Previous: Operating System
Up: Creating a Release Disc under DOS
Previous Page: Operating System
Next Page: Comparison of DOS and p-System disc components

Dialogue Files

At present, dialogue programs may take either of two forms: one program per activity (also called module or activity) of the dialogue; or a single monolithic program containing all the dialogue's code.

multiple programs

In this form, the menu and each activity of the dialogue is a separate .EXE file. Each one is executed from its predecessor by loading it entirely into memory to replace the previous program.

Because DOS has no support for runtime libraries or dynamic linking, all the code each .EXE file may use must be contained within it, so each one on the disc will contain another copy of most of the units from the Turbo Pascal library. So chaining to successive .EXE files necessarily includes re-reading into memory code that was actually already there.

In addition, the average size of instructions for the 80*86 family, as for most physical machines, is 3 or 4 times that of a p-machine instruction. The individual program is therefore several times as large under DOS as under the p-System. This means that reading the file from disc will usually take much longer. And it will often lead to the situation, not previously encountered, where a dialogue must be spread over more than one disc, and the student may be required to change discs during the dialogue.

These problems suggest a different organisation:

monolithic program

The inherent modularity of a dialogue leads well to implementing it as a single program in which each activity, and the menu, is a unit which the program uses. Each unit simply exports the procedure which comprises its activity. This document calls a dialogue using this form a monoprogram dialogue, or just monoprogram. This method requires no more than one copy of each library unit, and drastically reduces the disc space required. It is not, however, without a certain complexity in its creation. There is therefore discussion in Turbo Pascal Dialog Library Arrangements, in the ETC archives, about:

• The details of converting global environments into more local ones;
• The details of calling procedures that used to be programs;
• considerations for using the overlay manager when overlayed units have initialisation code;
• moving from module to module when exiting procedures is restricted to local or global exits.
• additional disc files required when overlaying units.