Oblivion Mod:Mod File Format

Outdated Tech: While these pages still provide the best public documentation of the Oblivion file formats, all mod makers should also be using Tes4View, which currently provides the most complete and correct understanding of the mod file format in a readily accessible manner.

Type Data Object ACHR Placed NPC ACRE Placed creature ACTI Activator ALCH Potion AMMO Ammo ANIO Animated Object APPA Alchem. Apparatus ARMO Armor BOOK Book BSGN Birthsign CELL Cell CLAS Class CLMT Climate CLOT Clothing CONT Container CREA Creature

Type Data Object CSTY Combat Style DIAL Dialog Topic DOOR Door EFSH Effect Shader ENCH Enchantment EYES Eyes FACT Faction FLOR Flora FURN Furniture GLOB Global GMST Game Setting GRAS Grass HAIR Hair IDLE Idle Animations INFO Dialog response INGR Ingredient

Type Data Object KEYM Key LAND Land LIGH Light LSCR Load Screen LTEX Land Texture LVLC Leveled Creature LVLI Leveled Item LVSP Leveled Spell MGEF Magic Effect MISC Misc. Item NPC_ Non-Player Character PACK AI Package PGRD Path grid QUST Quest RACE Race REFR Placed object

Type Data Object REGN Region ROAD Road SBSP Subspace SCPT Script SGST Sigil Stone SKIL Skill SLGM Soul Gem SOUN Sound SPEL Spell STAT Static TES4 Tes4 Header TREE Tree WATR Water Type WEAP Weapon WRLD Worldspace WTHR Weather

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Overview

Mod files for CS4 (TES4 Construction Set) are essentially collections of records, which are further divided into subrecords. Records generally correspond to objects in the construction set (e.g., a creature, a GMST setting, a dialog entry), with the fine details of the object (e.g., health of a creature, a dialog entry test) being handled by the subrecords of the record. Records themselves are organized into groups by GRUP records. At the highest grouping level, the TES4 file is simply:

• A single TES4 record
• A collection of top groups.

While CS4 seems to have some flexibility in the ordering and structure of records and groups in the files it reads, it also clearly likes to write files in a more specific ordering, which is described below. If your application is writing a mod file, it is suggested that you follow this preferred format if possible.

For a comparison with Morrowind (TES3) file format, see: Mod File Format/Vs Morrowind.

Groups

GRUPs are new (compared to Tes3), and seem to have been introduced largely to improve scanning of files, since they make it easier to skip over blocks of records that the reading program is not interested in. In addition to this, subgroups for WRLD and CELLS provide some useful structural information (e.g., the division of cell data into persistent and non-persistent references.)

Name	Type/Size	Info
type	char[4]	Always "GRUP"
groupSize	ulong	Size of the entire group, including the group header (20 bytes). This is in contrast to records and subrecords, whose sizes does not include their header sizes.
label	ubyte[4]	Format depends on group type (see next field). In the CS4 Details view, you can mark a group as ignored, but CS4 ignores this setting and reads the group anyway. If you subsequently save, the group will be written without the ignore markings. The ignore flag interferes with what should ordinarily be in the label field. E.g., "HAIR" becomes "HQIR". This mislabeling has no effect on record loading. In short, the label field of a group is not reliable.
groupType	long	Group type... Type Info Label Label 0 Top (Type) char[4] Record type 1 World Children formid Parent 2 Interior Cell Block long Block number 3 Interior Cell Sub-Block long Sub-block number 4 Exterior Cell Block short[2] Grid Y, X (Note the reverse order) 5 Exterior Cell Sub-Block short[2] Grid Y, X (Note the reverse order) 6 Cell Children formid Parent 7 Topic Children formid Parent 8 Cell Persistent Childen formid Parent 9 Cell Temporary Children formid Parent 10 Cell Visible Distant Children formid Parent
stamp	ulong	Date stamp, presumably of the last file modification. Stamp uses the MS-DOS date format. For a given esp, the stamp is the same for all groups, and seems to increase with date (and possibly time). Oblivion.esm stamps vary. Possibly due to a merging process?

