Short for "Microsoft CD-ROM
Extensions." On MS-DOS
systems, a TSR
software module which was loaded to provide access to CD-ROM
drives. Loaded after a lower-level device driver
module, MSCDEX was essentially an installable filesystem
driver that understood the ISO 9660
filesystem and allowed MS-DOS
to access it.
MSCDEX is still present in Windows 95, but is not usually required except for compatibility with older MS-DOS programs.