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.