Hi guys,

Just wondering if you could explain the difference between 16 bit and 32 bit memory addressing?

I'm struggling to understand the differences and why it is different :(

Would be very greatful if you could help me with this.

"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."*

*http://webster.cs.ucr.edu/Page_TechDocs/Doc386/c16.html