External Memory Devices:

It is desirable that the operating speed  of the primary storage of a computer be as fast as possible because most of the transfers of data to and from the processing unit are via the main memory. For this reason, storage devices with very fast access times, such as semiconductors, are generally chosen for the design of main memory. These high-speed storage devices are expensive and hence the cost per bit of storage is also high for a primary storage. Thus the storage capacity of the main memory of a computer system is limited. Unfortunately, the storage capacity of the primary storage of today's computers is not sufficient to store the large volume of data. As a result, additional memory, called the auxiliary memory or secondary storage, is used with most computer system.

This section of the computer's memory is characterized by low cost per bit stored, but generally it very slow than that of the primary storage. This section of the memory is also referred to as backup storage, bulk storage, permanent storage, nonvolatile storage, and external storage because it is used to store large volume of data on a permanent basis which can be partially transferred  to the primary storage as and when required for processing. Data are stored in secondary storage in the same binary codes as in main storage and are made available to main storage as needed. A wide range of secondary storage devices are available typical hardware devices are magnetic tape and magnetic disk.

External Memory Devices is further subdivided in to two main categories called:

1. Sequential Access Memory

2. Direct Access Memory

1. Sequential Access Memory:

Sequential access storage involves examining all the records in sequence from the beginning to the desired records. This form of storage is non addressable because an operator can not directly refer to the contents of particular storage location. Such sequential access is made with magnetic tapes. Writing on tape is destructive i.e. new information is written and old information is destroyed data can be erased and reused  and provide a relatively low cost storage medium over a long period of time.

i. Punched Paper Tape:

It is a sequential-access secondary storage device. Data is coded on paper tape in the form of punched holes combination. The tape is normally one inch wide, comes in rolls, and may be used in any length up to several hundred feet. There is a line of sprocket holes in the middle of the tape for the purpose of feeding the tape through the tape punching and reading device. Information is recorded as holes punched in rows across the width of the tape, with one row representing one character. The maximum number of holes per row is referred to as the number of channels on the tape. The particular coding system used to record information on the tape depends on the number of channels. An eight channel tape is the one most commonly used.
The number of characters that can be stored in a given physical space is low for paper tape storage. Moreover, the paper tape media is easily torn and mutilate. Hence, the use of paper tapes as a secondary storage device is very rare today and is vanishing day by day. However, the storage capacity of punched paper tape is virtually unlimited, and the cost per bit stored is very low.

ii. Magnetic Tape:

Magnetic tape is one of the most popular storage mediums for large data that are sequentially accessed and processed. The tape is a plastic ribbon usually 1/2 inch wide that is coated on one side with an iron-oxide material which can be magnetized. The tape ribbon itself is stored in a small cartridge or cassette. It is similar to the tape used on a tape recorder except that it is of higher quality and more durable. Like recorder tape, computer tape can be erased and reused indefinitely (destructive media). Old data on a tape are automatically erased as new data are recorded in the same area.
information is recorded on the tape in the form of tiny invisible magnetized and non-magnetized spots representing 1's and 0's on the tape. The tape is divided into vertical columns called frames and horizontal rows called channels or tracks.

Disadvantages of Magnetic Tape:

• No direct access
  Magnetic tape is a sequential access device and hence data recorded on tape can't be addressed directly. They can only be retrieved serially. So if a data item is at the end of a tape, all the earlier parts have to be read before accessing the required information.
   
• Slow
  If random access is frequently required then magnetic tape is not a suitable storage media for such type of data. Too much operator time would be required to load and unload tapes, and to much machine time would be wasted in fetching the exact data that is needed.
   
• Indirect Interpretation
  Data stored on magnetic tape are in the form of tiny invisible magnetized and non-magnetized spots. Hence the contents of a tape can't be interpreted and verified directly. This calls for the need of machine interpretation of the stored data.
   
• Environmental problems
  Specks of dust un controlled humidity or temperature levels can cause tape reading errors. Moreover, tapes and reel containers must be stored in a dust free environment and should be properly labeled so that some useful data stored on a particular tape is not erased by mistake.

iii. Tape Cassettes and Cartridges:

Magnetic tapes are suitable for use only with large and medium size computers. The same magnetic is used in smaller systems in the form of cassettes and cartridges.

