hw/i386.reference/s06_02.htm

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   4 <TITLE>80386 Programmer's Reference Manual -- Section 6.2</TITLE>
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   7 <B>up:</B> <A HREF="c06.htm">
   8 Chapter 6 -- Protection</A><BR>
   9 <B>prev:</B> <A HREF="s06_01.htm">6.1  Why Protection?</A><BR>
  10 <B>next:</B> <A HREF="s06_03.htm">6.3  Segment-Level Protection</A>
  11 <P>
  12 <HR>
  13 <P>
  14 <H1>6.2  Overview of 80386 Protection Mechanisms</H1>
  15 Protection in the 80386 has five aspects:
  16 <OL>
  17 <LI> Type checking
  18 <LI> Limit checking
  19 <LI> Restriction of addressable domain
  20 <LI> Restriction of procedure entry points
  21 <LI> Restriction of instruction set
  22 </OL>
  23 The protection hardware of the 80386 is an integral part of the memory
  24 management hardware. Protection applies both to segment translation and to
  25 page translation.
  26 <P>
  27 Each reference to memory is checked by the hardware to verify that it
  28 satisfies the protection criteria. All these checks are made before the
  29 memory cycle is started; any violation prevents that cycle from starting and
  30 results in an exception. Since the checks are performed concurrently with
  31 address formation, there is no performance penalty.
  32 <P>
  33 Invalid attempts to access memory result in an exception. Refer to
  34
  35 <A HREF="c09.htm">Chapter 9</A>
  36    for an explanation of the exception mechanism . The present
  37 chapter defines the protection violations that lead to exceptions.
  38 <P>
  39 The concept of "privilege" is central to several aspects of protection
  40 (numbers 3, 4, and 5 in the preceeding list). Applied to procedures,
  41 privilege is the degree to which the procedure can be trusted not to make a
  42 mistake that might affect other procedures or data. Applied to data,
  43 privilege is the degree of protection that a data structure should have
  44 from less trusted procedures.
  45 <P>
  46 The concept of privilege applies both to segment protection and to page
  47 protection.
  48 <P>
  49 <HR>
  50 <P>
  51 <B>up:</B> <A HREF="c06.htm">
  52 Chapter 6 -- Protection</A><BR>
  53 <B>prev:</B> <A HREF="s06_01.htm">6.1  Why Protection?</A><BR>
  54 <B>next:</B> <A HREF="s06_03.htm">6.3  Segment-Level Protection</A>
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