"The 80386 functions most efficiently when it is possible to distinguish between pure 16-bit modules and pure 32-bit modules.
A pure 16-bit module has these characteristics:
- All segments occupy 64 Kilobytes or less.
- Data items are either 8 bits or 16 bits wide.
- Pointers to code and data have 16-bit offsets.
- Control is transferred only among 16-bit segments.
A pure 32-bit module has these characteristics:
- Segments may occupy more than 64 Kilobytes (zero bytes to 4 gigabytes).
- Data items are either 8 bits or 32 bits wide.
- Pointers to code and data have 32-bit offsets.
- Control is transferred only among 32-bit segments.
Pure 16-bit modules do exist; they are the modules designed for 16-bit microprocessors. Pure 32-bit modules may exist in new programs designed explicitly for the 80386.
However, as systems designers move applications from 16-bit processors to the 32-bit 80386, it will not always be possible to maintain these ideals of pure 16-bit or 32-bit modules. It may be expedient to execute old 16-bit modules in a new 32-bit environment without making source-code changes to the old modules if any of the following conditions is true:
- Modules will be converted one-by-one from 16-bit environments to 32-bit environments.
- 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.
- The source code of 16-bit modules is not available for modification.
- The specific data structures used by a given module inherently utilize 16-bit words.
- The native word size of the source language is 16 bits.
On the 80386, 16-bit modules can be mixed with 32-bit modules. To design a system that mixes 16- and 32-bit code requires an understanding of the mechanisms that the 80386 uses to invoke and control its 32-bit and 16-bit features."*