Microsoft Compiled HTML Help based on HTML
Microsoft Compiled HTML Help 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, for Compiled HTML. The format is often used for software documentation.
It was introduced as the successor to Microsoft WinHelp with the release of Windows 95 OSR 2.5 and consequently, Windows 98. Within the Windows NT family, the CHM file support is introduced in Windows NT 4.0 and is still supported in Windows 11. Although the format was designed by Microsoft, it has been successfully reverse-engineered and is now supported in many document viewer applications.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 in with 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.