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Software Drivers and File System Extensions
CD-ROM drives generally require two pieces of software in order to function properly. These are a driver, and a file system extension. The driver is responsible for controlling access to the CD-ROM drive. The file system extension is what allows the CD-ROM drive to appear to the system as a regular file system volume, with directories and files, etc.
Virtually all CD-ROM drives come new with a floppy disk that includes the software driver designed for the drive. The driver is loaded in the CONFIG.SYS system file when the PC boots up. In most cases these drivers are unique to the drive and cannot be interchanged with a different one; if you install a new CD-ROM you need to install a new driver as well. The floppy disk will normally include an installation program that will copy the driver to the hard disk and insert the "DEVICE=" command into your CONFIG.SYS file for you.
There are two common file system extensions used to enable CD-ROMs to work on the PC. The first is "MSCDEX.EXE" (which stands for Microsoft Compact Disk Extension). This program is used to permit CD-ROM access under DOS and Windows and is normally loaded in the AUTOEXEC.BAT system file. The second common file system extension is the one built into Windows 95. When using Windows 95, it is not necessary to load MSCDEX.EXE in the AUTOEXEC.BAT file, because Windows 95 includes native support for CD-ROMs (in fact, you shouldn't load it there under Windows 95).
One complicating factor under Windows 95 is CD-ROM access when booting into Windows 95 "DOS mode". When you boot Windows 95, its native file system extension will allow the CD-ROM to work. However, if you shut down to DOS mode, that file system extension is unloaded and the CD-ROM drive won't be accessible. The way to fix this problem is to load MSCDEX.EXE after shutting down to DOS mode. This section of the Troubleshooting Expert gives more details on how to do this.
Tip: After setting up a new
CD-ROM drive, I strongly encourage that you change the default drive letter that is
selected for the drive. By default, the CD-ROM is normally set up to take the next
available drive letter after the hard disk volumes. If you leave this at the default, then
if you later add a hard disk volume the drive letter for the CD-ROM will shift upwards and
your installed CD-ROM software will stop working properly. See here for more on this.