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Making and Using the Overlays

Even if the entire code of the dialogue could fit in memory at once, it would take a long time to load all its code from disc. All monoprogram dialogues so far have therefore used Turbo Pascal's overlay manager, the unit Overlay, to make each module a separately loadable unit. These units are contained not in the .EXE file of the program itself, but in a file ending in .OVR which the Overlay manager administers.

To import all the modules and make them into overlays:

• Add to the USES list of the Attract program:
  • the names of the units to which the modules have now been converted;

  • the names Overlay and AttrXtra. They should precede the first overlaid unit. AttrXtra is needed for the initialization of the unit Overlay; it is described in the document ``Turbo Pascal Dialogue Library Arrangements''.
  • the name MarkJump, which allows Ports' routine ChainTo to jump back into the Attract from any point in the code.

• Following the USES list, for each unit listed which is to be made an overlay, add a comment of the form {$O <name-of-unit>}. Other preparations have only readied the unit to be used as an overlay; it is this directive which actually causes it to be placed in the overlay file.

  Every unit to be overlaid must have such a directive, whether it is used directly from the Attract's USES list, or indirectly, by one of the USE'd units. Regrettably, this form of the {$O } directive can only be used in the main program, not in units.

  NOTE: poor choices for overlaying:

  • units with initialization code, because the execution of the initialization code causes heavy swapping as each is loaded in turn.

  • units with heavy user interaction (one per keystroke, for instance) may also cause heavy swapping if they are overlaid. How heavy the swapping gets for a given unit should be determined by observation.

EXAMPLE:

USES AttrXtra, Overlay, MarkJump, Ports, PDisplay, 
         MenuDisplay, BasketWv1, BasketWv2, BasketWv3, BasketWv4;
    {$O MenuDisplay}
    {$O BasketWv1}
    {$O BasketWv2} {$O BW2Utils} {$O BW2Graph}
    {$O BasketWv3}
    {$O BasketWv4}

This is from the Attract program for the imaginary dialogue ``Basket Weaving''. All four modules are overlaid. Also, Basket Weaving 2 has two support units of its own which are to be overlaid.