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<!DOCTYPE HTML PUBLIC "-//IETF//DTD HTML 2.0//EN"><HTML><HEAD><TITLE>80386 Programmer's Reference Manual -- Section 14.5</TITLE></HEAD><BODY><B>up:</B> <A HREF="c14.htm">Chapter 14 -- 80386 Real-Address Mode</A><BR><B>prev:</B> <A HREF="s14_04.htm">14.4 Entering and Leaving Real-Address Mode</A><BR><B>next:</B> <A HREF="s14_06.htm">14.6 Real-Address Mode Exceptions</A><P><HR><P><H1>14.5 Switching Back to Real-Address Mode</H1>The processor reenters real-address mode if software clears the PE bit inCR0 with a <A HREF="MOVRS.htm">MOV</A> to CR0 instruction. A procedure that attempts to do this,however, should proceed as follows:<OL><LI> If paging is enabled, perform the following sequence:<UL><LI> Transfer control to linear addresses that have an identity mapping;i.e., linear addresses equal physical addresses.<LI> Clear the PG bit in CR0.<LI> Move zeros to CR3 to clear out the paging cache.</UL><LI> Transfer control to a segment that has a limit of 64K (FFFFH). Thisloads the CS register with the limit it needs to have in real mode.<LI> Load segment registers SS, DS, ES, FS, and GS with a selector thatpoints to a descriptor containing the following values, which areappropriate to real mode:<UL><LI> Limit = 64K (FFFFH)<LI> Byte granular (G = 0)<LI> Expand up (E = 0)<LI> Writable (W = 1)<LI> Present (P = 1)<LI> Base = any value</UL><LI> Disable interrupts. A <A HREF="CLI.htm">CLI</A> instruction disables INTR interrupts. NMIscan be disabled with external circuitry.<LI> Clear the PE bit.<LI> Jump to the real mode code to be executed using a far <A HREF="JMP.htm">JMP</A>. Thisaction flushes the instruction queue and puts appropriate values inthe access rights of the CS register.<LI> Use the <A HREF="LGDT.htm">LIDT</A> instruction to load the base and limit of the real-modeinterrupt vector table.<LI> Enable interrupts.<LI> Load the segment registers as needed by the real-mode code.</OL><P><HR><P><B>up:</B> <A HREF="c14.htm">Chapter 14 -- 80386 Real-Address Mode</A><BR><B>prev:</B> <A HREF="s14_04.htm">14.4 Entering and Leaving Real-Address Mode</A><BR><B>next:</B> <A HREF="s14_06.htm">14.6 Real-Address Mode Exceptions</A></BODY>⌨️ 快捷键说明
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