Computer Storage Device

Storage device is a device which stores the data. Data and instruction entered into a computer system thorough i/p system has to be stored inside the computer before actual procession starts. Then after procession again need to store output or result to at storage device. Types

1. Primary Storage
2. Secondary Storage

Primary Storage

• Also known as main memory
• RAM(Random access memory)
• Volatile memory
• Faster
• Very expensive
• Low storage capacity

Secondary Storage

• Also known as auxiliary storage.
• Hard Disk
• Non-volatile
• Slower
• Cheaper
• Very High Storage.

  Random Access Memory (RAM)

• Primary storage of a computer is often referred to as RAM because of its random access capability
• RAM chips are volatile memory
• A computer’s motherboard is designed in a manner that the memory capacity can be enhanced by adding more memory chips
• The additional RAM chips, which plug into special sockets on the motherboard, are known assingle-in-line memorymodules (SIMMs)

  Read Only Memory (ROM)

• ROM a non-volatile memory chip
• Data stored in a ROM can only be read and used –they cannot be changed
• ROMs are mainly used to store programs and data, which do not change and are frequently used.
• For example,system boot program

  PROM(Programmable Read only memory).

• Embedded system.
• Small chip on the circuit which is programmed
• Eg.
  • Printer,
  • Bios chip
  • Microwave ,
  • washing machine

EPROM

• Erasable programmable Read only memory
• Once information is stored in a ROM or PROM chip it cannot be altered.
• EPROM ever comes this problem.
• As the name suggest chip can be reprogrammed to store new information.

  Cache Memory

• It is commonly used for minimizing the memory-processor speed mismatch.
• It is an extremely fast, small memory between CPU and main memory whose access time is closer to the processing speed of the CPU.
• It is used to temporarily store very active data and instructions during processing.
• Cache is pronounced as “cash”

  Limitations of Primary Storage

1. Limited capacity because the cost per bit of storage is high
2. Volatile -data stored in it is lost when the electric power is turned off or interrupted

 

Secondary Storage

• Used in a computer system to overcome the limitations of primary storage
• Has virtually unlimited capacity because the cost per bit of storage is very low
• Has an operating speed far slower than that of the primary storage
• Used to store large volumes of data on a permanent basis
• Also known as auxiliary memory

   Sequential-access Storage Devices

• Arrival at the desired storage location may be preceded by sequencing through other locations
• Data can only be retrieved in the same sequence in which it is stored
• Access time varies according to the storage location of the information being accessed
• Suitable for sequential processing applications where most, if not all, of the data records need to be processed one after another
• Magnetic tape is a typical example of such a storage device

  Direct-access Storage Devices

• Devices where any storage location may be selected and accessed at random
• Permits access to individual information in a more direct or immediate manner
• Approximately equal access time is required for accessing information from any storage location
• Suitable for direct processing applications such as on-line ticket booking systems, on-line banking systems
• Magnetic, optical, and magneto-optical disks are typical examples of such a storage device

  Magnetic Tape Basics

• Commonly used sequential-access secondary storage device
• Physically, the tape medium is a plastic ribbon, which is usually ½inch or ¼inch wide and 50 to 2400 feet long
• Plastic ribbon is coated with a magnetizable recording material such as iron-oxide or chromium dioxide
• Data are recorded on the tape in the form of tiny invisible magnetized and non-magnetized spots (representing 1s and 0s) on its coated surface
• Tape ribbon is stored in reels or a small cartridge or cassette

  Magnetic Tape –Tape Drive

• Used for writing/reading of data to/from a magnetic tape ribbon
• Different for tape reels, cartridges, and cassettes
• Has read/write heads for reading/writing of data on tape
• A magnetic tape reel/cartridge/cassette has to be first loaded on a tape drive for reading/writing of data on it
• When processing is complete, the tape is removed from the tape drive for off-line storage

Types of Magnetic Tape

1. ½-inch tape reel
2. ½-inch tape cartridge
3. ¼-inch streamer tape
4. 4-mm digital audio tape (DAT)

