2.

5 COMPUTER MEMORY

The term memory usually refers to a form of semiconductor storage

and sometimes other forms of fast but temporary
storage. Similarly, today
Computer Application in Business 1)
Data Storage Techniques

the term storage more commonly refers to mass storage such as optical
discs, forms of magnetic storage like hard disks and other types. All
these
are slower than memory, but of a more
permanent nature. Generally , ne
computer memory is divided into two category : primary
memory arnd
secondary memory. The term primary memory is used to represent the
memory and the term secondary memory is used to represent the
storage.
Primary memory is directly accessible to the CPU. The CPU
continuously reads instructions stored there and executes them. Any data
actively operated on is also stored there in uniform manner. It is a
form of
semiconductor storage known as random access memory (RAM) and
sometimes other forms of fast but temporary storage. It is small-sized, light,
but quite expensive. This type of memory is divided
into the following two
types:
a) RAM (Random Accress Memory)
b) ROM (Read Only Memory)

2.5.1 Random Access Memory (RAM)

Amachine's main memory is often referred to as RAM- an

area in the computer system unit that temporarily holds user data,
operating system instructions and program instructions. Every time
we turn on our computer, a set of operating system instructions is
copied from hard disk into RAM. These instructions, which help
control basic computer functions, remain in RAM until we turn the
computer off.
RAM features :
a) Data that needs to be processed and the instructions which are
used for processing are held in the RAM.
b) RAM is a cluster of semi-conductor devices. The elements of
RAM change with the proper application (changes) of the electric
current.
c) Each element of RAM is a memory location in which data can
be stored. Each location has a unique address. Using this
address data can be directly retrieved and stored.
 Data Storage Techniques

must hold both the data to be processed anrt

d) Since RAM the
processing, its size or capacity is one of
instructions for
computer.
measure of power of the periodically onto a
RAM need to be stored
e) The contents of failure will erase the
contents
since power
secondary memory also called volatile
which this memory is
of RAM because of
memory.
categories of random access
There are generally two broad
memory
Random Access Memory)
) DRAM memories (Dynamic
SRAM memories (Static Random Access Memory)
ii)

2.5.2 Read Only Memory

used in PCs is called read
One major type of memory that is
is atype of memory that
only memory, or ROM for short. ROM
which can be both
normally can only be read, as opposed to RAM
whether the
read and write. The instructions in ROM are permanent
unless we
power is on or off. We have no way to change them,
remove the ROM chips from the main board and replace them with
another set.
Read-only memory is most commonly used to store system
level programs that we want to have available to the PC at alltimes.
The most common example is the ROM-BIOS (ROM basic input
and output services) program, which is use to boot up the system.
The term boot is used todescribe the initial loading of an
operating
system or of some other program into a computer. The word "boot
comes from the term "bootstrap".
The following are the different types of
ROMs with a
description of their relativemodifiability:
"
Programmable ROM (PROM): This is a type of ROM that cail
be programmed using
specialequipment: it can be written to,
but only once.
Data Storage Tochniques
Erasable Programmable ROM (EPROM) : An EPROM is a
ROM that can be erased and reprogrammed. Ultraviolet light of
a specific frequency is used for erasing the EPROM and allow
it to be reprogrammed again.
" Electrically Erasable Programmable ROM (EEPROM):The
next level of erasability is the EEPROM, which can be erased
under software control. This is the most flexible type of ROM,
and is nowcommonly used for holding BIOS programs.

2.5.3 Cache Memory and Virtual Memory

Another two types of memory used in a computers are -

cache memory and virtual memory.
Cache Memory :Cache memory isa special type of RAM which
is the faster memory used in a computer system in between the
CPUand main memory. Itmeans that the accessing speed from
the cache memory is faster than the main memory used in
computer. So, the use of the cache memory reduces the average
access time and helps the processor for fast processing. The
Cache memory stores the instructions and data that are
frequently needed by the processor for execution. Cache memory
chip is a static memorychip It is much expensive than the main
etc.
memory. Cache size varies from 32 MB to 128MB or 512MB
Virtual Memory : An operating system technique that uses
swapping techniques to increase the apparent size of actual
use disk
memory. Virtual Memory is the ability of a computer to
computers
storage to simulate RAM. Virtual Memory allows
large
without enough RAM torun large programs, manipulate
at a time. One
data files, and run more than one program
performance. With
disadvantage of virtual memory is reduced
computer uses space on
most of today's operating systems the
computer's hard disk drive as an extension of RAM. It takes
our
retrieve data from virtual memory than from RAM because
longer to
so access time is slower.
the disk is a mechanical device
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 Data Storage Technique,

2.5.4 Secondary Memory

thefacility of
Secondary memory or storage provides storing
information and programs permanently. It differs from primary
CPU. The computer
memory in that it is not directly accessible by the
secondary storage
usually uses its input/output channels to access
area in primary
and transfers desired data using intermediate
the data when the device
memory. Secondary storage does not lose
primary
is powered down; it is non-volatle. t is less expernsive than
memory.
In modern computers, hard disks are usually used as
given byte of
secondary storage. The time taken to access a
milliseconds. By
information stored on a hard disk is order of few
information stored
contrast, the time taken to acCcess a given byte of
in nanoseconds. Some
in random accesS memoy is measured
optical disk.
other examples of secondary storage devices are:
Cards,
floppy disks, magnetic tape, Paper tape, Punch
standalone RAM disks, flash memory (e.g. USBsticks or keys),
and Zip drives.
The characteristics that are used for the classification of
various types of storage are:
" Storage Capacity : It is a representative of the size of the
memory. The capacity of internal memory and main memory
can be expressed in terms of number of words or bytes. The
storage capacity of external memory is normally measured in
terms of bytes.
" Access Modes :A memory is considered to consists of various
memory locations. The information from these memory locations
can be accessed in the following ways:
Random Access :lt is the mode in which memory locations Can
be accessed in any order in the same amount of time.
Sequential Access:Sequential access also known as serlal
access where the device must read or move through a
Data Storage Techniques

information up to the point it is attempting to read or

write.Atape
drive is an example of a sequential
access drive, where the drive
must move the tape forward or
backwards until it reaches its
destination.
DirectAccess:Adirect access storage device is one in which
any location in the device may be selected at random, access to
the information stored is direct i.e. no need of scanning a series
of records and approximately equal access time is required for
each location. Hard disk, optical disk are the examples of direct
access storage device.
Access Time: The access time is the time required between
the request made for aread or write operation till the time the
data is made available or written at the requested location.