Table of Contents
Various Storage Devices
Data & program instructions are stored in computer either temporarily or permanently for quick retrieval & use during computer processing. Mainly there are 2 types of storages.
Primary (Main Memory): Data & instructions are held in it temporarily for immediate access & use by CPU. It’s a volatile form of storage.
Secondary storage: Data & instructions are stored permanently. It’s a non-volatile form of memory.
Magnetic Memory
Magnetic memory store date in the form of magnetic charge. If charge available is denoted as 1 bit binary information and charge not available is called 0 bit binary information. So magnetic memory is type of non volatile memory that store data in the digital format. Following types of magnetic memory used in computer system.
Magnetic Tap
- Magnetic tape was first invented for recording sound by German electronics company AEG in 1933.
- Magnetic tape is a plastic tape coated with magnetisable iron oxide.
- Tape is ½ inch wide & its length varies from 200 to 3600 feet.
- Data recording using ASCII or EBCDIC code.
- Tape divided into 9 column or track for storing data (8 character bits & 1 parity bits).
- It is used sequential access.
- Used for recording audio or video or for computer data storage.
- Recording density was 128 characters per inch on eight tracks.
Magnetic Floppy
Floppy disks are commonly used as secondary memory and backup in personal computer. They are also called diskettes. These disks are very thin and flexible so, are called floppy. They were introduced by IBM in 1972. It is made up of special type of flexible plastic which is coated with magnetic oxide. The disk is enclosed within a square plastic or cardboard jacket. The most popular sizes of floppy disks are 5.25 inches square and 3.5 inches square. A floppy disk is a surface device. The surface is divided into a number of concentric circles Called tracks. Each track is divided into a number of sectors. Tracks and sectors are logical Areas on disk, not physical area. Data are recorded on the tracks of a spinning disk surface and Read from the surface by read write head as the head comes in contact with the disk.
Megnetic Disk (Hard Disk)
- A floppy disk is basically a circular sheet of plastic, coated with magnetic material.
- Before a disk can be used it must be formatted.
- The surface of the disk is divided up into a number of concentric tracks.
- Each of which is subdivided into sectors.
- Floppy disks have 80 tracks on each side and each track is split into 18 sectors.
- A 3.5″ floppy disk with 80 tracks and 18 sectors will have 80 x 18 = 1,440 storage units.
- Each uniquely identified by its track and sector position.
- Each storage unit can hold 512 bytes of data.
- So the disk has a capacity of 1,440 x 512 = 737,280 bytes (720 KBytes) per side, or 1,400 KBytes (1.4 MBytes) per disk.
- A hard disk is a secondary storage device used for mass (size of) storage of data.
- HDD introduced in 1956 as data storage for an IBM accounting computer.
- It is a smooth metal plate coated on both sides with magnetic material.
- A set of such magnetic plates is fixed to a spindle one below the other to make up a disk pack. The disk pack is mounted on the disk drive.
- The drive rotates the pack at a speed of 2400 or more (3600) round per min.
- Thus all the disks of a disk pack move simultaneously in the same direction at equal speed.
- Information is recorded on both the surfaces of each disk except the upper surface of top plate and lower surface of bottom plate.
- Each disk consists of a number of invisible concentric circles called tracks.
- A track is divided into No. of tracks & sector on the disk varies.
- A set of corresponding tracks in all the surfaces is called a cylinder.
- Data is recorded on disk with the help of read/write head.
- Read/write operations on disk start at sector boundaries.
- Typically 512 bytes are stored per sector.
- Total of bytes that can be stored = No. of cylinders * Tracks per cylinder * Sector per track * Bytes per Sector.
- Capacity of HDD’s is 10 GB (IBM PC/XTs in the early 1980s) to 2 TB (As of April 2009).
Magnetic Memory (Zip Disk)
- A The Zip drive is a medium-capacity removable disk storage system that was introduced by Iomega in late 1994.
- Originally, Zip disks launched with capacities of 100 MB, but later versions increased this to first 250 MB and then 750 MB.
Optical Memory
- Optical disk is more popular data storage media.
- Optical disk plastic (polycarbonate) plate coded with special material (aluminium).
- In optical disk data read and write using laser beam.
- In optical disk used high power laser beam to burn/write data on disk surface.
- Used low power laser beam to read data from disk surface.
- Data is represented by the presence of light reflection (binary 1/land) and the absence of light reflection (binary 0/pit) in the storage locations.
- Optical disk store data in various size like-100MB to 50GB.
CD (Compact Disk)
- CD mean’s compact disk.
