DIRECT.DAT
While most files used in a tactical game are loaded from the missdat folder, some are loaded from a save folder (if a save slot was used).
This file contains nine bytes, dictating the name of the folder where the save files are located. Any unused bytes are left "empty" (filled with zeroes).
In theory, a small tool could be written allowing the use of more save slots, by pointing the game towards folders other then the default ten, and manually populating the missdat folder with the files it needs.
Files used from the save game folder:
When you start a new battle, the information contained in these files is generated by the battlescape engine. It is not required to save this to the drive at the end of battle, as the geoscape engine does not need to know this information to calculate the battle results.
The only time it is written to the drive is when you manually save the game, as it is required to continue it at a later point.
(Note to self - since the map isn't saved to the harddrive unless a savegame is made, how is terrain stock collection stored at the end of battle? Alloys, Alien Navigation, etc...)