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LMSW.htm
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<!DOCTYPE HTML PUBLIC "-//IETF//DTD HTML 2.0//EN">
<HTML>
<HEAD>
<TITLE>80386 Programmer's Reference Manual -- Opcode LMSW</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="LLDT.htm"> LLDT Load Local Descriptor Table Register</A><BR>
<B>next:</B><A HREF="LOCK.htm"> LOCK Assert LOCK# Signal Prefix</A>
<P>
<HR>
<P>
<H1>LMSW -- Load Machine Status Word</H1>
<PRE>
Opcode Instruction Clocks Description
0F 01 /6 LMSW r/m16 10/13 Load r/m16 in machine status word
</PRE>
<H2>Operation</H2>
<PRE>
MSW := r/m16; (* 16 bits is stored in the machine status word *)
</PRE>
<H2>Description</H2>
LMSW loads the machine status word (part of CR0) from the source
operand. This instruction can be used to switch to Protected Mode; if so,
it must be followed by an intrasegment jump to flush the instruction
queue. LMSW will not switch back to Real Address Mode.
<P>
LMSW is used only in operating system software. It is not used in
application programs.
<H2>Flags Affected</H2>
None
<H2>Protected Mode Exceptions</H2>
#GP(0) if the current privilege level is not 0; #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
<H2>Notes</H2>
The operand-size attribute has no effect on this instruction. This
instruction is provided for compatibility with the 80286; 80386 programs
should use <A HREF="MOVRS.htm">MOV</A> CR0, ... instead.
<P>
<HR>
<P>
<B>up:</B> <A HREF="c17.htm">
Chapter 17 -- 80386 Instruction Set</A><BR>
<B>prev:</B><A HREF="LLDT.htm"> LLDT Load Local Descriptor Table Register</A><BR>
<B>next:</B><A HREF="LOCK.htm"> LOCK Assert LOCK# Signal Prefix</A>
</BODY>