63 lines
2.3 KiB
HTML
63 lines
2.3 KiB
HTML
<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>
|
|
|