Direct access storage is addressable . A given item can be selected from any place in the storage simply by specifying the address where it is located. such a direct access is made with magnetic disk. All types of disks can be included in direct access storage.

i. floppy disk:

A popular direct access secondary storage medium for micro and mini computer is the flexible or floppy disk. Floppy disks are also referred as diskettes are floppies. They were introduced by IBM in 1972. A floppy disk is made of flexible plastic which is coated with magnetic oxide. The flexible disk is enclosed within a squire plastic of cardboard jacket. The jacket gives handling protection to the disk surface. Moreover, it has a special liner that provides a wiping action to remove dust particles that are harmful for the surface and the read/write head. The floppy can be very easily loaded into, and unloaded from, a drive unit.

The capacity of diskettes varies depending on their size and type. floppy disks are very cheap as compared to other storage devices.

A magnetic is a thin, circular metal plate coated on both slides with a magnetic material. It is very similar in appearance to a gramophone record. A disk pack consists of a number of these disks. All the disks of a disk pack move simultaneously in the same direction and at equal speed. Magnetic disks are the most popular medium for direct-access secondary storage.

• Storage of Information
  In the disk pack, information is stored on both the surfaces of each disk plate except the upper surface of the top plate and the lower surface of the bottom plate which are not used. Each disks consist of a number of invisible concentric circles called tracks. A set of corresponding tracks in all the surfaces is called a cylinder. Thus a-disk pack heaving 10 disks plates will have 18 recording surfaces and hence it will have18 tracks per cylinder. Each track is further subdivided into sectors. Information is recorded on the track of a disks surface in the form of invisible tiny magnetic spots. The presence of a magnetized spot represents a 1 bit and its absence represents a 0 bit. In some systems, the outer track contain more bits than the inner tracks, because the circumference of an outer track is greater than that of an inner track. However in most systems, each track contains the same number of characters, which means that the outer track of the disk are less thickly packed with characters than those towards the center.
  
  The information stored on a disk can be read many times without affecting the stored data. So the reading operation is non-destructive. But the writing of new data at the same location erases previously stored data. The data stored on a magnetic disk remains indefinitely until they are erased.
   
• Storage Capacity
  The more disk surfaces a particular disk pack has, the greater will be its storage capacity. But the storage capacity of disk system also depends on the tracks per inch of surface and the bit per inch of track. The total number of bytes that can be stored in a disk pack is:
  
  hard disk capacity = Number of cylinders × Track per cylinder × Sectors per tracks × Bytes per sector
   
• Accessing of Data
  Data are recorded on the track of a spinning disk surface and read from the surface by one or more read/write heads. There are two basics types of disks systems: moving-head and fixed-head.
   

  Terms

  Cylinder: Each high capacity hard disk may have several surfaces and tracks per surfaces, numbered consecutively from outside to inside. Corresponding tracks on each surface is called a cylinder. Reading and writing is done cylinder wise in the hard disk. Track: The concentric circles (rings) on a diskette along which data is recorded. These tracks be neither visible nor a single spiral, rather, they are closed concentric circles. Each track is divided onto sectors. Sectors: On a diskette, invisible wedge shaped sections  used by the computer for storage reference purpose is called sector.

  
  The moving head-system consists of one read/write head for each disk surface mounted on an access arm which can be moved in and out. So in this system, each read/write head moves horizontally across the surface of the dick so that it is able to access to each track individually.
  
  Each usable surface of the disk pack has its own head and all the heads move together. Information stored on the tracks which constitute a cylindrical shape through the disk pack is therefore accessed simultaneously.
  
  In the fixed-hard system, the access arm is non-moveable. A large number of read/write heads are distributed over the disk surface, one head for each track. As a result, no head movement is required and therefore information is accessed more quickly.
   

