MS-DOS CD-ROM Extensions

An additional device and an interface module developed to meet the demands of CD-ROM Extensions.

            Prior to the CD-ROM Extensions, the situation was all rather chaotic, with each drive manufacturer supplying programs that would swap individual 32 Mbyte data chunks. Commercial solutions to this problem did emerge, however. One of which came from TMS Inc., by way of LaserDos. This permitted access to the full data capacity, and also supported conventional DOS commands such as DIR.

Technical Overview

            MS-DOS consists of a number of block device drives able to support rewritable storage devices such as floppy disk drives and hard disks. These, however, are unsuitable for CD-ROM for several reasons:

1. The full capacity of CD-ROM is not made available;

2. MS-DOS utilities become inoperative because files and directories are not in the correct format;

3. CD-ROM is not a rewritable medium;

4. CD-ROM cannot take advantage of a standard File Allocation Table (FAT), because      blocks are addressed differently;

5. The method of assignment of drive letters to individual drivers is inappropriate;

MS-DOS, however can accommodate a character device driver, which reads data as a stream of bytes, and is used to support serial devices such as modems

In essence, an effective CD-ROM device driver is a character device driver which has been modified to accommodate drive letters, an eight-byte character device name, and number of devices or units supported.