i386/i386.reference/s14_01.htm

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   4 <TITLE>80386 Programmer's Reference Manual -- Section 14.1</TITLE>
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   8 Chapter 14 -- 80386 Real-Address Mode</A><BR>
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  14 <H1>14.1  Physical Address Formation</H1>
  15 The 80386 provides a one Mbyte + 64 Kbyte memory space for an 8086 program.
  16 Segment relocation is performed as in the 8086: the 16-bit value in a
  17 segment selector is shifted left by four bits to form the base address of a
  18 segment. The effective address is extended with four high order zeros and
  19 added to the base to form a linear address as
  20 <A HREF="#fig14-1">Figure 14-1</A>
  21   illustrates. (The
  22 linear address is equivalent to the physical address, because paging is not
  23 used in real-address mode.) Unlike the 8086, the resulting linear address
  24 may have up to 21 significant bits. There is a possibility of a carry when
  25 the base address is added to the effective address. On the 8086, the carried
  26 bit is truncated, whereas on the 80386 the carried bit is stored in bit
  27 position 20 of the linear address.
  28 <P>
  29 Unlike the 8086 and 80286, 32-bit effective addresses can be generated (via
  30 the address-size prefix); however, the value of a 32-bit address may not
  31 exceed 65535 without causing an exception. For full compatibility with 80286
  32 real-address mode, pseudo-protection faults (interrupt 12 or 13 with no
  33 error code) occur if an effective address is generated outside the range 0
  34 through 65535.
  35 <P>
  36 <A NAME="fig14-1">
  37 <IMG align=center SRC="fig14-1.gif" border=0>
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