Chapters

Storage Devices

Computer can store a large amount of data. Two types of storage devices are used in computer:

  1. Primary Storage
  2. Secondary Storage

Primary Storage

It is the main memory of computer which keeps track of what is currently being processed. It is a volatile memory means if power is switched off all the data of primary storage will be erased off. RAM is an example of primary memory.

Secondary Storage

It is the auxiliary memory of computer. Data stored in primary memory will never be lost unless we delete them. It is a permanent kind of memory. Examples of Secondary storage are Magnetic disk, Magnetic Tapes, Optical Disks like DVD etc.

Magnetic Disks

They are the most used auxiliary memory. They have large storage capacity, reliable and provide direct access to data. It contains a drive that spins the disk very quickly under a read/write head. All the magnetic disks have a similar format. They are divided into:

  1. Tracks
  2. Sectors
  3. Clusters
  4. Cylinders

When a disk is formatted all data of disk is erased and the surface of magnetic disk is checked for physical and magnetic defects. A root directory is created to list where things are of the disk.

Accessing Data in Magnetic Disks

In order to access the data, the following four steps are required

  1. Seek: It moves the head to proper track that is measured as seek time in terms of milliseconds
  2. Rotate: it rotates the disk under the head to the correct sector that is measured as rotational delay in terms of milliseconds
  3. Settle: It lowers the head to disk and waits for vibrations from moving to stop. It is measured as settling time in terms of milliseconds.
  4. Data Transfer: It copies data to main memory and is measured as data transfer rate in terms of Kbps.

Magnetic Tape

Magnetic tapes are just similar to the VCR tapes in the way of storing data. The speed of data access can be quite slow, however, when the tape is long and whatever desired is not near the start. The magnetic tape is divided into the following formats:

  1. Density: Higher density means more data on shorter tape. It is measured in BPI (Bits per Inch).
  2. Block: It is divided into logical blocks, one file can take up a number of logical blocks, but it must take up one whole block at least
  3. Gap: There are two type of blocks gaps one is Interblock Gap (It separates logical blocks) and other is Interrecord Gap (It is wider and separates the records).

Optical Disks

They are formed of layers and data is arranged in a spiral way which starts from the centre of the disk. These works on the methods of absorb and reflect the light. The absorption and reflection of laser light is read as ‘1’ or ‘0’ by the computer. Types of Optical Disks are:

  1. Read Only: CD-ROM (Compact Disk Read Only memory) is the most common type of optical disk. The data in these disks can be read only we cannot erase and rewrite the data in these disks.
  2. Write Once: The optical disks (CD-R, DVD-R) are well known type of write once disks. We can write data only once in these disks.
  3. Rewrite: In a rewritable disk we can erase and rewrite the data as many times as we require.

Flash Memory

They solid state devices that read and write data electrically, instead of magnetically, these are generally used in Mobile Phones, Digital cameras etc.


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