Compact Disk – Read Only Memory Overview

Compact Disk – Read Only Memory

According to, Compact Disk – Read Only Memory is abbreviated as CD-ROM.

Ways to accommodate data on a CD-ROM.

The most popular is defined in the international standard ISO 9660. This standard specifies a minimal file system, which is even more crude than the one used by MS-DOS. On the other hand, it is so minimal that any operating system should be able to map it to its native system. For normal use UNIX cannot use the ISO 9660 file system, so an extension to the standard, called the Rock Ridge extension, has been developed. Rock Ridge allows long file names, symbolic links, and a host of other goodies, making a CD-ROM look more or less like any contemporary UNIX file system. Better yet, a Rock Ridge file system is still a valid ISO 9660 file system, making it possible to use it on non-UNIX systems as well. Linux supports both ISO 9660 and the Rock Ridge extension; both are recognized and used automatically. However, the file system is only half the battle. Most CD-ROMs contain data that requires a special program to access it, and most of these programs do not work on Linux (except possibly under dosemu, the Linux MS-DOS emulator, or wine, the Windows emulator.). There is also VMWare, a commercial product that software emulates a full x86 machine) A CD-ROM device is accessed via the corresponding device file. There are several ways to connect a CD-ROM to a computer: via SCSI, via a sound card, or via EIDE. The hardware techniques necessary to do this are outside the scope of this book, but the type of connection determines the device file.

Item most coveted by users

The CD-ROM is the second most coveted item by personal computer users. The great dream of having a lot of information stored in very little space comes true with multimedia encyclopedias on CD-ROM disc that, in addition to including textual and graphic information like traditional encyclopedias, incorporate sound, video and a powerful search system, which is really the most useful for a user of encyclopedias. Something similar happens with CD-ROM drives as with sound cards. It is harder to get lost because there are fewer features to consider. The characteristics that you should look for when buying a CD-ROM drive are the following:


CD-ROM drives can be internal or external installation. The advantages of one over the other depend on the use that is going to be given to it. If we have several computers we can have an external drive to transport it and use it with everyone. If we do not have available space on the computer to install it (which is rare but it happens sometimes) the solution is an external installation (or a bigger box ;-)). Most cases will apply an internal installation.


It is the type of connection and electrical operating mode they use. The CD-ROM needs an interface to transfer the data to the computer and there are different types: Creative, Panasonic, Sony, E-IDE, Mitsumi, DMA / 33 and SCSI. If you want to add a CD-ROM and you already have a sound card, check the technical specifications and check what kind of interface it incorporates in order to choose the correct CD drive. The E-IDE interface allows you to connect the CD-ROM to the hard disk controller as if it were a second hard disk.


It is one of the most important aspects. It is clear that the higher the speed, the better the response of the system when it comes to reading data and playing sound and video from the CD. The values ​​that have been taken are 1x, 2x, 3x, 4x, 6x, 8x, 10x, 12x, 14x, 16x, 18x, 24x, 28x, 32x, 36x and 40x. The x must be replaced by 150 Kb / sec. We recommend, today, that you buy from a 32x since the others are hardly manufactured and the prices of the fastest are increasingly lower.

Access speed

It is the average time it takes for the unit to access the data when we ask for it. Typical values ​​range from 100-250 ms. It is clear that the lower the value, the better.

Buffer size

The buffer is a special memory that is responsible for transferring the information from the CD to the interface. It is not a cache, but it allows you to send data in larger packets, thus achieving greater (but not miraculous) transfers. Typical values ​​range from 64 to 512 Kb.


CD-XA, CD-1 (M2, F2), PhotoCD, multisession, recordable and rewritable CD, are different types of CD-ROM that can be read in a drive that specifies what is compatible with these systems. For example CD-XA stands for advanced architecture; CD-I can read Phillips CD-I and Video CD. PhotoCD reads the multisession format of Kodak photo discs. There are some drives that allow you to read Macintosh discs for use in Macintosh drives.

CD insertion

By tray and by Caddy. The Caddy is a kind of box in which the CD is inserted and then inserted into the unit. The main advantage is that the Caddy units pick up less dust and can be placed in an upright position, which is impossible to do with a tray unit. The latter are those that we may find in stores.

Own controller

There are some CD-ROMs that include their own controller, either because they do not fit the most used interfaces, or because they use an E-IDE interface and we have four hard drives installed, not being possible to connect them to the hard drive controller.

Compact Disk – Read Only Memory