The System Unit is the most important part of a Pc. It may be called the brain behind every action of a PC, since it controls and executes all the operations performed by the Pc. The keyboard, monitor, mouse, printer, etc. are all linked to the system unit through cables which are plugged into the back of the System Unit. In this section, we will discuss the vital components of the system unit so as to enable you understand the working process of the System. For better understanding, we will discuss the System Unit in three ways:

• Front Side of the System Unit
• Back Side of the System Unit
• Inside the System Unit

Front side of the system unit

Let us now observe the front visible portion of the system unit. The various components seen in the front part of a System Unit and their functions are as follows:

• Power switch: The power on/off switch is used to turn on or off the power to the PC.

• Reset button: This button helps you restart your computer without disconnecting the power supply.

• Lights: The front panel 'of the system unit may display a variety of colored indicator lights. These lights are used to indicate whether the hard disk, the floppy disk or the CO¬RaM is being read or written.

• Roppy disk drive: The floppy disk drive is used to read the information stored in floppy disks (also called a diskette). The System Unit has a slit in which the floppy disk can be inserted. Floppy disk drive is a component that enables us spin the disk, and a recording/reading device that moves across the disk to read or write data.

• CD-ROM Drive: CD-ROM drive is a device that reads the information stored on CD. A CD-ROM is an abbreviated term for Compact Disk-Read Dn/y Memory. The information stored in a CD-ROM can neither be changed nor can new information be added to it. That is why, it is called Read Only Memory (ROM). A number followed by the alphabet 'x' indicates the speed of a CD¬ROM drive. One x means 150 KB (kilo bytes) of data can be transferred from CD to computer's memory. Higher the number before x and faster the data that can be transferred to the computer The 56X CD-ROM is currently the most popular drive specification. For recording information on a CD, a special CD drive known as CD-RW is used. The CD¬ROM disk is flat and circular in shape and can store a lot more information as compared to a floppy disk.

• CD-Writer: In 1990, the CD-ROM technique was advanced to include writing data on CD. To make your own CD-ROMs, you use a CD-Writer drive, which can write on CD-ROM disks. While using the CD-Writer drive you must have seen numbers like 12x2x24 (three numbers separated by the letter "x'l these numbers indicate the drive speeds of the CD drive. A CD-R drive has two actions - recording onto and reading from compact discs. A CD-RW drive has three actions - recording, rewriting (erasing and recording over) and reading. When looking at the drive speeds, the first number ("12" in the above example) indicates the speed at which the CD drive will record data onto a CD-R compact disc.

So, in the above example, the CD drive will record data at 12 times 150 KB/second. The second number ("2" in the above example) indicates the speed at which the CD drive will rewrite data onto a CD¬RW compact disc. So in the above example, the CD drive will rewrite data onto the compact disc at 2 times 150 KB/second. Typically, the second number, the rewrite speed, is lower than the first number, the writing speed, because rewriting is a slower process than writing. The last number ("24" in the above example) indicates the speed at which the drive will read data from a compact disc. So in the above example, the CD drive will read data from a compact disc at 24 times 150 KB/second. CD-R compact discs are discs that can be recorded on only once, i.e. the matter once recorded is permanent. CD-RW compact discs are discs that can be recorded, erased and re- recorded over.