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01: Skyrim SE

General Info

  • Name: Skyrim Special Edition
  • Release Date: 2016
  • Engine: Creation (Gamebryo)

Code Injection Dependency Chart

For a typical modded game.

flowchart TD

    SE[skse_loader64] -->|"Start Process #40;Suspended#41;"| A
    SE -->|"Init#40;#41;"| B{SKSE}

    A[Skyrim SE] -->|"Load via Import Table #40;d3dx9_42.dll#41;"| C
    C{Preloader} -. Implicit Dependency .-> B

    B -- load+init plugin --> PA{Plugin A \ne.g. Engine Fixes}
    B -- load+init plugin --> PB{Plugin B}
    B -- load+init plugin --> PC{Plugin C}

    C -- Preload --> PA{Plugin A \ne.g. Engine Fixes}

Game setup starts with SKSE doing DLL Injection into suspended process; calling Init, and unsuspending.

In some cases behaviour of engine needs to be augmented ahead of SKSE loading. In those cases the preloader dll hijacking stub is used.

For our purposes; no special functionality/integration is required here we can simply push files out to game directory; run skse_loader64.exe and call it a day.

Uploaded Files Structure

Using popular mods as examples.

Uploaded Skyrim SE mods can appear in the following form:

  • Files Targeting Data Subfolder

    • SkyUI
    • Root of archive maps to data.
    • e.g. /SkyUI_SE.bsa maps to Data/SkyUI_SE.bsa.

  • Loose Files Targeting Data Subdirectory

  • Files Targeting Game Root Directory

Mods can ship as 'loose files' or '.esp+.bsa' pairs

Additional Considerations for Manager

Plugins.txt

Found in: %LOCALAPPDATA%\Skyrim Special Edition.

The plugins.txt file in Skyrim lists plugin files with the ".esp" and ".esm" extensions that are loaded after the game's own.

  • .esm stands for "Elder Scrolls Master" and is mainly used for adding database data other plugins (.esp, .esl) rely upon; such as models, terrain, mechanics. Mainly used for large scale overhauls, and loaded first before .esp(s).
  • .esp stands for "Elder Scrolls Plugin" and is used for most mods that add new content to the game, i.e. weapons, armor, or quests. They can have dependencies on .esm(s).
  • .esl stands for "Elder Scrolls (Master) Light" and is exclusive to >= Special Edition. These are effectively .esm(s) with limitations.

Example of file looks like: 

*Unofficial Skyrim Special Edition Patch.esp
*LegacyoftheDragonborn.esm
*MajesticMountains_Landscape.esm
*DwemerGatesNoRelock.esl
AbandonedPrisonTweaks.esp
*UHDAP - en0.esp

* denotes a file that is enabled; and will be loaded by the game, if missing, the game will ignore the plugin. This makes the presence of loadorder.txt superfluous, but could still be used to keep track of ghosted plugins.

Each .esp/.esm has a 'Mod Index':

  • Using naming convention xxYYYYYY, where xx is plugin slot.
  • Therefore there is a 255 implicit item limit.
  • However... slot 0xFE is reserved for .esl files.

The newer .esl files have a limit of 4096 items; and have their own limit.

  • They have naming convention FExxxYYY where xxx is plugin slot.
  • Therefore there is a 4096 limit.
  • i.e. They use the 0xFE slot previously reserved for .esp & .esm.

.esm(s) and .esl(s) are always loaded before .esp(s) by the engine.

Please note however. The game engine has a 512 open file handle limit; so you can never realistically consume all possible ~4350 items without hitting this cap.

This can however be increased with engine-fixes.

Masters (Dependencies)

Plugins (.esp, .esl), can have 'Masters'; these are effectively dependencies.

To load a given plugin, all masters present in the plugin file's header must also be enabled. These masters are usually .esm, files but can also technically be other .esp files.

BSA Archives

BSAs are a collection of archived files that are loaded either if there is a plugin with the same name

Unofficial Skyrim Special Edition Patch.esp -> Unofficial Skyrim Special Edition Patch.bsa

or same name followed by ellipsis: <pluginName> - <something>.bsa:

eg. Unofficial Skyrim Special Edition Patch - Textures.BSA

(Note: different bethesda games have different loading behaviour for BSA files)

or if the archive is listed in the Archive section of the Skyrim.ini file (see sResourceArchiveList).

The files listed here are loaded first, and then the files from the plugins, in the order listed in plugins.txt. Later archives overwrite earlier archives.

For list of BSA formats compatible with Skyrim see: Bethesda mod archives.

Loose Files Load Order

Files can include loose files that need to be pushed out to Data subfolder. These files will take priority over those packed in .bsa archives.

Usually this is only done for texture mods and shouldn't cause issues with other mods; but it is worth taking into account.

Essential Mods & Tools

Tooling for this game is built ; i.e. users are expected to modify the game folder directly, and use tools on the game folder.

SKSE

Adds additional scripting capabilities and engine changes not present in vanilla. Also acts as code loader; loading DLLs from inside Data/SKSE/Plugins.

This mod is considered essential; as it is a required dependency for many mods out there. It should ideally be preinstalled automatically for most mod setups.

Preloader

A DLL stub (entry point: d3dx9_42.dll) which forces itself to load before SKSE using DLL Hijacking approach. This is done as some mods, e.g. Engine Fixes need to kick in before SKSE loads to make engine changes.

Engine Fixes

General suite of fixes for game code/logic.

This one is considered essential for large mod setups because the game engine has a 512 open file handle limit; which means in practice you're hard capped at around 400-500 mods. This mod can extend that limit to 2048.

LOOT

A Load Order Optimisation Tool (LOOT). Inspects mod archives and figures out optimal order that plugins .esp(s) should be loaded in.

Very Useful information about load ordering here.

Considered essential in scope of Skyrim and requires implementation for MVP.

xEdit (SSEEdit)

xEdit is a modding tool for Skyrim that allows you to edit masters, plugins and the data structured contained within those files.

In the context of mod management, it includes various functions for comparing and merging mods (merging mods helps preventing you from hitting plugin limits); resolving conflicts between mods and cleaning up records.

This tool is essential for complex Skyrim mod setups.

FNIS

A third-party modding tool that allows for more advanced and complex animations to be used in the game. Usually you would run this utility after deploying a new mod setup, but before running the game.

It detects supported mods in game folder; autogenerates new files and off it goes. Ideally we should detect FNIS-supported mods and prompt the user if they want to run FNIS before starting the game.

Deployment Strategy

Standard deployment: Push files out to game folder.

Not suitable for VFS; tools work on game folder directly, and thus having VFS would affect user experience; this is a no-go.

Work To Do

Refer to the relevant Epic for the game.

Misc Notes

Code injection approach is fundamentally flawed; there is a lack of distinction between 'mod loader' and 'mod'. SKSE tries to be both; but by being a 'mod' itself, it opens the need for components like preloader to augment engine logic before the 'mod' part of SKSE kicks in.