Chapter 16 Mixing 16-Bit and 32 Bit Code

The 80386 running in protected mode is a 32-bit microprocessor, but it is designed to support 16-bit processing at three levels:

Executing 8086/80286 16-bit programs efficiently with complete compatibility.
Mixing 16-bit modules with 32-bit modules.
Mixing 16-bit and 32-bit addresses and operands within one module.

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:

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.

16.1 How the 80386 Implements 16-Bit and 32-Bit Features
16.2 Mixing 32-Bit and 16-Bit Operations
16.3 Sharing Data Segments Among Mixed Code Segments
16.4 Transferring Control Among Mixed Code Segments

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