The 80386 running in protected mode is a 32-bit microprocessor, but it is designed to support 16-bit processing at three levels:
compatibility.
The first level of support for 16-bit programs has already been discussed in Chapter 13, Chapter 14, and Chapter 15. This chapter shows how 16-bit and 32-bit modules can cooperate with one another, and how one module can utilize both 16-bit and 32-bit operands and addressing.
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:
A pure 32-bit module has these characteristics:
gigabytes).
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:
32-bit environments.
utilized in the new32-bit operating environment until new 32-bit
versions can be created.
16-bit words.
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.