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<!DOCTYPE HTML PUBLIC "-//IETF//DTD HTML 2.0//EN"><HTML><HEAD><TITLE>80386 Programmer's Reference Manual -- Section 14.1</TITLE></HEAD><BODY><B>up:</B> <A HREF="c14.htm">Chapter 14 -- 80386 Real-Address Mode</A><BR><B>prev:</B> <A HREF="c14.htm">Chapter 14 -- 80386 Real-Address Mode</A><BR> <B>next:</B> <A HREF="s14_02.htm">14.2 Registers and Instructions</A><P><HR><P><H1>14.1 Physical Address Formation</H1>The 80386 provides a one Mbyte + 64 Kbyte memory space for an 8086 program.Segment relocation is performed as in the 8086: the 16-bit value in asegment selector is shifted left by four bits to form the base address of asegment. The effective address is extended with four high order zeros andadded to the base to form a linear address as <A HREF="#fig14-1">Figure 14-1</A> illustrates. (Thelinear address is equivalent to the physical address, because paging is notused in real-address mode.) Unlike the 8086, the resulting linear addressmay have up to 21 significant bits. There is a possibility of a carry whenthe base address is added to the effective address. On the 8086, the carriedbit is truncated, whereas on the 80386 the carried bit is stored in bitposition 20 of the linear address.<P>Unlike the 8086 and 80286, 32-bit effective addresses can be generated (viathe address-size prefix); however, the value of a 32-bit address may notexceed 65535 without causing an exception. For full compatibility with 80286real-address mode, pseudo-protection faults (interrupt 12 or 13 with noerror code) occur if an effective address is generated outside the range 0through 65535.<P><A NAME="fig14-1"><IMG align=center SRC="fig14-1.gif" border=0><P><HR><P><B>up:</B> <A HREF="c14.htm">Chapter 14 -- 80386 Real-Address Mode</A><BR><B>prev:</B> <A HREF="c14.htm">Chapter 14 -- 80386 Real-Address Mode</A><BR><B>next:</B> <A HREF="s14_02.htm">14.2 Registers and Instructions</A></BODY>⌨️ 快捷键说明
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