Advantages of Magnetic Tapes

1. Storage capacity is virtually unlimited because as many tapes as required can be used for storing very large data sets
2. Cost per bit of storage is very low for magnetic tapes.
3. Tapes can be erased and reused many times
4. Tape reels and cartridges are compact and light in weight
5. Easy to handle and store.
6. Very large amount of data can be stored in a small storage space
7. Compact size and light weight
8. Magnetic tape reels and cartridges are also easily portable from one place to another
9. Often used for transferring data and programs from one computer to another that are not linked together

Limitations of Magnetic Tapes

1. Due to their sequential access nature, they are not suitable for storage of those data that frequently require to be accessed randomly
2. Must be stored in a dust-free environment because specks of dust can cause tape-reading errors
3. Must be stored in an environment with properly controlled temperature and humidity levels
4. Tape ribbon may get twisted due to warping, resulting in loss of stored data
5. Should be properly labeled so that some useful data stored on a particular tape is not erased by mistake

  Magnetic Disk -Basics

• Commonly used direct-access secondary storage device.
• Physically, a magnetic disk is a thin, circular plate/platter made of metal or plastic that is usually coated on both sides with a magnetizablerecording material such as iron-oxide
• Data are recorded on the disk in the form of tiny invisible magnetized and non-magnetized spots (representing 1s and 0s) on the coated surfaces of the disk
• The disk is stored in a specially designed protective envelope or cartridge, or several of them are stacked together in a sealed, contamination-free container

 

    Floppy Disks

• Round, flat piece of flexible plastic disks coated with magnetic oxide
• So called because they are made of flexible plastic plates which can bend
• Also known as floppies or diskettes
• Plastic disk is encased in a square plastic or vinyl jacket cover that gives handling protection to the disk surface
• The two types of floppy disks in use today are:
• 5¼-inch diskette, whose diameter is 5¼-inch. It is encased in a square, flexible vinyl jacket
• 3½-inch diskette, whose diameter is 3½-inch. It is encased in a square, hard plastic jacket
• Most popular and inexpensive secondary storage medium used in small computers

  Hard Disks

• Round, flat piece of rigid metal (frequently aluminium) disks coated with magnetic oxide
• Come in many sizes, ranging from 1 to 14-inch diameter.
• Depending on how they are packaged, hard disks are of three types:
• Zip/Bernoulli disks
• Disk packs
• Winchester disks
• Primary on-line secondary storage device for most computer systems today

  o  Zip/Bernoulli Disks

• Uses a single hard disk platter encased in a plastic cartridge
• Disk drives may be portable or fixed type
• Fixed type is part of the computer system, permanently connected to it
• Portable type can be carried to a computer system, connected to it for the duration of use, and then can be disconnected and taken away when the work is done
• Zip disks can be easily inserted/removed from a zip drive just as we insert/remove floppy disks in a floppy disk drive

              o  Disk Packs

• Uses multiple (two or more) hard disk platters mounted on a single central shaft
• Disk drives have a separate read/write head for each usable disk surface (the upper surface of the top-most disk and the lower surface of the bottom most disk is not used)
• Disks are of removable/interchangeable type in the sense that they have to be mounted on the disk drive before they can be used, and can be removed and kept off-line when not in use

                o  Winchester Disks

• Uses multiple (two or more) hard disk platters mounted on a single central shaft
• Hard disk platters and the disk drive are sealed together in a contamination-free container and cannot be separated from each other

Advantages of Magnetic Disks

1. More suitable than magnetic tapes for a wider range of applications because they support direct access of data
2. Random access property enables them to be used simultaneously by multiple users as a shared device. A tape is not suitable for such type of usage due to its
3. sequential-access property
4. Suitable for both on-line and off-line storage of data
5. Except for the fixed type Winchester disks, the storage capacity of other magnetic disks is virtually unlimited as many disks can be used for storing very large data sets
6. Due to their low cost and high data recording densities, the cost per bit of storage is low for magnetic disks.
7. An additional cost benefit is that magnetic disks can be erased and reused many times
8. Floppy disks and zip disks are compact and light in weight. Hence they are easy to handle and store.
9. Very large amount of data can be stored in a small storage space
10. Due to their compact size and light weight, floppy disks and zip disks are also easily portable from one place to another
11. They are often used for transferring data and programs from one computer to another, which are not linked together
12. Any information desired from a disk storage can be accessed in a few  milliseconds because it is a direct access storage device
13. Data transfer rate for a magnetic disk system is normally higher than a tape  system
14. Magnetic disks are less vulnerable to data corruption due to careless handling or unfavorable temperature and humidity conditions than magnetic tapes

