The CHM file format, short for Compiled HTML, was introduced by Microsoft as an approach for packaging HTML-based documentation into a single file. It arrived on the scene with the release of Windows 98, and quickly established itself as the standard for software documentation on Windows platforms. The format leverages a compressed HTML structure, along with a set of navigation tools, to deliver an interactive user experience.
Architecture of CHM Files
CHM files are comprised of a binary format that houses HTML documents, images, and other multimedia content. They include an index and a table of contents, allowing users to search and navigate the information efficiently. The underlying technology uses Microsoft's proprietary Indexed Sequential Access Method (ISAM), which enables rapid access to data points within the file.
While CHM was widely used in software documentation, its compatibility spanned various applications. Microsoft's HTML Help executable program (hh.exe) is the most common utility for opening CHM files. There are also third-party readers available for platforms outside of Windows, such as xCHM for UNIX-based systems.
Security Concerns and Decline
Security vulnerabilities led to a decline in the CHM format's popularity. The capability of executing embedded scripts made it a target for malicious use, prompting many to migrate to safer documentation formats. Despite these concerns, CHM is still supported on modern Windows systems, yet it's advised to open files from trusted sources only.
Alternatives to CHM
As technology has evolved, so have documentation formats. Popular alternatives to CHM include PDF, which offers wide compatibility, and web-based documentation, which is accessible across various devices and platforms. The rise of online help systems and content management systems also gives authors more dynamic and secure means to distribute information.