# PAK · resource packages

Resource container with a 40-byte header and a directory whose names share prefixes between entries.

**Documented edition: Imperivm HD · Partial documentation**

## Purpose

A .pak file groups resource files. Here the extension refers to Imperivm's HMMSYS PackFile container; other games use different formats with the same PAK extension.

The complete signature is 18 bytes: the text HMMSYS PackFile followed by a newline, byte 26 and a zero byte. Identify the container before interpreting its directory.

## Header

LE means little-endian. Positions in this table are zero-based offsets within the file.

| Byte position | Size | Contents |
| --- | --- | --- |
| 0 | 18 bytes | HMMSYS PackFile signature and its three trailing bytes |
| 18 | 14 bytes | Area whose complete meaning is not documented here |
| 32 | uint32 LE | Entry count |
| 36 | uint32 LE | Directory length in bytes |
| 40 | Variable | Directory start |

## Directory entries

Reconstruct a name using the indicated prefix of the previous name followed by the current suffix. Do not add the field at position 36 to Offset: that field measures the directory, and entry offsets are absolute.

- TotalLen: one byte containing the complete name length.
- PrefixLen: one byte containing the length of the prefix shared with the previous name.
- Suffix: TotalLen − PrefixLen bytes containing the remaining name.
- Offset: a 32-bit little-endian integer containing the absolute position of the entry data in the PAK file.
- Size: a 32-bit little-endian integer containing the entry size.

## When inspecting or extracting

- Check the signature and that the complete header is present.
- Validate lengths and positions before reading each entry; do not accept a prefix longer than the previous name.
- Check that Offset and Size describe a range within the file.
- When extracting, preserve the relative hierarchy and reject absolute paths or .. traversal outside the output directory.

## Known limits

This entry describes the header, directory shape and current interpretation of its offsets. It is not a complete specification of malformed-file behavior or every possible variant.

The area between the end of the directory and the first entry is not characterized here. A complete game-compatible repacking workflow is not documented either.

## Read and share

- [Markdown](/imperivm/exports/en/formatos/pak.md)
- [JSON](/imperivm/exports/en/formatos/pak.json)

## Related pages

- [BFHP / HPFS · map containers](/imperivm/en/formatos/bfhp/)
- [LZIS · block stream](/imperivm/en/formatos/lzis/)