hw/i386.reference/c16.htm

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   4 <TITLE>80386 Programmer's Reference Manual -- Chapter 16</TITLE>
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  12 <A HREF="s16_01.htm">
  13 16.1  How the 80386 Implements 16-Bit and 32-Bit Features</A>
  14 <P>
  15 <HR>
  16 <P>
  17 <H1>Chapter 16  Mixing 16-Bit and 32 Bit Code</H1>
  18 <P>
  19 The 80386 running in protected mode is a 32-bit microprocessor, but it is
  20 designed to support 16-bit processing at three levels:
  21 <OL>
  22 <LI>Executing 8086/80286 16-bit programs efficiently with complete
  23       compatibility.
  24
  25 <LI>Mixing 16-bit modules with 32-bit modules.
  26
  27 <LI>Mixing 16-bit and 32-bit addresses and operands within one module.
  28 </OL>
  29 The first level of support for 16-bit programs has already been discussed
  30 in <A HREF="c13.htm">Chapter 13</A>,
  31 <A HREF="c14.htm">Chapter 14</A>,
  32 and <A HREF="c15.htm">Chapter 15</A>. This chapter shows how 16-bit
  33 and 32-bit modules can cooperate with one another, and how one module can
  34 utilize both 16-bit and 32-bit operands and addressing.
  35 <P>
  36 The 80386 functions most efficiently when it is possible to distinguish
  37 between pure 16-bit modules and pure 32-bit modules. A pure 16-bit module
  38 has these characteristics:
  39 <UL>
  40 <LI>All segments occupy 64 Kilobytes or less.
  41 <LI>Data items are either 8 bits or 16 bits wide.
  42 <LI>Pointers to code and data have 16-bit offsets.
  43 <LI>Control is transferred only among 16-bit segments.
  44 </UL>
  45 A pure 32-bit module has these characteristics:
  46 <UL>
  47 <LI>Segments may occupy more than 64 Kilobytes (zero bytes to 4
  48    gigabytes).
  49
  50 <LI>Data items are either 8 bits or 32 bits wide.
  51
  52 <LI>Pointers to code and data have 32-bit offsets.
  53
  54 <LI>Control is transferred only among 32-bit segments.
  55 </UL>
  56 Pure 16-bit modules do exist; they are the modules designed for 16-bit
  57 microprocessors. Pure 32-bit modules may exist in new programs designed
  58 explicitly for the 80386. However, as systems designers move applications
  59 from 16-bit processors to the 32-bit 80386, it will not always be possible
  60 to maintain these ideals of pure 16-bit or 32-bit modules. It may be
  61 expedient to execute old 16-bit modules in a new 32-bit environment without
  62 making source-code changes to the old modules if any of the following
  63 conditions is true:
  64 <UL>
  65 <LI>Modules will be converted one-by-one from 16-bit environments to
  66      32-bit environments.
  67
  68 <LI>Older, 16-bit compilers and software-development tools will be
  69      utilized in the new32-bit operating environment until new 32-bit
  70      versions can be created.
  71
  72 <LI>The source code of 16-bit modules is not available for modification.
  73
  74 <LI>The specific data structures used by a given module inherently utilize
  75      16-bit words.
  76
  77 <LI>The native word size of the source language is 16 bits.
  78 </UL>
  79 On the 80386, 16-bit modules can be mixed with 32-bit modules. To design a
  80 system that mixes 16- and 32-bit code requires an understanding of the
  81 mechanisms that the 80386 uses to invoke and control its 32-bit and 16-bit
  82 features.
  83 <P>
  84 <A HREF="s16_01.htm">
  85 16.1  How the 80386 Implements 16-Bit and 32-Bit Features</A><BR>
  86 <A HREF="s16_02.htm">16.2  Mixing 32-Bit and 16-Bit Operations</A><BR>
  87 <A HREF="s16_03.htm">16.3  Sharing Data Segments Among Mixed Code Segments</A>
  88 <BR>
  89 <A HREF="s16_04.htm">16.4  Transferring Control Among Mixed Code Segments</A>
  90 <P>
  91 <HR>
  92 <P>
  93 <B>up:</B> <A HREF="toc.htm">
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  96 <A HREF="s15_07.htm">15.7  Differences From 80286 Real-Address Mode</A><BR>
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  98 <A HREF="s16_01.htm">
  99 16.1  How the 80386 Implements 16-Bit and 32-Bit Features</A>
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