Data storage & retrieval methods
Q5)Explain various
Data storage and retrieval methods?
Ans:
Data storage and
retrieval refers to the ways in which data is stored, accessed and retrieved
to/from the computer.
Various storage
and retrieval methods are as follows,
1.Sequential
Access Method
Sequential file access is the most straightforward method of accessing files. This method accesses data as one record at a time by starting from the beginning of the file to its end. Moreover, the records are read or written in the order they appear in the file
2. Direct Access
Method
Direct file access, also known as random access. It allows us to access data directly from any location within the file, without the need to read or write all the records that come before it. Furthermore, this method accesses records within the file by using their physical addresses or positions.
3. Indexed
Access Method
Indexed file access is a method that incorporates the benefits of both sequential and direct file access. This method involves creating an index file that maps logical keys or data elements to their corresponding physical addresses within the file. Moreover, the system stores the index separately from the data file, enabling quick access to locate the desired data.
4. Indexed
Sequential Access Method
ISAM method is an advanced sequential file organization. In this method, records are stored in the file using the primary key. An index value is generated for each primary key and mapped with the record. This index contains the address of the record in the file.
Memory/Storage:
A memory is just like a human brain. It is used to store data and instructions. Computer memory is the storage space in the computer, where data is to be processed and instructions required for processing are stored. The memory is divided into large number of small parts called cells. Each location or cell has a unique address, which varies from zero to memory size minus one.
Primary Storage
(Primary Memory (Main Memory)):
Q6)Explain
Primary storage types?(VIMP)
Ans: Primary memory holds only those data and instructions on which the computer is currently working. It has a limited capacity and data is lost when power is switched off. It is generally made up of semiconductor device. It is divided into two subcategories RAM and ROM.
Characteristics
of Main Memory
• It is known as
the main memory.
• Usually
volatile (Temporary) memory.
• Data is lost
in case power is switched off.
• Faster than
secondary memories.
• A computer cannot run without the primary memory.
A) RAM (Random
Access Memory):
It is the internal memory of the CPU for storing data, program, and program
result. It is a read/write memory which stores data until the machine is
working. As soon as the machine is switched off, data is erased.
RAM is volatile
(Temporary), i.e. data stored in it is lost when we switch off the computer or
if there is a power failure.
There are two
types of RAM:
Dynamic RAM
Static RAM.
Dynamic RAM
(DRAM):
Its data has to be refreshed after every few milliseconds. DRAM has higher
storage capacity. It is cheaper than Static RAM.
Static RAM (SRAM): Its data need not be refreshed. SRAM has higher speed. It is costlier.
B) ROM (Read Only Memory) : The memory from which we can only read but cannot write on it. This type of memory is non-volatile (Permanent). A ROM stores such instructions that are required to start a computer. This operation is referred to as bootstrap. ROM chips are not only used in the computer but also in other electronic items like washing machine and microwave oven.
The various
types of ROMs and their characteristics:
PROM
(Programmable Read Only Memory) :
PROM is
read-only memory that can be modified only once by a user. The user buys a
blank PROM and enters the desired contents using a PROM program. Inside the
PROM chip, there are small fuses which are burnt open during programming.
EPROM (Erasable
and Programmable Read Only Memory) : EPROM allows us to write our own BIOS
programs. These programs can be erased by using an ultraviolet light.
EEPROM
(Electrically Erasable and Programmable Read Only Memory) :
EEPROM is
programmed and erased electrically. It can be erased and reprogrammed about ten
thousand times. In EEPROM, any location can be selectively erased and
programmed. EEPROMs can be erased one byte at a time, rather than erasing the
entire chip. Hence, the process of reprogramming is flexible but slow.
The advantages
of ROM are as follows
−
• Non-volatile
in nature
• Cannot be
accidentally changed
• Cheaper than
RAMs
• Easy to test
• More reliable than RAMs
Cache Memory:
This is a very
special type of high speed memory. This memory cannot be accessed by the user.
The main function of this cache memory is to make the programs and data
available to the CPU very fast.
Access time of
memory is generally very high as compared to the execution time of the GPU.
Therefore a cache, which is a very small but fast memory, is used between the
CPU and the main memory. This memory also called a high speed buffer.
