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DAT Specs Document

You can see Fallout's DAT files as if they where common ZIPs, they're used to store the files that will be used later inside the game, you can store anything you want, compressed or not, having the length you want from a common TXT to a immense MVE, and if you want, you can include a nice but useless DLL file. At the bottom of the DAT it's included all the information about each one of these files and some information about the DAT itself.

The DAT Format

DATs are divided in 3 parts, Data Block, Directory Tree and Fixed DAT Information block. Data Blocks contains all the files stored in the DAT, some of them needs to be GZipped, others don't. The Directory Tree contains all the information about each file stored in Data Block, as well as the offset where it's located, if it's compressed or not, packed/unpacked sizes, etc. And finally the Fixed DAT Information block that contains the size in bytes of both full DAT and the Directory Tree.

Here you can see a small scheme of how DAT's structure:

Part Location Description


Files stored in the archive
FilesTotal X+1 Number of files in the archive
DirTree X+5
Tree information about the files stored in the archive
TreeSize Z+1 Size of DirTree in bytes
DataSize Z+5 Full size of the archive in bytes

*FilesTotal + DirTree corresponds to Directory Tree block
**TreeSize + DataSize corresponds to Fixed DAT Information block

The Data Block

The Data Block contains just plain files, their technical information is located in the Directory Tree.

Data Block starts from the very beginning of a DAT file. They can be compressed or not, (Fallout engine uses zlib stream data compression), if they're compressed the signature 0x78DA appears at the begin of the file, if not, there is no signature, the file starts without signature.

The 0x78DA compression signature has an integer (2 bytes/WORD) nature.

0x78DA in ASCII is "x" as char is 120 for 'x' and 218 for ''

Compressed files are "zlib stream data" (RFC-1950(zlib format), RFC-1951(deflate format), RFC-1952(gzip format)).

However, if you attach this header  1F 8B 08 08 9F E8 B7 36 02 03 to the file, such file could been easily decompressed with WinZip.

The Directory Tree

Directory Tree contains entries that specifies about a file stored in the Data Block. These entries can be varying depending on the FilenameSize of the file (Path + Filename).

Like you saw in the scheme located at the beginning of this document, Directory Tree has been divided into 2 parts, FilesTotal and the DirTree. FilesTotal contains how many files are stored in the DAT, DirTree contains all the information about these files.

FilesTotal is declared as a DWORD (4 bytes/Long) type and is readed in INTEL L-H format.

Format of DirTree entries

DirTree has a private structure. The length of this structure can vary depending on the length of the Filename (path + filename). All the entries are DWord types unless it's specified. At the end of this chapter you can find a scheme on the structure and the way it's declared on C and Visual Basic programming languages.

All the directories and files are stored in DOS 8.3 format, that is 8 characters for the file name and 3 characters for the file extension. All the entries are sorted alphabetically in a descendent direction.

Structure scheme: all Dwords are readed in INTEL L-H format.

Name Type Description
Length of the ASCII filename.
Path and name of the file, For example, "text\english\game\WORLDMP.MSG".

The length of the Filename is FilenameSize.

Compression boolean:

1 = Compressed
0 = Decompressed

Size of the file without compression.
Size of the compressed file.
Address/Location of the file.

*Dword stands for 4 bytes/long integers 0xNN NN NN NN
*Word stands for 2 bytes integers 0xNN NN
*Byte stands for 1 byte integer 0xNN
*String stands for common string bytes "ABCDEF123456!@#$%/][\", etc.

Declaration of a DirEntry

C decorated structure:

struct DirEntry
    DWORD FilenameSize;
    char Filename[FilenameSize];
    BYTE Type;
    DWORD RealSize;
    DWORD PackedSize;
    DWORD Offset;

Visual Basic decorated structure:

Type DirEntryId
    FilenameSize As Long
    Filename As String * 255
End Type

Type DirEntry
    Type As Byte
    RealSize As Long
    PackedSize As Long
    Offset As Long
End Type

Entry Example

Offset Values Description
13CC46AD 16 00 00 00 FilenameSize, that is, 0x16
13CC46B1 61 72 74 5C
62 61 63 6B
67 72 6E 64
5C 41 64 6F
62 65 2E 66
72 6D
ASCII Filename, in this case:


13CC46C7 01 Type: 0x1 = compressed
13CC46C8 6A 2F 01 00 RealSize
13CC46CC 94 85 00 00 PackedSize
13CC46D0 00 00 00 00 Offset, location of the file. 0x00 1st file

*This exact example can be found on the Team X DAT specs document.

How to find a DirTreeAddr (starting location of Directory Tree)

To find the beginning of Directory Tree you can use this calculation:

DirTreeAddr = DataSize - TreeSize - 4



Copyright 2000 by MatuX ( unless it's specified.