• Basic units of Measurement
• Storage versus Memory
III. Auxiliary Storage Devices-Magnetic Tape, Hard Disk, Floppy Disk
IV.Optical Disks: CD-R Drive, CD-RW disks, DVD, Blue ray Discs
The computer system essentially comprises three important parts – input device,
central processing unit (CPU) and the output device. The CPU itself is made of three
components namely, the arithmetic logic unit (ALU), memory unit, and the control
unit.In addition to these, auxiliary storage/secondary storage devices are used to store data
and instructions on a long-term basis.
The objective of this chapter is to introduce the concept of Memory units of the
computer which are shown in the above figure as main memory and secondary
memory. The Memory unit is linked with other parts of the computer specifically as
shown in the figure2.
Input Unit Output Unit
Central Processing Unit
All storage devices are characterized with the following features:
• Access method
• Cost and capacity
1.1 Basic Units of Measurement
All information in the computer is handled using electrical components like the
integrated circuits, semiconductors, all of which can recognize only two states –
presence or absence of an electrical signal. Two symbols used to represent these two
states are 0 and 1, and are known as BITS (an abbreviation for BInary DigiTS). 0
represents the absence of a signal, 1 represents the presence of a signal. A BIT is,
therefore, the smallest unit of data in a computer and can either store a 0 or 1.
Since a single bit can store only one of the two values, there can possibly be only four
00 01 10 11
Bits are, therefore, combined together into larger units in order to hold greater range
BYTES are typically a sequence of eight bits put together to create a single computer
alphabetical or numerical character. More often referred to in larger multiples, bytes
may appear as Kilobytes (1,024 bytes), Megabytes (1,048,576 bytes), GigaBytes
(1,073,741,824), TeraBytes (approx. 1,099,511,000,000 bytes), or PetaBytes
(approx. 1,125,899,900,000,000 bytes).
Bytes are used to quantify the amount of data digitally stored (on disks, tapes) or
transmitted (over the internet), and are also used to measure the memory and
The Term Computer Memory is defined as one or more sets of chips that store
Data/program instructions, either temporarily or permanently. It is critical processing
component in any computer. The PCs use several different types. They are :
• Main Memory / Primary Memory units
– Two most important are
• RAM(Random Access Memory)
• ROM(Read-only Memory)
– They work in different ways and perform distinct functions
– CPU Registers
– Cache Memory
• Secondary Memory/Auxiliary Memory
Also termed as ‘auxiliary’ or ‘backup’ storage, it is typically used as a
supplement to main storage. It is much cheaper than the main storage and
stores large amount of data and instructions permanently. Hardware devices
like magnetic tapes and disks fall under this category.
Computer’s memory can be classified into two types – RAM and ROM.
RAM or Random Access Memory is the central storage unit in a computer system. It
is the place in a computer where the operating system, application programs and the
data in current use are kept temporarily so that they can be accessed by the
computer’s processor. The more RAM a computer has, the more data a computer can
Random access memory, also called the Read/Write memory, is the temporary
memory of a computer. It is said to be ‘volatile’ since its contents are accessible only
as long as the computer is on. The contents of RAM are cleared once the computer is
ROM or Read Only Memory is a special type of memory which can only be read and
contents of which are not lost even when the computer is switched off. It typically
contains manufacturer’s instructions. Among other things, ROM also stores an initial
program called the ‘bootstrap loader’ whose function is to start the computer software
operating, once the power is turned on.
Read-only memories can be manufacturer-programmed or user-programmed.
While manufacturer-programmed ROMs have data burnt into the circuitry, userprogrammed
ROMs can have the user load and then store read-only programs. PROM
or Programmable ROM is the name given to such ROMs.
Information once stored on the ROM or PROM chip cannot be altered. However,
another type of memory called EPROM (Erasable PROM) allows a user to erase the
information stored on the chip and reprogram it with new information. EEPROM
(Electrically EPROM) and UVEPROM (Ultra Violet EPROM) are two types of
Storage Vs. Memory
RAM is volatile memory having a limited storage capacity. Secondary/auxiliary
storage is storage other than the RAM. These include devices that are peripheral and
are connected and controlled by the computer to enable permanent storage of
programs and data.
Magnetic medium was found to be fairly inexpensive and long lasting medium and,
therefore, became the preferred choice for auxiliary storage. Floppy disks and hard
disks fall under this category. The newer forms of storage devices are optical storage
devices like CDs, DVDs, Pen drive, Zip drive etc.
