Microsoft Compiled HTML Help (CHM) is a
Microsoft
Microsoft Corporation is an American multinational corporation and technology company, technology conglomerate headquartered in Redmond, Washington. Founded in 1975, the company became influential in the History of personal computers#The ear ...
proprietary
online help
Online help is a form of user assistance that provides topic-oriented, procedural or reference information designed to assist in using a software application, web application or operating system. Online help linked to the application's state ( ...
format, consisting of a collection of
HTML
Hypertext Markup Language (HTML) is the standard markup language for documents designed to be displayed in a web browser. It defines the content and structure of web content. It is often assisted by technologies such as Cascading Style Sheets ( ...
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
Windows 11 is a version of Microsoft's Windows NT operating system, released on October 5, 2021, as the successor to Windows 10 (2015). It is available as a free upgrade for devices running Windows 10 that meet the #System requirements, Windo ...
.
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
hyperlink
In computing, a hyperlink, or simply a link, is a digital reference providing direct access to Data (computing), data by a user (computing), user's point and click, clicking or touchscreen, tapping. A hyperlink points to a whole document or to ...
ed 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
ASCII ( ), an acronym for American Standard Code for Information Interchange, is a character encoding standard for representing a particular set of 95 (English language focused) printable character, printable and 33 control character, control c ...
), 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
)
* Built-in search engine
* Ability to merge multiple .chm help files
* Extended character support, although it does not fully support
Unicode
Unicode or ''The Unicode Standard'' or TUS is a character encoding standard maintained by the Unicode Consortium designed to support the use of text in all of the world's writing systems that can be digitized. Version 16.0 defines 154,998 Char ...
.
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.
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 an
arCHMagecan convert CHM into another format.
See also
*
WinHelp
*
Microsoft Help 2
*
Comparison of documentation generators
References
External links
HTML Help Web Page on MSDNMicrosoft Help 2 Reference (part of Visual Studio SDK for VS7.1 and VS8.0)
(the linked ITSF specification has been moved t
{{DEFAULTSORT:Microsoft Compiled Html Help
Internet Explorer
Technical communication
Computer file formats