bsr.htm
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<!DOCTYPE HTML PUBLIC "-//IETF//DTD HTML 2.0//EN"><HTML><HEAD><TITLE>80386 Programmer's Reference Manual -- Opcode BSR</TITLE></HEAD><BODY><B>up:</B> <A HREF="c17.htm">Chapter 17 -- 80386 Instruction Set</A><BR><B>prev:</B><A HREF="BSF.htm"> BSF Bit Scan Forward</A><BR><B>next:</B><A HREF="BT.htm"> BT Bit Test</A> <P><HR><P><H1>BSR -- Bit Scan Reverse</H1><PRE>Opcode Instruction Clocks Description0F BD BSR r16,r/m16 10+3n Bit scan reverse on r/m word0F BD BSR r32,r/m32 10+3n Bit scan reverse on r/m dword</PRE><H2>Operation</H2><PRE>IF r/m = 0THEN ZF := 1; register := UNDEFINED;ELSE temp := OperandSize - 1; ZF := 0; WHILE BIT[r/m, temp] = 0 DO temp := temp - 1; register := temp; OD;FI;</PRE><H2>Description</H2>BSR scans the bits in the second word or doubleword operand from the mostsignificant bit to the least significant bit. The ZF flag is cleared if thebits are all 0; otherwise, ZF is set and the destination register is loadedwith the bit index of the first set bit found when scanning in the reversedirection.<H2>Flags Affected</H2>ZF as described above<H2>Protected Mode Exceptions</H2>#GP(0) if the result is in a nonwritable segment; #GP(0) for an illegalmemory operand effective address in the CS, DS, ES, FS, or GS segments;#SS(0) for an illegal address in the SS segment; #PF(fault-code) for a pagefault<H2>Real Address Mode Exceptions</H2>Interrupt 13 if any part of the operand would lie outside of the effectiveaddress space from 0 to 0FFFFH<H2>Virtual 8086 Mode Exceptions</H2>Same exceptions as in Real Address Mode; #PF(fault-code) for a page fault<P><HR><P><B>up:</B> <A HREF="c17.htm">Chapter 17 -- 80386 Instruction Set</A><BR><B>prev:</B><A HREF="BSF.htm"> BSF Bit Scan Forward</A><BR><B>next:</B><A HREF="BT.htm"> BT Bit Test</A> </BODY>⌨️ 快捷键说明
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