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.
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
like IDE drives, they are normally configurable to act as master
or slave drives, with slave often being the default.
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.
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.
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.
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.
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
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.
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