The memory is specifically meaning the RAM. This keeps the information for a
shorter period of time (usually volatile), is faster and more expensive.
By Storage we mean the Hard disk. Here the information is retained longer (nonvolatile),
It’s Slower and Cheaper
III. Auxiliary Storage Devices-Magnetic Tape, Floppy Disk, Hard Disk.
The Magnetic Storage Exploits duality of magnetism and electricity. It converts
electrical signals into magnetic charges, captures magnetic charge on a storage
medium and then later regenerates electrical current from stored magnetic charge.
Polarity of magnetic charge represents bit values zero and one.
The Magnetic Disk is Flat, circular platter with metallic coating that is rotated beneath
read/write heads. It is a Random access device; read/write head can be moved to any
location on the platter.
These are small removable disks that are plastic coated with magnetic recording
material. Floppy disks are typically 3.5″ in size (diameter) and can hold 1.44 MB of
data. This portable storage device is a rewritable media and can be reused a number of
Floppy disks are commonly used to move files between different computers. The
main disadvantage of floppy disks is that they can be damaged easily and, therefore,
are not very reliable. The following figure shows an example of the floppy disk.
Figure 3 shows a picture of the floppy disk.
Figure 3: Floppy Disk
Another form of auxiliary storage is a hard disk. A hard disk consists of one or
more rigid metal plates coated with a metal oxide material that allows data to be
magnetically recorded on the surface of the platters. The hard disk platters spin at
a high rate of speed, typically 5400 to 7200 revolutions per minute (RPM).Storage
capacities of hard disks for personal computers range from 10 GB to 120 GB (one
billion bytes are called a gigabyte).
Figure 4: Hard Disk
IV.Optical Disks: CD-R Drive, CD-RW disks, DVD, Blue ray Discs
Optical Mass Storage Devices Store bit values as variations in light reflection. They
have higher area density & longer data life than magnetic storage. They are also
Standardized and relatively inexpensive. Their Uses: read-only storage with low
performance requirements, applications with high capacity requirements & where
portability in a standardized format is needed.
Example of the Optical Drives
• CD's (Compact Disk)
~ 700 MB storage
– CD-ROM (read only)
– CD-R: (record) to a CD
– CD-RW: can write and erase CD to reuse it (re-writable)
– DVD(Digital Video Disk)
Compact Disk (CD) is portable disk having data storage capacity between 650-700
MB. It can hold large amount of information such as music,
full-motion videos, and text etc. It contains digital information that can be read, but
cannot be rewritten. Separate drives exist for reading and writing CDs.
Since it is a very reliable storage media, it is very often used as a medium for
distributing large amount of information to large number of users. In fact today most
of the software is distributed through CDs.
Digital Versatile Disk (DVD) is similar to a CD but has larger storage capacity and
enormous clarity. Depending upon the disk type it can store several Gigabytes of data
(as opposed to around 650MB of a CD). DVDs are primarily used to store music or
movies and can be played back on your television or the computer too. They are not
rewritable media. Its also termed DVD (Digital Video Disk)
– Over 4 GB storage (varies with format)
– DVD- ROM (read only)
– Many recordable formats (e.g., DVD-R, DVD-RW; ..)
– Are more highly compact than a CD.
– Special laser is needed to read them
The name is derived from the blue-violet laser used to read and write data. It was
developed by the Blu-ray Disc Association with more than 180 members. Some
companies with the technology are Dell, Sony, LG.The Data capacity is very large
because Blu-ray uses a blue laser(405 nanometers) instead of a red laser(650
nanometers) this allows the data tracks on the disc to be very compact. This allows for
more than twice as small pits as on a DVD. Because of the greatly compact data Bluray
can hold almost 5 times more data than a single layer DVD. Close to 25 GB!.Just
like a DVD Blu-ray can also be recorded in Dual-Layer format. This allows the disk
to hold up to 50 GB!!
The Variations in the formats are as follows:
• BD-ROM (read-only) - for pre-recorded content
• BD-R (recordable) - for PC data storage
• BD-RW (rewritable) - for PC data storage
• BD-RE (rewritable) - for HDTV recording
The chapter has introduced the concept of memory units and its categories as the main
and the second memory and their characteristic features. It discusses the features of
RAM, ROM, PROM, and EPROM. Auxiliary Storage Devices like -Magnetic Tape,
Hard Disk, Floppy Disk are also discussed .The chapter concludes with an
introduction to the Optical Disks such as CD-R Drive,CD-RW disks,DVD,Blue ray