This is an introduction to mod files: their uses, differences, and importance both to using and making mods. Understanding how mod files work is fundamentally important to understanding how to mod.
- Note: This page does not cover the use of assets such as mesh and texture files. See BSA Files for more information.
Master Files (*.esm) and Plugins (*.esp)
Master Files (files with .esm - ie, Fallout3.esm) and Plugin files ("TES files" or "mods" for short - files with .esp - ie, Fallout3mod.esp) are the core data files of Fallout 3. A master file acts as a database of all of the data for the world, including object data, dialogue, gameplay settings, object placements, AI settings, landscape, script commands, cells, etc. The GECK is the tool we use to view and edit master files and plugins.
(However, not all game data files are master and plugin files. Textures, meshes, sounds, videos, etc. are all part of the full game. More about these files elsewhere.)
Master files and plugins are largely identical in format, but have some important distinctions in practice. The main practical difference is that GECK will not create master files. Nor will it allow plugin files to modify other plugin files.
In order to create new .esms, you can use FOMM or FO3Edit to convert an .esp to an .esm. To load multiple .esms in the GECK, you need to change this setting in the GECKCustom.ini file:
This file is NOT found in the fallout3 folder in the BethesdaSoftworks folder. It is found in the user folders under
My Documents/My Games/Fallout 3 My Documents/My Games/FalloutNV
Further this file typically can not be found under a normal search in windows.
NOTE: If an enabled .esp has a master file, and that .esm is not enabled in the load order, the game will crash on startup.
Master files, plugins and savegames are usually dependent on other master files or plugins (the exception being the original master file that shipped with the game - that master file has no dependencies, and runs on its own). Typically, a Fallout 3 mod will be dependent on the original Fallout 3 master file. The masters of a given mod or savegame is simply the list of "parent" mods that the mod or savegame depends on. The masters of a mod can be seen in the GECK details view.
Masters lists are important because they help identify where objects in a mod or a savegame come from. I.e. when a savegame remembers an object, it essentially remembers it as something like "the 2,034th object defined in my fourth master". If the master list were somehow to be erased, then the savegame would lose the ability to match the object in the player's inventory with original object defined in the mod.
In regular gameplay, it's not really necessary to know about masters, but modders need to understand them, since gameplay debugging often requires working with FormIDs, and FormIDs are the way the game encodes information like "the 2,034th object defined by my fourth master".
Records and Overrides
As mentioned before, a mod file (master file or plugin) is basically a database of all the objects in the gameworld. To be more specific, the mod file is a collection of records. The records in a given mod are mixture of 1) new records, and 2) modifications of existing records (i.e. of records defined by the mod's masters). E.g. "Cool Guns.esp" might introduce three new guns, as well as modify two existing guns defined by Fallout3.esm. Each of these guns is represented by a single weapon record (so 3+2 = 5 records in total).
The important thing about records is that each record is a single indivisable object as far as GECK and the game engine are concerned. Thinks of the game world as a table with stacks of playing cards. You can add a new card, or you can override an existing card by putting a new card on top of it. So, in the example above, the three new guns form three new stacks, and the two modified guns go on top of existing stacks overriding the original cards/guns/records
Now, suppose that there's a second mod "Heavy Guns" which adds two new guns and redefines four guns from Fallout3 to be more "heavy". Suppose that of these four overridden guns, one conflicts with an override done by "Cool Guns" -- i.e. they both try to change the same gun.
Again, this is just like the card game. The two new guns form two new stacks, and the four redefined guns go on top of existing cards, overriding them. But in this case, one of those cards goes on top of the card added by Cool Guns -- thus overriding its changes as well as as original Fallout3.esm gun/card. Note in particular, that they changes are not merged. Each newly added card/record completely replaces the card/record under it.
The card game above is why load order is important. Load order determines the order in which mods lay their cards down on the table. Fallout3 goes first, then the next mod in load order, up to the last mod. Whenever two mods conflict, the later loading mod will win.
And as for the order, its fairly simple: master files (.esm) always load before plugins (.esp) Then within each groups, files are ordered by modified date.
So the following load rules exist:
- Plugins always trump master files
- Newer plugins trump older plugins
- Newer master files trump older master files.
To change load order, change the modified date of the files.
For general use, mods which add new content such as weapons, new worldspaces and armors, can be placed practically anywhere in your load order. However, mods which overwrite records from their masters, such as a weather or lighting mod, typically have to be placed at the bottom of the load order to maintain compatibility. Failing to do so may cause the mod to be overwritten by other mods which potentially unintentionally change some aspect of that mod.
Remember the the game engine is effectively playing a game of stacked cards. And when you create a mod, you're simply creating a hand of cards that will be played in that game. From that ending table, with its various stacks of cards, arises the gameworld that the player enters and lives in.