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<!DOCTYPE HTML PUBLIC "-//IETF//DTD HTML 2.0//EN"><HTML><HEAD><TITLE>80386 Programmer's Reference Manual -- Chapter 16</TITLE></HEAD><BODY><B>up:</B> <A HREF="toc.htm">Table of Contents</A><BR><B>prev:</B><A HREF="s15_07.htm">15.7 Differences From 80286 Real-Address Mode</A><BR><B>next:</B><A HREF="s16_01.htm">16.1 How the 80386 Implements 16-Bit and 32-Bit Features</A><P><HR><P><H1>Chapter 16 Mixing 16-Bit and 32 Bit Code</H1><P>The 80386 running in protected mode is a 32-bit microprocessor, but it isdesigned to support 16-bit processing at three levels:<OL><LI>Executing 8086/80286 16-bit programs efficiently with complete compatibility.<LI>Mixing 16-bit modules with 32-bit modules.<LI>Mixing 16-bit and 32-bit addresses and operands within one module.</OL>The first level of support for 16-bit programs has already been discussedin <A HREF="c13.htm">Chapter 13</A>, <A HREF="c14.htm">Chapter 14</A>, and <A HREF="c15.htm">Chapter 15</A>. This chapter shows how 16-bitand 32-bit modules can cooperate with one another, and how one module canutilize both 16-bit and 32-bit operands and addressing.<P>The 80386 functions most efficiently when it is possible to distinguishbetween pure 16-bit modules and pure 32-bit modules. A pure 16-bit modulehas these characteristics:<UL><LI>All segments occupy 64 Kilobytes or less.<LI>Data items are either 8 bits or 16 bits wide.<LI>Pointers to code and data have 16-bit offsets.<LI>Control is transferred only among 16-bit segments.</UL>A pure 32-bit module has these characteristics:<UL><LI>Segments may occupy more than 64 Kilobytes (zero bytes to 4 gigabytes).<LI>Data items are either 8 bits or 32 bits wide.<LI>Pointers to code and data have 32-bit offsets.<LI>Control is transferred only among 32-bit segments.</UL>Pure 16-bit modules do exist; they are the modules designed for 16-bitmicroprocessors. Pure 32-bit modules may exist in new programs designedexplicitly for the 80386. However, as systems designers move applicationsfrom 16-bit processors to the 32-bit 80386, it will not always be possibleto maintain these ideals of pure 16-bit or 32-bit modules. It may beexpedient to execute old 16-bit modules in a new 32-bit environment withoutmaking source-code changes to the old modules if any of the followingconditions is true:<UL><LI>Modules will be converted one-by-one from 16-bit environments to 32-bit environments.<LI>Older, 16-bit compilers and software-development tools will be utilized in the new32-bit operating environment until new 32-bit versions can be created.<LI>The source code of 16-bit modules is not available for modification.<LI>The specific data structures used by a given module inherently utilize 16-bit words.<LI>The native word size of the source language is 16 bits.</UL>On the 80386, 16-bit modules can be mixed with 32-bit modules. To design asystem that mixes 16- and 32-bit code requires an understanding of themechanisms that the 80386 uses to invoke and control its 32-bit and 16-bitfeatures.<P><A HREF="s16_01.htm">16.1 How the 80386 Implements 16-Bit and 32-Bit Features</A><BR><A HREF="s16_02.htm">16.2 Mixing 32-Bit and 16-Bit Operations</A><BR><A HREF="s16_03.htm">16.3 Sharing Data Segments Among Mixed Code Segments</A><BR><A HREF="s16_04.htm">16.4 Transferring Control Among Mixed Code Segments</A><P><HR><P><B>up:</B> <A HREF="toc.htm">Table of Contents</A><BR><B>prev:</B><A HREF="s15_07.htm">15.7 Differences From 80286 Real-Address Mode</A><BR><B>next:</B> <A HREF="s16_01.htm">16.1 How the 80386 Implements 16-Bit and 32-Bit Features</A></BODY>⌨️ 快捷键说明
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