MS-DOS installation compression

The MS-DOS installation compression system is used for system installation files when installing or upgrading MS-DOS starting with version 5.0a. It replaces the last character of the file extension with an underscore. The EXPAND command decompresses the files, and is run as part of the installer. Apparently the Google Chrome installation process also needs it.

This format apparently uses some form of Lempel-Ziv compression.

Links

 * Documentation in Microsoft site