Game Memory Layout

Civilization keeps the main game loop, unit operations, city simulation and tactical AI resident in memory. Screens and less frequently used operations occupy 23 overlays, which share a 9,648-byte code window. Loading another overlay replaces the code in that window.

Shared state remains available across overlay changes. City and unit records, civilization finances, diplomacy and rule tables use the common data area, DGROUP. Larger arrays such as map_visible_to, power_history and place_name occupy separate data storage. The execution stack is 2,048 bytes.

Tile attributes also use the page 2 map layout, accessed through the game’s graphics and map routines. This storage links the map engine to the screen-page abstraction.

Executable Layout

These addresses are relative to the executable’s unrelocated load image.

The main module initializes the engine and enters the turn loop. The overlay catalog identifies the modules loaded for menus, reports, world generation and other operations.