Opening up new possibilities
The Compact Disk - Read Only Memory (CD-ROM)
The Compact Disk is one device that we take for granted even though it is arguably one of the most remarkable inventions in recorded human history. In a few short years, the CD-ROM drive has gone from high-priced luxury to economical necessity on modern PCs. It opened up new computing vistas due to its high capacity and broad pertinence. In many ways, the CD-ROM has replaced the floppy disk drive and allowed us to use our computers in ways that we never used them before. In fact, the ‘multimedia revolution’ was largely a result of the availability of cheap CD-ROM drives.

CD-ROMs are a huge topic in the computer world today, so large that I cannot spend as much time as I would like discussing every aspect. This is especially true of the recordable formats such as CD-R and CD-RW, which are evolving fast and represent a new use of the CD-ROM that most people still are not taking advantage of.

How do they work?
In terms of construction and basic components, CD-ROMs are rather similar to other storage devices that use circular, spinning media. The differences is the way the information is recorded on the media, and the way it is read from the media. Unlike the single ‘head’ of a floppy disk or a hard disk drive, the CD-ROM lens is only one part of an assembly of components that together read the information off the surface of the disk.

The detailed workings of the CD-ROM are too complicated for a discussion of this nature, but here’s a simplified version of what goes on within the box and underneath the CD tray.

1. A beam of light energy is emitted from an infrared laser diode and aimed toward a reflecting mirror. The mirror is part of the head assembly, which moves in a line along the surface of the disk.
2. The light reflects off the mirror and through a focusing lens and shines onto a specific point on the disk.
3. A certain amount of light is reflected back from the disk. The amount of light reflected depends on which part of the disk the beam strikes: each position on the disk is encoded as a one or a zero based on the presence or absence of ‘pits’ in the surface of the disk.
4. A series of ‘collector’ mirrors and lenses accumulate and focus the reflected light from the surface of the disk and send it towards a photo-detector.
5. The photo-detector transforms the light energy into electrical energy. The strength of the signal is dependent on how much light was reflected from the disk.

Most of these components are fixed in place; only the head assembly containing the mirror and read lens moves. This makes for a relatively simplified design. CD-ROMs are, of course, single-sided media and the drive, therefore, has only one ‘head’ to go with this single data surface. Since the read head on a CD-ROM uses light to perform its actions, it avoids many of the problems associated with magnetic heads.

There is also no contact with the media as with floppy disks so there are no wear or dirt build-up problems. There is no intricate close-contact flying height as with a hard disk so there is no concern about head crashes. However, since the mechanism uses light, it is important that the path used by the laser beam be unhindered. Dirt on the media can cause problems for CD-ROMs, and over time dust can also accumulate on the focus lens of the read head, thus causing errors.

Like all spinning-disk media, the CD-ROM drive includes a spindle motor that turns the media containing the data to be read. The spindle motor of a standard CD-ROM is very different from that of a hard disk or floppy drive in one very important way: it does not spin at a constant speed. Rather, the speed of the drive varies depending on what part of the disk (inside vs. outside) is being read. Standard hard disks and floppy disks spin the disk at a constant speed. Regardless of where the heads are, the same speed is used to turn the media.

This is called constant angular velocity (CAV) because it takes the same amount of time for a turn of the 360 degrees of the disk at all times. CD-ROMs take a different approach. They adjust the speed of the motor so that the linear velocity of the disk is always constant. When the head is on the outside of the disk, the motor runs slower, and when it is on the inside, it runs faster.

This is done to ensure that the same amount of data always goes past the read head at a given period of time. This is called constant linear velocity or CLV. The reason for this is historical and linked to certain limitations faced by audio CDs of earlier ages. The speed of the spindle motor is controlled by the microcontroller, tied to the positioning of the head actuator.

The data signals coming from the disk are used to synchronize the speed of the motor and make sure that the disk is turning at the correct rate.

Next week we will focus on how it integrates with the system and the issues arising out of it. Most of the problems arising out of CD-ROMs are with regard to its integration with the rest of the system so watch out next week for the answers to some of the problems you may be facing with your hardware.

Why are CD- ROMs such a ‘big deal’?

• Software support: The most important reason why a CD-ROM drive is such a vital component in computers is the large number of software titles that are only available on CD-ROM. At one time there were only a few software products that came on CD-ROM, and those few generally came on floppy disks as well. In contrast, today not having a CD-ROM means losing out on a large segment of the PC software market. That is not all; some CD-ROMs require a drive that meets certain minimum performance requirements.
  
• Performance: Since so much software uses the CD-ROM drive today, the performance level of the drive is important. It usually isn’t as important as the performance of the hard drive or system components such as the processor or system memory, but it is still important, depending on what you use the drive for. Obviously, the more you use the CD-ROM, the more essential it is that it performs well.

Computer literacy
System resource - A tool used by hardware to alert software of a need or by software to control a function of hardware. Hardware and software need a way to communicate with each other, and they do so by using a combination of four system resources:

• IRQ - Hardware devices use the IRQ bus on a motherboard to signal the CPU for attention.
• Port addresses - Software addresses a hardware device using the device’s port, or I/O address. The device “listens” to the bus to determine if it is being requested.
• Memory addresses - Software communicates with physical memory located in either RAM or ROM chips using memory addresses.
• DMA channel - Data travels back and forth between memory and a hardware device using this channel.

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