System Unit & Storage Devices

System Unit
The system unit, sometimes called a chassis, is a box-like case made from metal or
plastic that protects the internal electronic components of the computer from damage
(see Figure 1-3). The circuitry in the system unit usually is part of or is connected to a
circuit board called the motherboard.
Two main components on the motherboard are the central processing unit and
memory. The central processing unit (CPU), also called a processor, is the electronic
device that interprets and carries out the basic instructions that operate the
computer.
During processing, the processor places instructions to be executed and data
needed by those instructions into memory. Memory is a temporary holding place for
data and instructions.
Both the processor and memory consist of chips. A chip is an electronic device that
contains many microscopic pathways that carry electrical current. Chips, which
usually are no bigger than one-half inch square, are packaged so they can be
attached to a motherboard or other circuit board.
Some computer components, such as the processor, memory, and most storage
devices, are internal and reside inside the system unit. Other components, such as
the key board, mouse, microphone, monitor, p rinter, scanner, digital camera, and PC
camera, usually are located outside the system unit. These devices are considered
external. A peripheral is any external device that attaches to the system unit.

Storage Devices
Storage holds data, instructions, and information for future use. Storage differs from
memory, in that it can hold these items permanently. Memory, by contrast, holds
items only temporarily while the processor interprets and executes instructions.
A storage medium (media is the plural) is the physical material on which a computer
keeps data, instructions, and information. A storage device records and retrieves
items to and from a storage medium. Storage devices often function as a source of
input because they transfer items from storage into memory.
Common storage devices are a floppy disk drive, a Zip® drive, a hard disk drive, a
CD-ROM drive, a CD-RW drive, a DVD -ROM drive, and a DVD+RW drive. A drive is
a device that reads from and may write on a storage medium. This media includes
floppy disks, Zip® disks, hard disks, and compact discs.
A floppy disk consists of a thin, circular, flexible disk enclosed in rigid plastic. A floppy
disk stores data, instructions, and information using magnetic patterns. You insert
and remove a floppy disk into and from a floppy disk drive (Figure 1-5). A Zip® disk is
a higher capacity disk that can store the equivalent of up to 170 standard floppy
disks.
A hard disk provides much greater storage capacity than a floppy disk. A hard disk
usually consists of several circular platters that store items electronically. These disks
are enclosed in an airtight, sealed case, which often is housed inside the system unit
.Some hard disks are removable, which enables you to insert and remove the hard
disk from a hard disk drive, much like a floppy disk . Removable disks are enclosed
in plastic or metal cartridges so you can remove them from the drive. The advantage
of removable media such as a floppy disk and removable hard disk is you can take
the media out of the computer and transport or secure it.
A compact disc is a flat, round, portable medium that stores data using microscopic
pits, which are created by a laser light. One type of compact disc is a CD-ROM,
which you can access using a CD -ROM drive. A Picture CD is a special type of CDROM
that stores digital versions of photographs for consumers.
A variation of the standard CD-ROM is the rewriteable CD, or CD-RW. In addition to
accessing data, you also can erase and store data on a CD-RW. To use a CD-RW,
you need a CD-RW drive. Another type of compact disc is a DVD-ROM, which has
tremendous storage capacities — enough for a full-length movie. To use a DVDROM,
you need a DVD drive (Figure 1 -8). A variation of the standard DYD-ROM is
the rewriteable DYD, or DVD+RW.
Some devices, such as digital cameras, use miniature storage media (Figure 1 -9).
PC Cards and memory cards are popular types of miniature storage media. You then
can transfer the items, such as the digital photographs, from the media to your
computer using a device called a card reader.