While performing a processing
operation, the CPU needs a place to temporarily hold instruction to be executed
and data to be used with those instructions. Memory, which is composed of one or
more chips on the motherboard, holds data and instruction while the CPU is
The two basic type of memory are volatile and nonvolatile. The contents of volatile memory, such as RAM are lost (erased) when the power to the computer is turned of. The contents of nonvolatile memory, however, are not lost when power is removed from the computer. For example, once instruction has been record onto a nonvolatile ROM chip, they usually cannot be erased or change, and the contents of the chips are not erased when power is turned off.
also called secondary storage, auxiliary storage, or mass
storage. Holds items such as data, instruction, and information for future
Think of storage as a filling
cabinet used to hold file folder, and memory as the top of your desk. When you
need to work with a file, you remove it from the filling cabinet (storage) and
place it on your desk (memory). When you are finished with the file, you return
it to the filling cabinet (storage).
Storage is nonvolatile, which means
that items in storage are retained even when power is removed from the computer
(fig 6-1). A storage medium (media is the plural) is the physical
material on which item are kept. One commonly used storage medium is a disk,
which is a round, flat piece of plastic or metal with a magnetic coating on
which items can be written. A storage device is the
mechanism used to record and retrieve items to and from a storage medium.
Storage devices can function is
sources of input and output. For example, each time a storage devices transfer
data, instruction, and information from a storage medium into memory-a process
called reading-it function as an input source. When a storage device transfer
these item from memory to a storage medium-a process called writing-it
function as an output source. The speed of the storage device is defined by its access
time, which is the minimum time it takes the devices to locate a single
item on a disk.
Compared to memory, storage devices
are slow. The access time of memory devices is measured in non second
(billionths of a second), while the access time of storage devices is measured
in million seconds (thousands of a second).
The size, or capacity,
of storage devices, is measured by the number of bytes (characters) it can hold,
fig 6-2 lists the terms used to define the capacity of storage devices. For
example, a typical floppy disk can store 1.44 MB of data (approximately
1,440,000 bytes) and a typical hard disk can store 8GB of data (approximately
Storage requirements among user
vary greatly. Users of smaller computer, such as small business users, might
need moth store a relatively small of amount data, for example, a field sales
representative might have a list of names, addresses, and telephone number of 50
customers, which he or she uses on a daily basis. Such a list might require no
more than several thousand bytes of storage. Users of larger computer, such as
banks, libraries or insurance companies, process data for millions of customers
and thus might need to store trillions of bytes worth of historical or financial
record in their achieves.
To meet the needs of a wide range
of users, numerous types of storage media and storage devices exist, figure 6-3
show how different types of storage media and memory compare in terms of
relative cost and speed. The storage media included in the pyramid are discussed
in this chapter.
As noted, a floppy disk is a portable storage medium. When discussing a storage medium, the term portable means you can remove the medium from one computer and carry it to another computer. For example, you can insert a floppy disk into and remove it from the floppy disk drive on many types of computer (figure 6-5). A floppy disk drive is a device that can read from and write to a floppy disk.
To protect them from accidentally
being erased, floppy disk have a write-protect notch. A write-protect
notch is a small opening in the corner of the floppy disk with a tab
that you slide to cover or expose the notch (figure 6-8). The write-protect
notch works much like the reading tab on VHS tape: if the recording tab is
removed, a VCR cannot record onto the VHS tape.
On a floppy disk, if the write protect notch is exposed, or open, the drive cannot write on the floppy disk. If the write-protect north is covered, or closed, the drive can write on the floppy disk. The write protect notch only affect the flopper disk drive’s capability of writing on the disk: a floppy disk drive car read from the floppy disk whether the write-protect notch is open or closed. Some floppy disk has a second opening on the opposite side of disk that does not have the small tab: this opening identifies the disk as a high-density floppy disk
As noted, a floppy disk drive
(FDD) is a device that can read from and write on a floppy disk. Desktop
personnel computers usually have a floppy disk drive installed inside the system
unit. Many lap top computer have removable floppy disk drives that can be
replaced with other types of drives or devices, or they use an external floppy
disk drive that plugs into the laptop (see figure 6-5 on page 6.4).
a computer has one floppy disk drive, the drive usually is designated drive
A; if the computer has two floppy disk drive; the second one usually is
designated drive B.
On any 3.5-inch floppy disk, a piece of metal called the shutter
covers an opening in the rigid plastic shell. When you insert a floppy disk into
a floppy disk drive, the drives slides shutter to the side to expose a portion
of both sides of the floppy disk’s recording surface.
The read/write head is the mechanism that
actually read items from a write items on the floppy disk. Figure 6-0
illustrates the steps for reading from and writing on a floppy disk. The average
access time for current floppy disk drives to locate an item on the disk is 84
ms, or approximately 1/12 of a second.
On the front of most floppy disk drives is a
light emitting diode (LED) that lights up when the drive is accessing the floppy
disk. You should not remove a floppy disk when the floppy disk drive is
accessing the disk.Sometimes, a floppy disk drive will malfunction when it is
attempting to access a
When personnel computer (PC) were
introduced, software programs and their related files required small amount of
storage and fit easily on floppy disk. As software became more complex and
included graphical user interfaces and multimedia, file size and storage
requirement increased. Today, hard disk-which provide far larger storage
capacities and much faster access time than floppy disk-are one of the primary
media for storing software programs and file. Current personnel computer hard
disk can store 4 to 50 gb of data, instruction and information.
A hard disk usually
consist of several inflexible, circular disk, called platter, on which item are
stored electronically. A platter in a hard disk is made of
aluminum, glass, or ceramic and is coated with a material that allow item to be
magnetically recorded on it surface. On hard disk, the platter, the read/write
heads, and the mechanism for moving the heads across the surface of the disk
enclosed in a airtight, sealed case that protect the platters from contamination
The hard disk in most desktop
personnel computer is housed inside the system unit. Such hard disk, which are
not portable, are considered fixed disk (figure 6-13). Hard disks
also can be removable. Removable hard disk is discussed later in the chapter.
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