Top Groups

In Oblivion.esm, the top, or highest level groups are stored in the following order:

GMST, GLOB, CLAS, FACT, HAIR, EYES, RACE, SOUN, SKIL, MGEF, SCPT, LTEX, ENCH, SPEL, BSGN, ACTI, APPA, ARMO, BOOK, CLOT, CONT, DOOR, INGR, LIGH, MISC, STAT, GRAS, TREE, FLOR, FURN, WEAP, AMMO, NPC_, CREA, LVLC, SLGM, KEYM, ALCH, SBSP, SGST, LVLI, WTHR, CLMT, REGN, CELL, WRLD, DIAL, QUST, IDLE, PACK, CSTY, LSCR, LVSP, ANIO, WATR, EFSH.

Whether the game engine expects this order is unknown, but it's probably safer to use this order than not.

All top groups contain records matching their label (e.g., the GMST top group contains GMST records). For most top groups, only the matching record types are present. However, in the CELL, WRLD and DIAL top groups, each main record can be followed by one or more child groups which contain additional records of a different type. Structure and ordering of those are as follows...

Hierarchical Top Groups

DIAL Top Group
DIAL Topic Children INFO
CELL Top Group
Interior Cell Block Interior Cell Sub-Block CELL Cell Childen Persistent children REFR, ACHR, ACRE Visible distant children REFR, ACHR, ACRE Temp Children PGRD REFR, ACHR, ACRE
WRLD Top Group
WRLD World Children ROAD CELL Cell Children Persistent Children REFR, ACHR, ACRE Visible Distant Children REFR, ACHR, ACRE Temp Children PGRD REFR, ACHR, ACRE Exterior World Block Exterior World Sub-block CELL Cell Childen Persistent Children REFR, ACHR, ACRE Visible Distant Children REFR, ACHR, ACRE Temp Children LAND PGRD REFR, ACHR, ACRE

Records

Name	Type/Size	Info
type	char[4]	Record type
dataSize	ulong	Size of data field.
flags	ulong	Flags... Flag Meaning 0x00000001 ESM file. (TES4.HEDR record only.) 0x00000020 Deleted 0x00000200 Casts shadows 0x00000400 Quest item / Persistent reference 0x00000800 Initially disabled 0x00001000 Ignored 0x00008000 Visible when distant 0x00020000 Dangerous / Off limits (Interior cell) 0x00040000 Data is compressed 0x00080000 Can't wait
formid	formid	Record identifier. Tes4 does not have a FormId. Some GMST records do not have a FormID.
Version Control Info	ulong	used for revision control by the CS (only if enabled) Low word is a timestamp: low byte is the day of the month, high byte is a month number, starting with 1 = Jan. 2003 The CS's algorithm for producing the timestamps is a bit strange, though, so that the month of December for a given year actually comes before January of that year, instead of January of the next year. One can only assume it was a bug ... High word marks ownership: low byte is user id that last had the form checked out, high byte is user id (if any) that currently has the form checked out
data	ubyte[dataSize]	Data For uncompressed records, this is a sequence of subrecords. Compressed data is the same, except that the subrecords are compressed using ZLIB level 6, and stored into the data field like so... Name Type/Size Info decompSize uint Size of decompressed data. compData ubyte[dataSize-4] Compressed collection of subrecords.

Subrecords

Name	Type/Size	Info
subType	char[4]	Subrecord type.
dataSize	ushort	Size of data field. If the preceding field has the type XXXX, then dataSize will be 0 and the size of the data is in fact the 32 bit quantity stored in the XXXX field. This happens once in Oblivion.esm.
data	ubyte[dataSize]	Data. Format depends on record and subrecord type.

There are a number of subrecord formats that are appear in several places.

Variable Length String (terminated)

Holds a variable length string including the nul (0x00) terminator. This means that an empty string would have a subrecord size of 1. Most variable string subrecords use this type.

Variable Length String (not terminated)

Holds a variable length string but does not include the nul (0x00) terminator. Only seen in SCTX subrecords in scripts so far.

byte/word/dword/int64/float

Basic data types which are used in a variety of places. The subrecord size is the same size of the native type.