COMPUTER STORAGE
PARTS OF THE REPORT
Primary storage
Secondary storage
Tertiary storage
Off-line storage
PRIMARY STORAGE
Primary storage (or main memory or internal memory), often referred to simply as memory, is the only one
directly accessible to the CPU. The CPU continuously reads instructions stored there and executes them as
required. Any data actively operated on is also stored there in uniform manner.
SECONDARY STORAGE
Secondary storage (also known as external memory or auxiliary storage), differs from primary storage in
that it is not directly accessible by the CPU. The computer usually uses its input/output channels to access
secondary storage and transfers the desired data using intermediate area in primary storage. Secondary
storage does not lose the data when the device is powered downit is non-volatile. Per unit, it is typically
also two orders of magnitude less expensive than primary storage. Modern computer systems typically
have two orders of magnitude more secondary storage than primary storage and data are kept for a longer
time there.
in modern computers, hard disk drives are usually used as secondary storage. The time taken to access a
given byte of information stored on a hard disk is typically a few thousandths of a second, or milliseconds.
By contrast, the time taken to access a given byte of information stored in random-access memory is
measured in billionths of a second, or nanoseconds
TERTIARY STORAGE
tertiary storage or tertiary memory,provides a third level of storage. Typically it involves a robotic
mechanism which will mount (insert) and dismount removable mass storage media into a storage device
according to the system's demands; this data is often copied to secondary storage before use. It is primarily
used for archiving rarely accessed information since it is much slower than secondary storage (e.g. 560
seconds vs. 110 milliseconds). This is primarily useful for extraordinarily large data stores, accessed
without human operators. Typical examples include tape libraries and optical jukeboxes.
OFF-LINE STORAGE
Off-line storage is a computer data storage on a medium or a device that is not under the control of
a processing unit. The medium is recorded, usually in a secondary or tertiary storage device, and then
physically removed or disconnected. It must be inserted or connected by a human operator before a
computer can access it again. Unlike tertiary storage, it cannot be accessed without human interaction.
Off-line storage is used to transfer information, since the detached medium can be easily physically
transported. Additionally, in case a disaster, for example a fire, destroys the original data, a medium in a
remote location will probably be unaffected, enabling disaster recovery. Off-line storage increases
general information security, since it is physically inaccessible from a computer, and data confidentiality or
integrity cannot be affected by computer-based attack techniques. Also, if the information stored for
archival purposes is rarely accessed, off-line storage is less expensive than tertiary storage
VOLATILITY
Non-volatile
Will retain the stored information even if it is not
constantly supplied with electric power. It is
suitable for long-term storage of information.
Volatile
Requires constant power to maintain the stored
information. The fastest memory technologies of
today are volatile ones (not a universal rule). Since
primary storage is required to be very fast, it
predominantly uses volatile memory.
CAPACITY
Raw capacity
he total amount of stored information that a
storage device or medium can hold. It is expressed
as a quantity of bits or bytes (e.g. 10.4 megabytes).
Memory storage density
The compactness of stored information. It is the
storage capacity of a medium divided with a unit of
length, area or volume (e.g. 1.2 megabytes per
square inch).
FUNDAMENTAL STORAGE TECHNOLOGIES
As of 2011, the most commonly used data storage technologies are semiconductor, magnetic, and optical,
while paper still sees some limited usage. Media is a common name for what actually holds the data in the
storage device. Some other fundamental storage technologies have also been used in the past or are
proposed for development.
SEMICONDUCTOR
Semiconductor memory uses semiconductor-
based integrated circuits to store information. A
semiconductor memory chip may contain millions of
tiny transistors or capacitors. Both volatile and non-
volatile forms of semiconductor memory exist. In
modern computers, primary storage almost exclusively
consists of dynamic volatile semiconductor memory
or dynamic random access memory. Since the turn of
the century, a type of non-volatile semiconductor
memory known as flash memory has steadily gained
share as off-line storage for home computers. Non-
volatile semiconductor memory is also used for
secondary storage in various advanced electronic
devices and specialized computers.
MAGNETIC
Magnetic storage uses different patterns of magnetization on a magnetically coated surface to store
information. Magnetic storage is non-volatile. The information is accessed using one or more read/write
heads which may contain one or more recording transducers. A read/write head only covers a part of the
surface so that the head or medium or both must be moved relative to another in order to access data. In
modern computers, magnetic storage will take these forms:
MAGNETIC DISK
Floppy disk Hard disk drive
used for off-line storage
used for secondary storage
OPTICAL
Optical storage, the typical optical disc, stores information in deformities on the surface of a circular disc
and reads this information by illuminating the surface with a laser diode and observing the reflection.
Optical disc storage is non-volatile. The deformities may be permanent (read only media), formed once
(write once media) or reversible (recordable or read/write media). The following forms are currently in
common use:
PAPER
Paper data storge, typically in the form of paper tape or punched cards, has long been used to store
information for automatic processing, particularly before general-purpose computers existed. Information
was recorded by punching holes into the paper or cardboard medium and was read mechanically (or later
optically) to determine whether a particular location on the medium was solid or contained a hole. A few
technologies allow people to make marks on paper that are easily read by machinethese are widely used
for tabulating votes and grading standardized tests. Barcodes made it possible for any object that was to be
sold or transported to have some computer readable information securely attached to it.
UNCOMMON
Vacuum tube memory
A Williams tube used a cathode ray tube, and
a Selectron tube used a large vacuum tube to store
information. These primary storage devices were
short-lived in the market, since Williams tube was
unreliable and the Selectron tube was expensive.
Electro-acoustic memory
Delay line memory used sound waves in a
substance such as mercury to store information.
Delay line memory was dynamic volatile, cycle
sequential read/write storage, and was used for
primary storage.
Optical tape
Is a medium for optical storage generally consisting
of a long and narrow strip of plastic onto which
patterns can be written and from which the
patterns can be read back. It shares some
technologies with cinema film stock and optical
discs, but is compatible with neither. The
motivation behind developing this technology was
the possibility of far greater storage capacities than
either magnetic tape or optical discs.
Phase-change memory
Uses different mechanical phases of Phase Change
Material to store information in an X-Y addressable
matrix, and reads the information by observing the
varying electrical resistance of the material. Phase-
change memory would be non-volatile, random-
access read/write storage, and might be used for
primary, secondary and off-line storage. Most
rewritable and many write once optical disks
already use phase change material to store
information.
Holographic data storage
stores information optically
inside crystals or photopolymers. Holographic
storage can utilize the whole volume of the storage
medium, unlike optical disc storage which is limited
to a small number of surface layers. Holographic
storage would be non-volatile, sequential access,
and either write once or read/write storage. It
might be used for secondary and off-line storage.
See Holographic Versatile Disc (HVD).
Molecular memory
stores information in polymer that can store
electric charge. Molecular memory might be
especially suited for primary storage. The
theoretical storage capacity of molecular memory
is 10 terabits per square inch.
THATS THE END. THANK YOU
