i386/i386.reference/LOOP.htm

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   4 <TITLE>80386 Programmer's Reference Manual -- Opcode LOOP</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>LOOP/LOOPcond -- Loop Control with CX Counter</H1>
  15
  16 <PRE>
  17 Opcode   Instruction  Clocks  Description
  18
  19 E2  cb   LOOP rel8    11+m    DEC count; jump short if count <> 0
  20 E1  cb   LOOPE rel8   11+m    DEC count; jump short if count <> 0 and ZF=1
  21 E1  cb   LOOPZ rel8   11+m    DEC count; jump short if count <> 0 and ZF=1
  22 E0  cb   LOOPNE rel8  11+m    DEC count; jump short if count <> 0 and ZF=0
  23 E0  cb   LOOPNZ rel8  11+m    DEC count; jump short if count <> 0 and ZF=0
  24 </PRE>
  25
  26 <H2>Operation</H2>
  27
  28 <PRE>
  29 IF AddressSize = 16 THEN CountReg is CX ELSE CountReg is ECX; FI;
  30 CountReg := CountReg - 1;
  31 IF instruction <> LOOP
  32 THEN
  33    IF (instruction = LOOPE) OR (instruction = LOOPZ)
  34    THEN BranchCond := (ZF = 1) AND (CountReg <> 0);
  35    FI;
  36    IF (instruction = LOOPNE) OR (instruction = LOOPNZ)
  37    THEN BranchCond := (ZF = 0) AND (CountReg <> 0);
  38    FI;
  39 FI;
  40
  41 IF BranchCond
  42 THEN
  43    IF OperandSize = 16
  44    THEN
  45       IP := IP + SignExtend(rel8);
  46    ELSE (* OperandSize = 32 *)
  47       EIP := EIP + SignExtend(rel8);
  48    FI;
  49 FI;
  50 </PRE>
  51
  52 <H2>Description</H2>
  53
  54 LOOP decrements the count register without changing any of the flags.
  55 Conditions are then checked for the form of LOOP being used. If the
  56 conditions are met, a short jump is made to the label given by the operand
  57 to LOOP. If the address-size attribute is 16 bits, the CX register is used
  58 as the count register; otherwise the ECX register is used. The operand
  59 of LOOP must be in the range from 128 (decimal) bytes before the
  60 instruction to 127 bytes ahead of the instruction.
  61 <P>
  62 The LOOP instructions provide iteration control and combine loop index
  63 management with conditional branching. Use the LOOP instruction by
  64 loading an unsigned iteration count into the count register, then code the
  65 LOOP at the end of a series of instructions to be iterated. The
  66 destination of LOOP is a label that points to the beginning of the
  67 iteration.
  68
  69 <H2>Flags Affected</H2>
  70
  71 None
  72
  73 <H2>Protected Mode Exceptions</H2>
  74
  75 #GP(0) if the offset jumped to is beyond the limits of the current code
  76 segment
  77
  78 <H2>Real Address Mode Exceptions</H2>
  79
  80 None
  81
  82 <H2>Virtual 8086 Mode Exceptions</H2>
  83
  84 None
  85
  86
  87 <P>
  88 <HR>
  89 <P>
  90 <B>up:</B> <A HREF="c17.htm">
  91 Chapter 17 -- 80386 Instruction Set</A><BR>
  92 <B>prev:</B><A HREF="LODS.htm"> LODS/LODSB/LODSW/LODSD Load String Operand</A><BR>
  93 <B>next:</B><A HREF="LSL.htm"> LSL Load Segment Limit</A>
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