Limitations of Magnetic Disks

1. Although used for both random processing and sequential processing of data, for applications of the latter type, it may be less efficient than magnetic tapes
2. More difficult to maintain the security of information stored on shared, on-line secondary storage devices, as compared to magnetic tapes or other types of magnetic disks
3. For Winchester disks, a disk crash or drive failure often results in loss of entire stored data. It is not easy to recover the lost data. Suitable backup procedures are suggested for data stored on Winchester disks
4. Some types of magnetic disks, such as disk packs and Winchester disks, are not so easily portable like magnetic tapes
5. On a cost-per-bit basis, the cost of magnetic disks is low, but the cost of magnetic tapes is even lower
6. Must be stored in a dust-free environment
7. Floppy disks, zip disks and disk packs should be labeled properly to prevent erasure of useful data by mistake

  Optical Disk –Basics

• Consists of a circular disk, which is coated with a thin metal or some other material that is highly reflective
• Laser beam technology is used for recording/reading of data on the disk
• Also known as laser disk / optical laser disk, due to the use of laser beam technology
• Proved to be a promising random access medium for high capacity secondary storage because it can store extremely large amounts of data in a limited space
• Has one long spiral track, which starts at the outer edge and spirals inward to the center
• Track is divided into equal size sectors

     Optical Disk Drive

• Uses laser beam technology for reading/writing of data
• Has no mechanical read/write access arm
• Uses a constant linear velocity (CLV) encoding scheme, in which the rotational speed of the disk varies inversely with the radius

Type:

1)CD Rom Drive

2)Combo DVD

3)DVD Drive

 

Advantages of Optical Disks

1. The cost-per-bit of storage for optical disks is very low because of their low cost and enormous storage density.
2. The use of a single spiral track makes optical disks an ideal storage medium for reading large blocks of sequential data, such as music.
3. Optical disk drives do not have any mechanical read/write heads to rub against or crash into the disk surface.  This makes optical disks a more reliable storage medium than magnetic tapes or magnetic disks.
4. Optical disks have a data storage life in excess of 30 years.
5. This makes them a better storage medium for data archiving as compared to magnetic tapes or magnetic disks.
6. As data once stored on an optical disk becomes permanent, danger of stored data getting inadvertently erased/overwritten is removed
7. Due to their compact size and light weight, optical disks are easy to handle, store, and port from one place to another
8. Music CDs can be played on a computer having a CD-ROM drive along with a sound board and speakers. This allows computer systems to be also used as music systems

Limitations of Optical Disks

1. It is largely read-only (permanent) storage medium. Data once recorded, cannot be erased and hence the optical disks cannot be reused
2. The data access speed for optical disks is slower than magnetic disks
3. Optical disks require a complicated drive mechanism

 

Memory Storage Devices

1)Flash Drive (Pen Drive)

• Relatively new secondary storage device based on flash memory, enabling easy transport of data from one computer to another
• Compact device of the size of a pen, comes in various shapes and stylish designs and may have different added features
• Plug-and-play device that simply plugs into a USB (Universal Serial Bus) port of a computer, treated as removable drive
• Available storage capacities are 8MB, 16MB, 64MB, 128MB, 256MB, 512MB, 1GB, 2GB, 4GB, and 8GB

2)Memory Card (SD/MMC)

• Similar to Flash Drive but in card shape
• Plug-and-play device that simply plugs into a port of a computer, treated as removable drive
• Useful in electronic devices like Camera, music player
• Available storage capacities are 8MB, 16MB, 64MB, 128MB, 256MB, 512MB, 1GB, 2GB, 4GB, and 8GB

 

Storage Hierarchy

• As a single type of storage is not superior in speed of access, capacity, and cost, most computer systems make use of a hierarchy of storage technologies as shown below.