Any storage unit of a computer system is classified on the basis of the following criteria:
Access time: This is the time required to locate and
retrieve stored data from the storage unit in response to
2. Storage capacity: It is the amount of data that can be stored in the storage unit.
3. Cost per bit of storage.
Units of memory:
The computer stores a character in the storage cells with
binary (0,1) mechanism. Thus the basic unit of memory is a
bit (binary digit – 0,1). To store a character, a
computer requires 8 bits or 1 byte. This is called the ?
word length of the storage unit. Hence the storage
capacity of the computer is measured in the number of words
it can store and is expressed in terms of bytes. The
different units of measurement are
8 Bits = 1 Byte
4 Bits = 1/2 Byte (also called Nibble or Semioctet or Quartet )
1/2 Bit is called Short Bit.
210 (or) 1024 Bytes = 1 Kilo Byte (KB)
210 (or) 1024 KB = 1 Mega Byte (MB)
210 (or) 1024 MB = 1 Gega Byte (GB)
|Traditional byte units|
Kilobyte ( x 1024) = Megabyte ( x 1024) = Gigabyte ( x 1024) = Terabyte ( x 1024) = Petabyte ( x 1024) = Exabyte ( x 1024) =Zettabyte (x 1024) = Yottabyte
Table of Nibbles
The sixteen nibbles and their equivalents in other numeral systems:
Types of Memory : A computer memory is of two types
1. Primary Memory ( Internal storage) 2. Secondary Memory ( External storage)
: Primary memory is also called internal memory and is
an important part of a computer. It is the main area in a
computer where the data is stored. The stored data can be
recalled instantly and correctly whenever desired. This memory
can be quickly accessed by the CPU for reading or storing
information. Primary memory is further classified into two
Random Access Memory (RAM) and Read- Only Memory (ROM ) RAM:
is also known as read/write memory as information can be
read from and written onto it. RAM is a place in a computer
that holds instructions for the computer, its programs and the
data. The CPU can directly access the data from RAM almost
immediately. However, the storage of data and instructions in
RAM is temporary, till the time the computer is running.
It disappears from RAM as soon as the power to the computer is switched off. i.e it is volatile memory.
Secondary memory: The
primary memory which is faster (and hence expensive) is
generally not sufficient for large storage of data. As a
result, additional memory, called the ?auxiliary? or ?secondary
memory? is used. It is also referred as ?backup storage as it
is used to store large volume of data on a
basis which can be transferred to the primary memory whenever
required for processing. Data are stored in secondary storage
in the same binary codes as in the main (primary memory)
storage. Some of the devices of secondary storages are
Floppy Disk, Hard Disk, CD-ROM, DVD and Flash drive.
1. Floppy Disk:
It is also referred as ?Diskette: and is made of flexible
Vinyl material. It has a small hole on one side
called ?Right protect notch, Which protects
accidental writing/deleting the information from the disk.
There is a hole in the centre through which the spindle of
drive unit rotates the disk. The disks are available in two
sizes of 5.25 and 3.5 inches and these could be either low-
density or high-density floppies. Storage capacity of
floppies are measured in kilobytes (KB) and megabytes (MB). The
details about the storage capacities of the floppies are
|Floppy Disk||Storage Capacity||Size (Diameter)|
|Low Density||360 KB||5.25 inches|
|High Density||1.2 MB||5.25 inches|
|High Density||1.44 MB||3.5 inches|
|Extended||2.8 MB||3.5 inches|
2. Hard Disk:
The hard disk can hold more information than the floppy
disk and the retrieval of information from hard disk is
faster when compared to floppies or tapes. A hard disk is
fixed inside the CPU and its capacity ranges from 20 MB
onwards. The hard disk is made up of a collection of discs
(one below the other) known as platters on which the data is
recorded. These platters are coated with magnetic material.
It is less sensitive to external environmental disorders and
hence the storage in hard disk is safe. A small hard disk
might be as much as 25 times larger than a floppy disk.
Storage Capacity of hard disks varies from 20 MB to several
Gega bytes like 80GB, 160GB.
CD-ROM stands for Compact Disk–Read Only Memory. It is used
to store a wide variety of information. Its main advantage is
that it is portable and can hold a large amount of data..
The storage capacity of most CD-ROMs is approximately 650 MB
or 700 MB.
CD-ROMs have the following variations:
CD-R(Compact disc Recordable): Data can be written onto it
just once. The stored data can be read. Data once written
onto it cannot be erased.
(ii) CD-RW(Compact disc
Rewritable): It is also called erasable CD. Data once written
onto it can be erased to write or record new information
To use a CD-ROM, a device called CD drive is needed.
DVD stands for Digital Versatile Disc. It is similar to a
CD-ROM, except that it can store larger amounts of data. The
storage capacity of a DVD is at least 4.7MB. DVDs that can
store up to 17GBs are also available. Because of their
capacity, DVDs are generally used to store a very large
multimedia presentations and movies that combine high quality
sound and graphics.
5. Flash Drive:
It is a small, portable device that can be used to store,
access and transfer data. Due to its small size, it is
commonly called Pen drive. It is also called USB (Universal Serial Bus)
drive. We can read, write, copy, delete, and move data from
computer to pen drive or pen drive to computer. It comes in
various storage capacities of 2GB, 4GB, 8GB etc. It is popular
because it is easy to use and small enough to be carried
in a pocket. This device is plugged into the USB port of
the computer and the computer automatically detects this device.