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Beginning with Patch 1.5.3 of Skyrim Special Edition, the game supports a new type of mod file known as a "light plugin" with the .esl extension. Elminster has spent a considerable amount of time studying how this works and has had a number of community members aid in verifying this information. Since questions about new plugin types come up all the time, I felt it was best to make sure our source here is accurate. Flag Types Plugins can have 3 different flag types: ESP, ESM, and ESL. This causes them to load in different ways, and in some cases handle their form IDs different from the old ways. ESP files are standard every day mods. There will be no type flag set on these and they have the .esp file extension. Mod managers all know how these work and various modding tools consider these to be the basics of modding. Files flagged with ESM are commonly referred to as "masters". They load early, immediately following the 5 hardcoded official files and any Creation Club content you may have. Files flagged with ESL are known as "light plugins" or "light masters". How these load is determined by their file extension. The exception being any Creation Club content you may have downloaded. Those files load immediately following the 5 official files, according to the order they're listed in the Skyrim.ccc file. File Extensions The engine treats files in a certain way based on their file extensions: A .esm file extension will always be treated as a master file. The ESM flag will automatically set in memory. A .esl file extension will always be treated as a light master. The ESM and ESL flags are automatically set in memory. A .esp file can function as either type of file depending on which flags are set in its file header. If the ESM flag is set, then the .esp is treated as a master file. If the ESL flag is set, it will be treated as a light plugin. If BOTH flags are set, then it is treated as a light master. It is important to note that as far as the game is concerned, a .esp file with no ESM flag will load as a normal .esp file would. All standard load order procedures apply to them, and any properly functioning mod manager should handle them the same way it does regular .esp files. The Golden Goose Files with the ESL flag are given special treatment with their form IDs. Upon loading into the game, the form IDs are remapped into the FE mod index. This is now considered a special slot in SSE and should not be occupied by any mods in your load order. Not even the Bashed Patch or any other form of merged data. As a result of this new support, the engine can allow you to bypass the previously hard limit of 255 plugin files. The theoretical limit now should be 4096 ESL flagged mods. Individual files flagged as ESL can in theory hold up to 4096 form ID records. In practice this ends up actually being 2048 form ID records because the engine reserves everything from $0x0000 to $0x07FF. All files using ESL flags must therefore contain their internal form IDs between xx000800 and xx000FFF. Anything exceeding this range is invalid and the game will either crash or you'll have severely corrupted data due to overruns. The reason for this is how the mapping works. We're all familiar with the normal 8 digit form ID. The first 2 digits are only used to handle load order, the other 6 are internal to their plugin file. What the ESL flag system does is it break this down even further. The First 3 digits get mapped into an internal table for the FE slot. The remaining 3 belong to the actual records in the plugin. You'll end up with something like this: FE000800 - FE000FFF ModFile1 FE001800 - FE001FFF ModFile2 FE002800 - FE002FFF ModFile3 FExxxyyy and so forth up until you reach FEFFFFFF and run out of IDs and mappings. This is where the 4096 files and 2048 form IDs per file comes from. The benefits of all of this should be fairly clear. No more need to merge the 20 weapon mods with 5 records each. Just flag them with ESL and you're good to go. This is much safer than merging and is fully supported by the game engine so it works the same way for everyone. Other Information Using ESL flagged files come with some important issues to consider: You must be playing on a copy of SSE version 1.5.3 or newer in order to use ESL flagged files. You must be using a copy of the SSE CK version 1.5.3 or higher in order to properly recognize these files for editing. The file cannot contain more than 2048 records in it or the form IDs will overflow into another FE slot and corrupt the save. Form IDs need to be compacted for the file to be considered valid. The CK has an option called "Compact Active File Form IDs" in the file menu. xEdit also has a function for this. When using either of these, PAY ATTENTION TO THE WARNINGS. You cannot compact IDs on a file you intend to continue using in an existing save or you WILL corrupt it due to the form IDs contained within it being rewritten. Only do this on files you have not yet begun using, and which do not yet exist as public releases. Other mods which reference form IDs in the mod you are compacting (compatiblity patches, script calls, etc) will break if you compact the form IDs on a mod that's been released to the public. NMM does not work with files that have the .esl extension or carry an ESL flag. It has been reported to completely lock up if even one of these is present in your Data folder. Use a functional mod manager such as Wrye Bash to manage these types of mods. If by some wierd twist of fate you end up with a ESL flagged file that's also ESM flagged and someone uses a standard .esp file as a master to it for some reason, that file will be forced to load into the wrong part of your setup. Normally a standard .esp file will never load before anything ESM flagged, this is the one exception to that now and it should be considered a bug to report to any mod's author who has done this. DO NOT attempt to patch anything inside of a CELL record that originates with a file flagged as ESL. Doing so will cause the cell to break and not load most of its contents when you enter it. This includes any official Creation Club content you may have. It doesn't matter who made the file, it will break. Mods that use the GetFormFromFile() function in their scripts to reference things within them cannot be compacted unless the scripts are also recompiled with the new form ID. Mods using recorded voice lines or that include NPC facegen data cannot be compacted because all of those asset files will need to be manually renamed to match the new dialogue and NPC form IDs. This post will be updated again as needed to reflect new information that may be uncovered.