| Microsoft Compiled HTML Help | |
| Extension: | .chm |
| Mime: | application/vnd.ms-htmlhelp[1] |
| Owner: | Microsoft |
| Released: | 1997 |
| Latest Release Version: | 1.4[2] |
| Extended To: | .lit |
| Microsoft Compiled HTML Help | |
| Screenshot Size: | 300px |
| Developer: | Microsoft |
| Replaces: | Microsoft WinHelp |
| Replaced By: | Microsoft Help 2 |
| Operating System: | Microsoft Windows |
| Genre: | Help system |
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[3] and consequently, Windows 98. Within the Windows NT family, the CHM file support is introduced in Windows NT 4.0[4] [5] [6] and is still supported in Windows 11.[7] Although the format was designed by Microsoft, it has been successfully reverse-engineered and is now supported in many document viewer applications.
| Month | Year | Description | |
|---|---|---|---|
| February | 1996 | Microsoft announces plans to stop development of WinHelp and start development on HTML Help. | |
| August | 1997 | HTML Help 1.0 (HH 1.0) is released with Internet Explorer 4. | |
| February | 1998 | HTML Help 1.1a ships with Windows 98. | |
| January | 2000 | HTML Help 1.3 ships with Windows 2000. | |
| July | HTML Help 1.32 releases with Internet Explorer 5.5 and Windows Me. | ||
| October | 2001 | HTML Help 1.33 releases with Internet Explorer 6 and Windows XP. | |
| March | At the WritersUA (formerly WinWriters) conference, Microsoft announces plans for a new help platform, Help 2, which is also HTML based. | ||
| January | 2003 | Microsoft decides not to release Microsoft Help 2 as a general Help platform. |
Microsoft has announced that they do not intend to add any new features to HTML Help.[8]
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.[9] [10]
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.[11]
CHM files support the following features:
The Microsoft Reader's .lit file format is a modification of the HTML Help CHM format. CHM files are sometimes used for e-books.[13]
Sumatra PDF supports viewing CHM documents since version 1.9.
Various applications, such as HTML Help Workshop and 7-Zip can decompile CHM files. The hh.exe utility on Windows and the extract_chmLib utility (a component of chmlib) on Linux can also decompile CHM files.
Microsoft's HTML Help Workshop and Compiler generate CHM files by instructions stored in a HTML Help project. The file name of such a project has the extension .HHP and the file is just a text with the INI file format.[14]
The Free Pascal project has a compiler (chmcmd) that can create CHM files in a multiplatform way.
Read support:
Read/write support: