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<!DOCTYPE HTML PUBLIC "-//IETF//DTD HTML 2.0//EN"><HTML><HEAD><TITLE>80386 Programmer's Reference Manual -- Opcode BSF</TITLE></HEAD><BODY><B>up:</B> <A HREF="c17.htm">Chapter 17 -- 80386 Instruction Set</A><BR><B>prev:</B><A HREF="BOUND.htm"> BOUND Check Array Index Against Bounds</A><BR><B>next:</B><A HREF="BSR.htm"> BSR Bit Scan Reverse</A><P><HR><P><H1>BSF -- Bit Scan Forward</H1><PRE>Opcode Instruction Clocks Description0F BC BSF r16,r/m16 10+3n Bit scan forward on r/m word0F BC BSF r32,r/m32 10+3n Bit scan forward on r/m dword</PRE><H2>Notes</H2> is the number of leading zero bits.<H2>Operation</H2><PRE>IF r/m = 0THEN ZF := 1; register := UNDEFINED;ELSE temp := 0; ZF := 0; WHILE BIT[r/m, temp = 0] DO temp := temp + 1; register := temp; OD;FI;</PRE><H2>Description</H2>BSF scans the bits in the second word or doubleword operand starting withbit 0. The ZF flag is cleared if the bits are all 0; otherwise, the ZF flagis set and the destination register is loaded with the bit index of thefirst set bit.<H2>Flags Affected</H2>ZF as described above<H2>Protected Mode Exceptions</H2>#GP(0) for an illegal memory 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 page fault<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="BOUND.htm"> BOUND Check Array Index Against Bounds</A><BR><B>next:</B><A HREF="BSR.htm"> BSR Bit Scan Reverse</A></BODY>⌨️ 快捷键说明
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