hw/i386.reference/BSF.htm

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   4 <TITLE>80386 Programmer's Reference Manual -- Opcode BSF</TITLE>
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   7 <B>up:</B> <A HREF="c17.htm">
   8 Chapter 17 -- 80386 Instruction Set</A><BR>
   9 <B>prev:</B><A HREF="BOUND.htm"> BOUND Check Array Index Against Bounds</A><BR>
  10 <B>next:</B><A HREF="BSR.htm"> BSR Bit Scan Reverse</A>
  11 <P>
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  13 <P>
  14 <H1>BSF -- Bit Scan Forward</H1>
  15
  16 <PRE>
  17 Opcode    Instruction          Clocks    Description
  18
  19 0F  BC    BSF r16,r/m16        10+3n     Bit scan forward on r/m word
  20 0F  BC    BSF r32,r/m32        10+3n     Bit scan forward on r/m dword
  21 </PRE>
  22
  23 <H2>Notes</H2>
  24
  25  is the number of leading zero bits.
  26
  27 <H2>Operation</H2>
  28
  29 <PRE>
  30 IF r/m = 0
  31 THEN
  32    ZF := 1;
  33    register := UNDEFINED;
  34 ELSE
  35    temp := 0;
  36    ZF := 0;
  37    WHILE BIT[r/m, temp = 0]
  38    DO
  39       temp := temp + 1;
  40       register := temp;
  41    OD;
  42 FI;
  43 </PRE>
  44
  45 <H2>Description</H2>
  46
  47 BSF scans the bits in the second word or doubleword operand starting with
  48 bit 0. The ZF flag is cleared if the bits are all 0; otherwise, the ZF flag
  49 is set and the destination register is loaded with the bit index of the
  50 first set bit.
  51
  52 <H2>Flags Affected</H2>
  53
  54 ZF as described above
  55
  56 <H2>Protected Mode Exceptions</H2>
  57
  58 #GP(0) for an illegal memory operand effective address in the CS, DS, ES,
  59 FS, or GS segments; #SS(0) for an illegal address in the SS segment;
  60 #PF(fault-code) for a page fault
  61
  62 <H2>Real Address Mode Exceptions</H2>
  63
  64 Interrupt 13 if any part of the operand would lie outside of the effective
  65 address space from 0 to 0FFFFH
  66
  67 <H2>Virtual 8086 Mode Exceptions</H2>
  68
  69 Same exceptions as in Real Address Mode; #PF(fault-code) for a page fault
  70
  71
  72 <P>
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  74 <P>
  75 <B>up:</B> <A HREF="c17.htm">
  76 Chapter 17 -- 80386 Instruction Set</A><BR>
  77 <B>prev:</B><A HREF="BOUND.htm"> BOUND Check Array Index Against Bounds</A><BR>
  78 <B>next:</B><A HREF="BSR.htm"> BSR Bit Scan Reverse</A>
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