</A> was executed), it decrements.
  59 The increment or decrement is 1 if a byte is loaded, 2 if a word is loaded,
  60 or 4 if a doubleword is loaded.
  61 <P>
  62 If the address-size attribute for this instruction is 16 bits, SI is used
  63 for the source-index register; otherwise the address-size attribute is 32
  64 bits, and the ESI register is used. The address of the source data is
  65 determined solely by the contents of ESI/SI. Load the correct index value
  66 into SI before executing the LODS instruction. LODSB, LODSW, LODSD are
  67 synonyms for the byte, word, and doubleword LODS instructions.
  68 <P>
  69 LODS can be preceded by the
  70 <A HREF="REP.htm">REP</A> prefix; however, LODS is used more typically
  71 within a
  72 <A HREF="LOOP.htm">LOOP</A> construct,
  73 because further processing of the data moved into
  74 EAX, AX, or AL is usually necessary.
  75
  76 <H2>Flags Affected</H2>
  77
  78 None
  79
  80 <H2>Protected Mode Exceptions</H2>
  81
  82 #GP(0) for an illegal memory operand effective address in the CS, DS,
  83 ES, FS, or GS segments; #SS(0) for an illegal address in the SS segment;
  84 #PF(fault-code) for a page fault
  85
  86 <H2>Real Address Mode Exceptions</H2>
  87
  88 Interrupt 13 if any part of the operand would lie outside of the effective
  89 address space from 0 to 0FFFFH
  90
  91 <H2>Virtual 8086 Mode Exceptions</H2>
  92
  93 Same exceptions as in Real Address Mode; #PF(fault-code) for a page
  94 fault
  95
  96
  97 <P>
  98 <HR>
  99 <P>
 100 <B>up:</B> <A HREF="c17.htm">
 101 Chapter 17 -- 80386 Instruction Set</A><BR>
 102 <B>prev:</B><A HREF="LOCK.htm"> LOCK Assert LOCK# Signal Prefix</A><BR>
 103 <B>next:</B><A HREF="LOOP.htm"> LOOP/LOOPcond Loop Control with CX Counter</A>
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