The Mm256.dat and Mm2048.dat files are cache files used by Internet Explorer. When you visit a Web page, Internet Explorer assigns the Web address a unique identification number and searches the Mm256.dat and Mm2048.dat files for that identification number. If the Web page's identification number is found, the contents of the Web page are stored locally on your computer's hard disk and Internet Explorer uses the locally stored content instead of downloading the information from the Internet. If the Web page's identification number is not found, the contents of the Web page must be downloaded from the Internet. This occurs if you have not visited the Web page before, the Web page has changed, or the Web page's identification number has expired. When the Web page's content has been downloaded to the hard disk, the Mm256.dat or Mm2048.dat file is updated with the Web page's identification number.
The Mm256.dat file is used to store the identification numbers of Web pages whose Web addresses are equal to or less than 256 characters. The Mm2048.dat is used to store the identification numbers of Web pages whose Web addresses are between 257 and 2048 characters.
Note that in Internet Explorer 4.0, the Index.dat file in the Temporary Internet Files folder performs this function.