- CD is plastic (polycarbonate) plate coded with special material (aluminium).
- Used red laser beam for data read and write.
- CD store data in various size like-100MB, 600MB, 700MB and 800MB.
- CD – CD, VCD, CD-ROM, CD-R/W.
DVD (Digital Versatile Disk)
- DVD mean’s digital versatile disk/ digital video disk.
- DVC is plastic (polycarbonate) plate coded with special material (aluminium).
- Used red laser beam for data read and write.
- Used compressed technology for data storing.
- DVD store data in various (4.7 GB in single sided and 8.54 in dual sided).
- DVD – DVD, DVD-ROM, DVD-RW.
Blu-ray Disk
- Blu-ray disk is also called BD.
- Blu-ray disk is plastic (polycarbonate) plate coded with special material.
- Used Blue laser beam for data read and Developed by – Blu-ray Disc Association (Apple, Dell, HP, LG, Philips, Samsung, Sony and Toshiba) in 2000.
- First BD player was released in April 2003 in Japan.
- Blu-ray disk storage capacity of data in single layer is 25GB and double Layer. Is 50 GB.
- BD-ROM – read-only format for distribution of HD movies, games, software etc.
- BD-R – recordable format for HD video recording and PC data storage.
- BD-RE – rewritable format for HD video recording and PC data storage.
Difference b/w CD/DVD/Blu-ray Disk
Difference | CD | DVD | Blu-ray |
Laser Wavelength (pit size) | 720 NM (Nanometre) | 650 NM | 405 NM |
Laser Color | Red | Red | Blue |
Storage Capacity | 100MB-800MB | 4.7GB(single layer) 8.5GB(dual layer) | 25GB(single layer) 50GB(dual layer) |
Disc Diameter | 120mm | 120mm | 120mm |
Disc Thickness | 1.2mm | 1.2mm | 1.2mm |
Protection layer Hard coating | No | 0.6mm No | 0.1mm Yes |
Track pitch | 1.6µm(microns) | 0.74µm | 0.32µm |
Semiconductor Memory
- Semiconductor memory is called flash memory.
- Semiconductor memory is non-volatile computer memory that can be electrically erased and reprogrammed.
- It is a technology that is primarily used in memory cards and USB flash drives for general storage and transfer of data between computers and other digital products.
- It is a specific type of EEPROM (Electrically Erasable Programmable Read-Only Memory)
- Example applications include PDAs (personal digital assistants), laptop computers, digital audio players, digital cameras and mobile phones.
- Semiconductor Memory – Memory Card, Pen Drive, SIMM.
Memory Card
- Memory cards are type of semiconductor memory.
- Developed By – SD Card Association in (ScanDisk and Toshiba).
- Usage – Portable devices, including digital cameras and handheld computers.
- Memory cards type is :-
- SD (SECURE DIGITAL)
- MMC (Multi Media Card)
- Size of memory card is:-
- Standard SD: 8 MB to 4 GB
- Standard MMC : 400MB to 32GB
- Memory card – SD, MMC.
Pen Drive
- Pen Drive is type of semiconductor Also called USB flash drive.
- The pen drive was invented in 1998 by IBM.
- Used in PC replacing the floppy drive.
- Size of pen drive is:-8 MB to 32 GB.
SIMM
- A SIMM, or single in-line memory module, is a type of memory module containing random access memory used in computers from the early 1980s to the late 1990s.
Data Storage and Retrieval Methods
- In computerized systems, collection of records is called as a file & collection of inter-related files is called as a database. There are mainly 3 methods of storing & retrieving data.
- Methods – Sequential, Direct Access, Indexed Sequential.
Sequential Storing & Retrieval Method
- In this case data is retrieved in the sequence in which it was recorded on the storage media. The records must be accessed one after the other. If a particular record (nth) has to be read, you have to start from the first record and have to read n-1 records.
- Magnetic tape & diskettes support this type of storage & retrieval.
Direct Storage & Retrieval Method
- Also called as random access.
- It is generally used where only a few records in a file are to be accessed and in no particular sequence.
- There is of ways to access records directly.
- Indexing
- Hashing
- Direct access method is ideal for applications like airline reservations system or computerized telephone information service where records need to be retrieved only one at a time & there is no fixed pattern or sequence to the requests for data & records.
Indexed Sequential Access Method
- ISAM is used almost exclusively with direct access micro computer storage devices to provide maximum flexibility for processing.
- In this each file contains an index of the records stored in it. It contains the record’s address on the disk. to retrieve a record from a file, file’s index is checked to get record’s address.
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