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Ref-docs/manual Intel386/I386Manual/BSF.HTM Normal file
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<html><!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" tppabs="http://webster.cs.ucr.edu/Page_TechDocs/Doc386/C17.HTM">Chapter 17 -- 80386 Instruction Set</a><br>
<b>prev:</b><a href="BOUND.HTM" tppabs="http://webster.cs.ucr.edu/Page_TechDocs/Doc386/BOUND.HTM"> BOUND Check Array Index Against Bounds</a><br>
<b>next:</b><a href="BSR.HTM" tppabs="http://webster.cs.ucr.edu/Page_TechDocs/Doc386/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" tppabs="http://webster.cs.ucr.edu/Page_TechDocs/Doc386/C17.HTM">Chapter 17 -- 80386 Instruction Set</a><br>
<b>prev:</b><a href="BOUND.HTM" tppabs="http://webster.cs.ucr.edu/Page_TechDocs/Doc386/BOUND.HTM"> BOUND Check Array Index Against Bounds</a><br>
<b>next:</b><a href="BSR.HTM" tppabs="http://webster.cs.ucr.edu/Page_TechDocs/Doc386/BSR.HTM"> BSR Bit Scan Reverse</a>
</body>