Microsoft Compiled HTML Help

Microsoft Compiled HTML Help (CHM) is a Microsoft proprietary online help format, consisting of a collection of HTML pages, an index and other navigation tools. The files are compressed and deployed in a binary format with the extension .CHM. The format was intended to succeed Microsoft WinHelp.

Although the format was designed by Microsoft, it has been successfully reverse-engineered and is now supported by many document viewers.

History

CHM was introduced as the successor to Microsoft WinHelp with the release of Windows 95 OSR 2.5. Within the Windows NT family, the CHM file support is introduced in Windows NT 4.0 and is still supported in Windows 11.

Microsoft has announced that they do not intend to add any new features to HTML Help.

File format

Help is delivered as a binary file with the .chm extension. It contains a set of HTML files, a hyperlinked table of contents, and an index file. The file format has been reverse-engineered and documentation of it is freely available.

The file starts with bytes "ITSF" (in ASCII), for "Info-Tech Storage Format", which is the internal name given by Microsoft to the generic storage file format used for CHM files.

CHM files support the following features:

  • Data compression (using LZX)
  • Built-in search engine
  • Ability to merge multiple .chm help files
  • Extended character support, although it does not fully support Unicode.

The Microsoft Reader's .lit file format is a modification of the HTML Help CHM format. CHM files are sometimes used for e-books.

Viewers

In addition to Microsoft Windows, the following apps support CHM:

Creators

Microsoft's HTML Help Workshop generates CHM files by instructions stored in a HTML Help project file, which bears a .HHP file name extension and is a specialized form of INI file.

Lazarus and Free Pascal provide a doxygen-like tool for CHM generation and a separate command-line compiler called chmcmd.

Other utilities

The official viewer in Microsoft Windows (hh.exe) can decompile a CHM file. So can Microsoft HTML Help Workshop and 7-Zip. Calibre and arCHMage can convert CHM into another format.

See also

References

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Uses material from the Wikipedia article Microsoft Compiled HTML Help, released under the CC BY-SA 4.0 license.