i386/i386.reference/s13_03.htm

   1 <!DOCTYPE HTML PUBLIC "-//IETF//DTD HTML 2.0//EN">
   2 <HTML>
   3 <HEAD>
   4 <TITLE>80386 Programmer's Reference Manual -- Section 13.3</TITLE>
   5 </HEAD>
   6 <BODY>
   7 <B>up:</B> <A HREF="c13.htm">
   8 Chapter 13 -- Executing 80286 Protected-Mode Code</A><BR>
   9 <B>prev:</B> <A HREF="s13_02.htm">13.2  Two ways to Execute 80286 Tasks</A><BR>
  10 <B>next:</B> <A HREF="c14.htm">Chapter 14 -- 80386 Real-Address Mode</A>
  11 <P>
  12 <HR>
  13 <P>
  14 <H1>13.3  Differences From 80286</H1>
  15 The few differences that do exist primarily affect operating system code.
  16
  17 <H2>13.3.1  Wraparound of 80286 24-Bit Physical Address Space</H2>
  18 With the 80286, any base and offset combination that addresses beyond 16M
  19 bytes wraps around to the first megabyte of the 80286 address space. With
  20 the 80386, since it has a greater physical address space, any such address
  21 falls into the 17th megabyte. In the unlikely event that any software
  22 depends on this anomaly, the same effect can be simulated on the 80386 by
  23 using paging to map the first 64K bytes of the 17th megabyte of logical
  24 addresses to physical addresses in the first megabyte.
  25
  26 <H2>13.3.2  Reserved Word of Descriptor</H2>
  27 Because the 80386 uses the contents of the reserved word (last word) of
  28 every descriptor, 80286 programs that place values in this word may not
  29 execute correctly on the 80386.
  30
  31 <H2>13.3.3  New Descriptor Type Codes</H2>
  32 Operating-system code that manages space in descriptor tables often uses an
  33 invalid value in the access-rights field of descriptor-table entries to
  34 identify unused entries. Access rights values of 80H and 00H remain invalid
  35 for both the 80286 and 80386. Other values that were invalid on for the
  36 80286 may be valid for the 80386 because of the additional descriptor types
  37 defined by the 80386.
  38
  39 <H2>13.3.4  Restricted Semantics of LOCK</H2>
  40 The 80286 processor implements the bus lock function differently than the
  41 80386. Programs that use forms of memory locking specific to the 80286 may
  42 not execute properly when transported to a specific application of the
  43 80386.
  44 <P>
  45 The <A HREF="LOCK.htm">LOCK</A>
  46 prefix and its corresponding output signal should only be used to
  47 prevent other bus masters from interrupting a data movement operation.
  48 <A HREF="LOCK.htm">LOCK</A>
  49 may only be used with the following 80386 instructions when they modify
  50 memory. An undefined-opcode exception results from using
  51 <A HREF="LOCK.htm">LOCK</A> before any
  52 other instruction.
  53 <UL>
  54 <LI> Bit test and change: <A HREF="BTS.htm">BTS</A>,
  55 <A HREF="BTR.htm">BTR</A>,
  56 <A HREF="BTC.htm">BTC</A>.
  57 <LI> Exchange:
  58 <A HREF="XCGH.htm">XCHG</A>.
  59 <LI> One-operand arithmetic and logical:
  60 <A HREF="INC.htm">INC</A>,
  61 <A HREF="DEC.htm">DEC</A>,
  62 <A HREF="NOT.htm">NOT</A>, and
  63 <A HREF="NEG.htm">NEG</A>.
  64 <LI> Two-operand arithmetic and logical:
  65 <A HREF="ADD.htm">ADD</A>,
  66 <A HREF="ADC.htm">ADC</A>,
  67 <A HREF="SUB.htm">SUB</A>,
  68 <A HREF="SBB.htm">SBB</A>,
  69  <A HREF="AND.htm">AND</A>,
  70 <A HREF="OR.htm">OR</A>,
  71 <A HREF="XOR.htm">XOR</A>.
  72 </UL>
  73 A locked instruction is guaranteed to lock only the area of memory defined
  74 by the destination operand, but may lock a larger memory area.  For example,
  75 typical 8086 and 80286 configurations lock the entire physical memory space.
  76 With the 80386, the defined area of memory is guaranteed to be locked
  77 against access by a processor executing a locked instruction on exactly the
  78 same memory area, i.e., an operand with identical starting address and
  79 identical length.
  80
  81 <H2>13.3.5  Additional Exceptions</H2>
  82 The 80386 defines new exceptions that can occur even in systems designed
  83 for the 80286.
  84 <DL>
  85 <DT>
  86 Exception #6 -- invalid opcode
  87 <DD>
  88 This exception can result from improper use of the
  89 <A HREF="LOCK.htm">LOCK</A> instruction.
  90 <DT>
  91 Exception #14 -- page fault
  92 <DD>
  93 This exception may occur in an 80286 program if the operating system
  94 enables paging. Paging can be used in a system with 80286 tasks as long
  95 as all tasks use the same page directory. Because there is no place in
  96 an 80286 TSS to store the PDBR, switching to an 80286 task does not
  97 change the value of PDBR. Tasks ported from the 80286 should be given
  98 80386 TSSs so they can take full advantage of paging.
  99 </DL>
 100 <P>
 101 <HR>
 102 <P>
 103 <B>up:</B> <A HREF="c13.htm">
 104 Chapter 13 -- Executing 80286 Protected-Mode Code</A><BR>
 105 <B>prev:</B> <A HREF="s13_02.htm">13.2  Two ways to Execute 80286 Tasks</A><BR>
 106 <B>next:</B> <A HREF="c14.htm">Chapter 14 -- 80386 Real-Address Mode</A>
 107 </BODY>