A cache stores
those segments of programs and data which are frequently needed. It makes
available this data to the CPU at a very fast rate thus increasing the
efficiency.
Secondary Storage(Secondary Memory (Auxiliary Memory)) :
Q7)Write about
secondary storage devices of computer?
(VIMP)
Ans: Auxiliary memory is much larger in size
than main memory but is slower. It normally stores system programs, instruction
and data files. It is also known as secondary memory. Secondary memories cannot
be accessed directly by a processor. First the data/information of auxiliary
memory is transferred to the main memory and then that information can be
accessed by the CPU.
Ex: Floppy disc,
Hard Disc CD-ROM, DVD, etc.
Characteristics
of Secondary Memory
• These are
magnetic and optical memories.
• It is known as
the backup memory.
• It is a
non-volatile memory.
• Data is permanently stored even if power is switched off.
Magnetic Tape :Magnetic tape
has been in use for more than 50 years. Modern magnetic tape is packaged in
cartridges or cassettes and is used for storing data backups, particularly in
corporate settings. The average amount of storage is 5 MB to 140 MB.
Advantages of
Magnetic Tapes
❖
They are cost-effective as they use low cost reels and cartridges. They also
support erasing and rewriting of data.
❖
They are light-weight and making them easy to manage physically.
❖
They offer storage of large amount of data within a small space.
❖
They are highly portable.
Limitations of
Magnetic Tapes
❖
They offer sequential access to data and therefore, they lack in accessing the
data randomly.
❖
They can generate read errors when they are exposed to dust, extreme
temperature or twisted.
❖ They need to be named manually to identify the data stored in the disks.
Some of the
common types of magnetic tapes are as follows,
1. Tape Reel: In this type of
magnetic tapes, the tape ribbon is rolled in a reel format. The size of this
ribbon is half-inch. The ribbon is moved/rolled up from one reel (supply) to
another reel (take-up).
2. Cartridge
Tape:
This type of tape carries a ribbon of length ranging from 60 to 90 meters and
width of 4mm. It is covered with a catridge. The storage capacity ranges from 4
GB to 14 GB.
3. Digital Audio Tape (DAT): The working and appearance of tape drive is very much similar to audio/video cassettes. It is considered as the compact and faster version of tape reels. The process of inserting and ejecting these tapes in the tape drives is same as that of audio and video cassettes.
Magnetic Disk :Magnetic disk is
a device that write/reads data to and from a magnetic tape. It has two heads.
These are write head and read head. The write head encodes the magnetic signals
on the magnetic tape. Similarly, the read head reads the stored data from
magnetic tape and transfers it into the RAM.
Magnetic disks are the most commonly used secondary storage medium.
Advantages of
Magnetic Disks
1. They provide
high storage capacity because of their high data recording densities and
compact size.
2. They are much
reliable.
3. They are used
to transfer the data from one computer to another.
4. Easily portable.
Limitations
1. Not all the
types of magnetic disks are portable.
2. The cost of
magnetic disks is high compared to magnetic tapes.
3. They are very
sensitive and they must be stored in dust-less environment.
4. Magnetic disks should be named properly while keeping them for offline storage. Otherwise, it leads to confusions.
Types of
Magnetic Disks are Floppy disks and Hard disks
Floppy Disks :
Floppy disks are a storage medium made of a thin magnetic disk. They were widely used from the 1970s to the early 2000s. They are very easy to use and are portable. Data is stored as electromagnetic charges on a metal oxide film coating the Mylar plastic. There exist several types of floppy disks but the most common one is the 3.5 inch floppy that can store data up to a capacity of 1.44 megabytes, something like 400 typewritten pages. A floppy drive is required to access a floppy disk.
Hard Disks:
Hard disk drive is made up of a series of circular disks called platters arranged one over the other almost ½ inches apart around a spindle. Disks are made of non-magnetic material like aluminium alloy and coated with magnetic material. Data is stored by magnetizing or demagnetizing the magnetic coating. A magnetic reader arm is used to read data from and write data to the disks. A typical modern HD has capacity in terabytes (TB). Hard disks store very high volume of data. Currently hard disks can store gigabytes of data (e.g. 500GB). Most operating systems are stored in hard disks and all materials (application software, documents, images, music files etc) that you save on your computer are stored in the hard disk. Similar to floppy disks, data is stored in closed concentric circles called tracks. The metal platter is sealed inside a disk drive. Sometimes some hard disk may fail due to a head crash, resulting in loss of data and this is due to various reasons such as: improper handling of hard disks, voltage problems, … etc.
