ESO Mod:DAT File Format

DAT files basically just contain a short header and then zero or more compressed files. They are found in a few places:

\game\client\

game0000.dat

\depot\

eso0000.dat to eso0215.dat

\vo_en\

esoaudioen0000.dat to esoaudioen0003.dat

For each of the three sets of files there is a single MNF file in the same directory with a matching name (game.mnf, eso.mnf, esoaudioen.mnf).

File Format

The overall DAT format is simply:

    [File Header (14 bytes)] 
    [0...N Compressed Files]

File Header

The DAT file header appears to be a fixed size of 14 (0x0E) bytes and contains a few fields:

    byte  MAGIC_WORD[4] = "PES2"
    word PES_VERSION  = 1 
    dword  Unknown2  = 0 /* seems to be 0x0 in all dat files */
    dword OffsetToFirstFile = 0x0E

It is assumed that the last field is the offset to the first sub-file as all DATs have a value of 0x0000000E in this field.

Compressed Files

Directly following the header are 0 or more files either uncompressed or compressed with the zLib or Snappy format. Files compressed with zLib seem to always have the first two bytes as 78 9C (default compression) but in theory this could take a variety of values. Files continue one after another with no extra data between them until the end of the DAT file.

The data can be uncompressed either using the ZLib library (exactly as shown in their usage example) or the Snappy library.

The actual content of the DAT sub-file appears to have one of two formats depending on the value of Unknown2 in the MNF file header.

Unknown2 0x06

This format, currently only used in the GAME.MNF file, has a small header and two variable sized records before the actual file data:

   dword Null1 = 0x00000000
   dword Size1                //Ranges from 0x1B8 to 0x1BC
   byte  Data1[Size1]
   dword Size2                //Always 0x28
   byte  Data2[Size2]
   byte  FileData[]

The Data1/Data2 formats are unknown. They look somewhat like compressed data but are not valid snappy or zlib formats.

This format was introduced in the 8/2/2014 beta patch.

Unknown2 Other

This is the typical/default format where the data is simply the entire sub-file (no header or footer bytes).

Notes

There doesn't appear to be any of directly accessing a random file in the DAT file without processing all previous files.
The MNF_File_Format contains additional information related to the DAT files including a file index holding offsets to sub-files with the DAT.
The last file in game0000.dat is a ZOSFT (Zenimax Online Studios File Table) which contains a nul-terminated list of filenames among other binary data. Unfortunately the number of filenames does not correlate exactly to the number of compressed files in the DAT file. The ZOSFT is itself composed of several compressed data blocks followed the the filename text.
Some files store uncompressed data (ex: eso0093.dat). These appear to have B8 D6 0A as the first three bytes of their data followed by the uncompressed file.
The patch for the 8/2/2014 beta appears to have changed the format of the sub-file data for game0000.dat (the overall DAT format hasn't changed). The new format appears to be (header bytes are in big endian bit format):

    dword Null1 = 0x00000000
    dword Size1
    byte  Data1[Size1]
    dword Size2
    byte  Data2[Size2]
    byte  FileData[]    //Same as pre-patch

The Data1/Data2 formats are unknown. They look like compressed data but are not valid snappy or zlib formats.
DataSize1 ranges from 0x1B8 to 0x1BC
DataSize2 is always 0x28
This DAT format might be identified by the value of 0x6 in Unknown2 of the MNF file header.

File Contents

Moved to ESOMod:DAT File Contents

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