• Access time
  In order to access information from a disk, the disk address of the desired data has to be specified. The disk address is specified in term of the track number, and the sector number. As soon as a read/write command is received by the disk unit, the read write heads are first positioned on to the specified track number by moving the arm assembly in the proper direction. This involve a mechanical motion of the arms and is slow. The time required to position the head over the proper track is called the seek time.
  
  The seek time varies depending on the position of the arm assembly when a read/write command is received. If the arm assembly is positioned on the outer most track and the track to be reached is the inner most one then the seek time will be maximum, and it will be zero if the arm assembly already happened to be on the desired track. The average seek time is thus specified for most systems which is generally somewhere between several milliseconds to fractions of a seconds. Note that seek time is associated only with movable-head systems. For a fixed-head system, the seek time is always zero because there is a head for each track and no movement of head is required for accessing a particular track.
  
  Once the heads are positioned on the desired track, the head on the specified surface is activated. Since the disk is continuously rotating, this head should wait for the desired data (specified sector) to come under it. This rotational waiting time, i. e, the time required to spin the needed data under the head is called the latency time. The latency time is also a variable and depends on the distance of the desired data from the initial position of the head  on the specified track. It also depends on the rotational speed of the disk.
  
  The total access time for a disk is equal to the seek time plus the latency time. Technically speaking, disk system have direct but not random access to the stored data. Random access refers to a storage device in which the access time is independent of the physical location of the data. For example, primary storage is random access storage. Since the disk access time is dependent on the physical location of data, it is more correct to say that disks provide direct access. This distinction is not always observed and hence disk system are some times referred to as random access storage devices.
   
• hard disk Connection
  There are two types of connection SCSI (pronounced skuzzy) and EIED. It is not true that a SCSI type drive is faster than an EIDE type drive. SCSI and EIDE a simply terms that describe two kinds of technological connections by which a hard disk is attached to a microcomputer.
  
  SCSI
  SCSI (small Co System Interface)
  EIDE
  
  EIDE (Enhanced integrated drive electronic) connects hard drives to a microcomputer by using a flat ribbon cable attached to an expansion board also called host adapter or card that plugs into an expansion slot on the motherboard.
  
  EIDE is popular because of its low cost, and it is increasingly being used to connect CD-ROM drives and tape drive. An inexpensive EIDE host adapter can control two to four hard drives, CD-ROM, two diskette drives, two serial ports, and a game port.
   

  Comparison Between Magnetic Tape and Magnetic Disk Storage

  Magnetic disk has the flexibility of being used as a sequential as well as a direct access storage device. On the other hand, magnetic tapes can be used only for sequential processing of data. Magnetic disks are less vulnerable to damage from dust or care less handling than magnetic tapes. Any information desired from disk storage, can be accessed in a few milliseconds because it is a direct access storage device. This is not possible in case of a tape storage which is a sequential access storage device. Data transfer rate for a magnetic disk system is normally higher than a tape system. A disk pack equal in storage capacity to a reel of magnetic tape may be about 25 times more expensive. On a cost-per-bit basis, the cost is disks is low. But the cost of magnetic tape is even less. Sequential processing using disks may be slower and less efficient than when tapes are used. It is easies to maintain the security of information stored on a tape as compared to information stored on disk. Disk packs are not so easily portable like magnetic tapes.

iii. Winchester disk:

There is a third type of relatively new disk storage unit known as Winchester disk. In this unit disks are permanently sealed in contamination free (airtight) containers. the disks are coated with a special lubricant which reduce the friction when the read/write heads land on the disk surface. The container is usually not removed from the disk drive. High capacity systems using these sealed housings are said to employ Winchester technology. The technology enables greater precision of alignment, an increase in the number of tracks on the disk surface and a higher storage density per track.

Winchester disks are fast and highly reliable, yet low priced compared which conventional hard-disk devices.

iv. Magnetic Drum:

like the magnetic disk, magnetic drum is a direct access storage device that can be used for both sequential and random processing. It consists basically of a cylinder whose outer surface is coated with a thin layer of magnetizable material. A motor rotates the cylinder on its axis at a constant and rapid rate. The surface of the drum is divided into tracks upon which data is stored as magnetized spots in the same manners as on the surface of the disk. Data is stored and read from the drum by a set of read/write heads, which are positioned a fraction of an inch from the drum surface.

v. Optical disks:

An optical disk storage system consists of rotating disk. Data recording is done by focusing a laser beam on the surface of the spinning disk. The laser beam is turned on and off at a varying rate because of which tiny holes are burnt into the metal coating of the disk along its tracks.