Optical Disks :
An optical disk
is primarily used as a portable and secondary storage device. It can store more
data than the previous generation of magnetic storage media, and has a
relatively longer lifespan. Compact disks (CD), digital versatile/video disks
(DVD) are currently the most commonly used forms of optical disks. Optical
disks have very less cost-per-bit. The mostly used optical disk with storage
capacity of 700MB/ 800MB. These disks are generally used to:
▪ Distribute
software to customers
▪ Store large
amounts of data such as music, images and videos
▪ Transfer data
to different computers or devices
▪ Back up data from a local machine
Optical Disk
Drive
Optical disk
drive is used to read and write data from an optical disk. It consists of all
the electrical, electronic and mechanical components to hold an optical disk
while reading or writing data from or to the optical disk.
Access Time
The optical
disks have continuous spiral track sectors. The accessing of data randomly is
very slow
Optical disk is
a removable media.
Types of Optical
disks are:
• Compact Disk
(CD)
• Digital Versatile Disk (DVD)
Compact Disks
(CD):
Compact
disks are circular disks that use optical rays, usually lasers, to read and
write data. They are very cheap as you can get 700 MB of storage. CDs are
inserted in CD drives built into CPU cabinet. They are portable as you can
eject the drive, remove the CD and carry it with you.
There are three
types of CDs
1. CD-ROM
(Compact Disk – Read Only Memory) − The data on these CDs are recorded by
the manufacturer. Proprietary Software, audio or video are released on CD-ROMs.
2. CD-R
(Compact Disk – Recordable) − Data can be written by the user only once on
the CD-R. It cannot be deleted or modified later. Some example applications
include creating a backup of your hard disk, creating your own music CD or
saving your family album.
3. CD-RW (Compact Disk – Rewritable) − Data can be written and deleted on these optical disks again and again. Also known as erasable disks. CR-RW (rewritable) can be used to save data several times. This type of optical disk is particularly useful to save large files that need to be updated regularly.
Digital Versatile Disk(DVD): DVD are optical devices that can store 15 times more than CDs. They are usually used to store rich multimedia files that need high storage capacity. DVDs also come in 3 varieties – read only, recordable and rewritable.
Zip Disk Drive: The drive which is used to play the zip disk is called Zip Disk Drive. The capacity of these disks ranges from 8GB to 500GB. The drive use to run or connect these disks with computer is called zip drive. These drives can be of fixed type or portable type. The fixed type of zip drives cannot be disconnected from the system while the portable drives can be easily removed and connected with other systems. The zip disks can be inserted and removed from the zip drives as easy as inserting and ejecting a CD drive in a CD player.
Flash Drive (Pen
Drive) Pen
drive is also called as USB flash drive which is a secondary storage device. It
comprises of the components like USB connectors. Pen drives use EEPROM
(Electrically Erasable Programmable Read Only Memory) that enables to read,
write and erase the data easily. Pen drives are available with 256MB, 512MB,
1GB, 2GB, 4GB, 8GB, 16GB, 32GB and 64GB capacities.
Working of Flash
Drive or Pen Drive: It
is often identified as a plug and play device as it does not require any kind
of external power supply, a software or a battery for its functioning. The user
can start using it by just directly plugging it into the system’s USB port
which is then automatically recognized by the operating system. As soon as the
operating system detects the device, a new drive is created in “My computer”
icon of the system. Now, the user can read, transmit or rewrite the data from a
pen drive to computer and vice versa. Finally, after completing the task,
eject/remove the device safely from the computer for future use.
Furthermore, a
pen drive is a readable, writable and removable device where the user can
access data (read and write), listen audio and watch videos as well. It can
have varied storage capacity. And due to its compact size, longer life span and
faster accessibility, pen drives are preferred more than floppy disks and
CD-ROMs.
Off-line Storage
Memory Card :
An electronic flash memory storage disk commonly used in consumer electronic
devices such as digital cameras, MP3 players, mobile phones, and other small
portable devices.
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