Mirror Magazine

Mirror Magazine Techno Page by Harendra Alwis

The problem of fitting in
More on CD - ROMs and how they ‘fit in’ to the system
CD-ROM drives made their way into the personal computer scene quite recently, at least compared to their predecessors: the floppy drive and the hard disk drive. They have also been relatively slow in adapting to the market. As a result, they tended at first to have non-standard interfaces to the rest of the PC. This means that, sometimes different manufacturers adopted their own different ways of integrating the CD - ROM into the rest of the computer system.

As the CD-ROM has become very common today, its interfaces have become comparatively more standardized than before. The trend so far has been to make the CD-ROM use the same interfaces that have traditionally been employed by hard disk drives because it is much more simple and reduces the cost. As a result of this, today’s CD-ROM drives generally use either IDE (actually ATAPI) or SCSI (see below), just as hard disks do.

But today as the role of the CD - ROM has changed from being a luxury to being an essential multimedia and portable storage tool, it is acknowledged that changes to its interface are necessary in order to enable the CD - ROM to perform specialized tasks.

In the early days, most of the CD-ROM drives were sold in so-called multimedia kits, to be added into existing systems that really had no way of interfacing them in a standard way. A typical multimedia kit came with a sound card, a CD-ROM drive, and a pair of small stereo speakers. Since the kits came with a sound card and a CD-ROM drive, the most natural way to interface the CD-ROM to the PC was to connect it to the sound card, and that is exactly what was done. In the early days there were only a few different types of CD-ROMs, and a sound card would typically be sold with the interface required for the specific CD – ROM drive it came with in a multimedia kit, or with multiple interfaces for two or three types of drives. Some of the drives that came out later were designed to emulate these types of drives so that they could use the same proprietary interfaces found on the sound card.

With time, the whole situation became rather confusing and messy, which is part of the reason why these proprietary interfaces were eventually done away with. The most common interface used in modern CD-ROM drives is the AT Attachment Packet Interface, more commonly called just ATAPI. This is a special protocol that was developed to allow devices like CD-ROM drives and tape drives to attach to regular IDE controllers normally used for hard disks.

CD-ROM drives that use ATAPI are often called “IDE CD-ROMs” but this terminology is not strictly correct even though physically ATAPI CD-ROM drives do connect to the system in the same way that IDE hard disks do. That is because ATAPI is only a derivative of the standard IDE interface and all IDE commands are compatible with ATAPI. But like IDE drives, they are normally configurable to act as master or slave drives, with slave often being the default.

Taxing system resources
CD-ROM drives do not use any system resources other than those that are used by their interface. If you require the addition of an interface (such as an IDE controller or a SCSI host adapter) then you will of course “spend” the resources that the interface requires. If you decide to slave an ATAPI CD-ROM drive off an IDE hard disk, then you are using the existing interface and there is no resource usage cost. The CD-ROM drive itself doesn’t use any resources of the PC, except for a bit of memory for the driver.

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 with a floppy disk that includes the software driver designed for the drive but some new computers have a base of drivers that support a wide variety of CD - ROMs so that you don’t have to install the drive. The driver is loaded in the CONFIG.SYS system file when the PC boots up.

Most of the time these drivers are unique to the drive and cannot be interchanged; if you install a new CD-ROM you may probably need to install a new driver as well. If your system doesn’t detect the CD-ROM then something has probably gone wrong with its driver. If it detects your CD-ROM but is unable to open its files, then something is probably wrong with the file system extension (unless it is a hardware fault). Next week: Compact Disk formats, CD-R and CD-RW.

Improve your computer literacy
SCSI - Acronym for small computer system interface. Pronounced “scuzzy”, SCSI is a parallel interface standard used by Apple Macintosh computers, PCs, and many UNIX systems for attaching peripheral devices to computers. Nearly all Apple Macintosh computers, excluding only the earliest Macs and the recent iMac, come with a SCSI port for attaching devices such as disk drives and printers.

SCSI interfaces provide faster data transmission rates (up to 80 megabytes per second) than standard serial and parallel ports. In addition, you can attach many devices to a single SCSI port, so that SCSI is really an I/O bus rather than simply an interface.
Although SCSI is an ANSI standard, there are many variations of it, so two SCSI interfaces may be incompatible. For example, SCSI supports several types of connectors.

While SCSI has been the standard interface for Macintoshes, the iMac comes with IDE, a less expensive interface, in which the controller is integrated into the disk or CD-ROM drive. Other interfaces supported by PCs include enhanced IDE and ESDI for mass storage devices, and Centronics for printers. You can, however, attach SCSI devices to a PC by inserting a SCSI board in one of the expansion slots. Many high-end new PCs come with SCSI built in. Note, however, that the lack of a single SCSI standard means that some devices may not work with some SCSI boards.

The following varieties of SCSI are currently implemented: SCSI-1: Uses an 8-bit bus, and supports data rates of 4 MBps SCSI-2: Same as SCSI-1, but uses a 50-pin connector instead of a 25-pin connector, and supports multiple devices. This is what most people mean when they refer to plain SCSI.

Wide SCSI: Uses a wider cable (168 cable lines to 68 pins) to support 16-bit transfers.
Fast SCSI: Uses an 8-bit bus, but doubles the clock rate to support data rates of 10 MBps. Fast Wide SCSI: Uses a 16-bit bus and supports data rates of 20 MBps.
Ultra SCSI: Uses an 8-bit bus, and supports data rates of 20 MBps.

SCSI-3: Uses a 16-bit bus and supports data rates of 40 MBps. Also called Ultra Wide SCSI. Ultra2 SCSI: Uses an 8-bit bus and supports data rates of 40 MBps.
Wide Ultra2 SCSI: Uses a 16-bit bus and supports data rates of 80 MBps.

Back to Mirror Magazine