hw/i386.reference/BSR.htm

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