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BSF.htm
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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 STYLE="width:80ch">
<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 Description
0F BC BSF r16,r/m16 10+3n Bit scan forward on r/m word
0F 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 = 0
THEN
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 with
bit 0. The ZF flag is cleared if the bits are all 0; otherwise, the ZF flag
is set and the destination register is loaded with the bit index of the
first 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 effective
address 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>