In order to read the stored data, a less powerful laser beam is focused on the disk surface; this beam is strongly reflected by the coated surface and weakly reflected by the holes, producing patterns of on-off reflections that can be converted into electronic signals. A serious problem to optical disk system is that they are permanent storage devices, data once recorded cannot be erased and hence the disk cannot be reused. This technology is also known as WORM (write once read many) technology.

vi. Rewritable Optical Disks:

Rewritable optical disks use several technologies, including magneto-optical technology (MO technology) to integrate optical and magnetic disk technology to enable read and write storage. The 5-1/4 inch rewritable disk cartridges can store up to 1GB. However , the technology must be improved be for the optical disks can be considered as a direct alternative to magnetic media.

At present, rewritable optical disk drives are more expensive and less reliable than magnetic media. in addition. the disk access times for rewritable optical disks are slow relative to magnetic media. For these reasons, most traditional information systems continue to rely on magnetic disks. Rewritable optical disks are beginning to find their place. Applications that call for large volumes of storage with relatively little update activity are made to order for rewritable optical disks. Also, applications that require hardware to operate in harsh environments may be candidates for rewritable optical disks. MO technology is very durable, able to resist shock, magnetic fields, and a wide range of temperatures, from below freezing to 100 degrees Fahrenheit. Magnetic storage media may malfunction under less than ideal conditions as optical laser disk technology natures to offer reliable, cost-effective read/write operation, it eventually may dominate secondary storage in the future as magnetic disks and tape do today.

vii. Magnetic Bubble Memory:

The magnetic bubble memory is an electronic secondary storage made with solid state electronic chips and heaving no moving part. Magnetic bubble can be thought as a tiny positively charged island in a sea of negatively charged film. They are formed by applying magnetic fields to their sheets of certain magnetic materials. The magnetic fields strengthen some regions in the material and weaken other. The strengthened regions break into isolated cylinders that resemble small positively charged island surrounded by a sea of negatively charges. Data is represented in bubble storage by the presence or absence of bubble just as it is represented in punched paper tapes by the presence or absence of holes. The presence or absence of bubble corresponds to 1 & 0 in the binary code. Bubble chips are non-volatile and retain their stored data even when the power is switched off.

viii. Storage Forecast: Is There a Disk in Your Future?

Storage is like money: No matter how much you have, you always want more. Each year, improvements are made in existing secondary storage devices as the storage industry struggle to meet our thirst for more storage.

Some scientists believe that holographic technology mat give users everything they want in a storage device. Holographic memory systems enable the mound of data on the recording surface. The different layers are read by changing the angle of the laser beam used for reading the data. Holographic memory systems will enable the entire encyclopedia Britannica to be stored in a space the size and thickness of a metal rupee.

Rotating storage may go the way of the steam engine when low-cost solid-state memory can stored as much in less space. If nonvolatile chip technology continues to improve at the current pace, the entire encyclopedia Britannica will fit into one tiny memory chips in the near future. Perhaps someday the only moving parts on PCs will be the keyboard, the mouse, and the cooling fan. Its not unreasonable to expect 1GB flash memory in a PC by the end of the decade. If this happens, rotating storage may be relegated to archival storage.

Store more information in less space means videophones that can be worn like wristwatches. It means that you can carry a diskette sized reader and all your college "text books" in your front pocket. We can expect at least one big leap in storage technology in the next 5 to 10 years. That leap will forever change much of what we do and how we do it.

Comparison of Internal & External Memory: 

Comparison of Internal & External Memory
Internal Memory	External Memory
Directly connected to the motherboard Basic part of computer Helps in starting computer Fast Electronic movements Limited size Volatile memory Expensive	Indirectly connected to the motherboard Slow Mechanical movement Large storage Permanent storage cheaper

 

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